David S. Strait
Doctoral Program in Anthropological
Sciences, State University of New York,
Stony Brook, New York, 11794-4364,
U.S.A.
Frederick E. Grine
Departments of Anthropology and
Anatomy, State University of New York,
Stony Brook, New York, 11794-4364,
U.S.A.
Marc A. Moniz
Department of Anthropology, Suffolk
Community College, 533 College Road,
Selden, New York, 11784-2899,
U.S.A.
Received 6 December 1995
Revision received 26 July 1996
Accepted 23 August 1996
Keywords: hominid phylogeny,
cladistics, Praeanthropus,
Paranthropus, Australopithecus, Homo.
A reappraisal of early hominid
phylogeny
We report here on the results of a new cladistic analysis of early hominid
relationships. Ingroup taxa included Australopithecus afarensis, Australopithecus
africanus, Australopithecus aethiopicus, Australopithecus robustus, Australopithecus boisei,
Homo habilis, Homo rudolfensis, Homo ergaster and Homo sapiens. Outgroup taxa
included Pan troglodytes and Gorilla gorilla. Sixty craniodental characters were
selected for analysis. These were drawn from the trait lists of other studies and
our own observations. Eight parsimony analyses were performed that differed
with respect to the number of characters examined and the manner in which
the characters were treated. Seven employed ordered characters, and included
analyses in which (1) taxa that were variable with respect to a character were
coded as having an intermediate state, (2) characters with variable states in any
taxon were excluded; (3) a variable taxon was coded as having the state
exhibited by the majority of its hypodigm, (4) variable taxa were coded as
missing data for that character, (5) some characters were considered irreversible, (6) masticatory characters were excluded, and (7) characters whose states
were unknown in some taxa were excluded. In the final analysis, (8) all
characters were unordered. All analyses were performed using PAUP 3.0s.
Despite the fact that the eight analyses differed with respect to methodology,
they produced several consistent results. All agreed that the ‘‘robust’’ australopithecines form a clade, A. afarensis is the sister taxon of all other hominids, and
the genus Australopithecus, as conventionally defined, is paraphyletic. All eight
also supported trees in which A. africanus is the sister taxon of a joint Homo+
‘‘robust’’ clade, although in one analysis an equally parsimonious topology
found A. africanus to be the sister of the ‘‘robust’’ species. In most analyses, the
relationships of A. africanus and H. habilis were unstable, in the sense that their
positions vary in trees that are marginally less parsimonious than the favored
one. Trees in which ‘‘robust’’ australopithecines are paraphyletic were found
to be extremely unparsimonious.
? 1997 Academic Press Limited
Journal of Human Evolution (1997) 32, 17–82
Introduction
Since the description of Australopithecus afarensis (Johanson et al., 1978), there has been a
proliferation of hypotheses concerning early hominid phylogeny (Johanson & White, 1979;
Tobias, 1980; White et al., 1981; Olson, 1981, 1985; Kimbel et al., 1984; Skelton et al., 1986;
Wood & Chamberlain, 1986; Chamberlain & Wood, 1987). The subsequent discovery of
KNM-WT 17000, and its assignment to Australopithecus aethiopicus (Walker et al., 1986; Kimbel
et al., 1988), has added to this debate. Disagreement has centered on the relationships of
A. aethiopicus, Australopithecus africanus, Homo habilis and Homo rudolfensis.
Initially, A. aethiopicus was reconstructed as being ancestral to Australopithecus boisei or to both
A. boisei and Australopithecus robustus (Delson, 1986; Kimbel et al., 1988; Walker & Leakey, 1988;
Grine, 1988a). A similar conclusion was reached by Wood (1991, 1992a,b) following a cladistic
analysis, although he did not formally include A. aethiopicus as a member of his ingroup. In
contrast, three cladistic analyses that have included this species have challenged this result.
Wood (1988) examined the characters described by Walker et al. (1986), while Skelton &
McHenry (1992) employed a much larger trait list. Both studies identified the ‘‘robust’’
australopithecines as a paraphyletic group; A. aethiopicus was identified as the sister of all
Correspondence to David S. Strait.
0047–2484/97/010017+66 $25.00/0/hu960097
? 1997 Academic Press Limited
18
. . 
ET AL.
hominids except A. afarensis, while A. robustus and A. boisei formed a clade that is the sister of
Homo. In a more recent study, Lieberman et al. (1996) examined the cladistic relationships of
H. habilis and H. rudolfensis. Although they did not endorse a tree, the four most parsimonious
cladograms that they generated also supported ‘‘robust’’ australopithecine paraphyly. In all
four, A. aethiopicus was reconstructed as being the sister of a clade that includes A. robustus,
A. boisei, A. africanus and Homo. The results of these three studies suggest that A. aethiopicus
represents a lineage that is distinct from that of other ‘‘robust’’ species (Skelton & McHenry,
1992).
Prior to the discovery of KNM-WT 17000, A. africanus was widely regarded either as the
ancestor of ‘‘robust’’ australopithecines (Johanson & White, 1978; White et al., 1981; Rak,
1983; Kimbel et al., 1985) or as the ancestor of all later hominids (Tobias, 1980; Skelton et al.,
1986). Following its discovery, it was recognized that those relationships might not be tenable
(Delson, 1986; Walker et al., 1986; Grine, 1988a; Kimbel et al., 1988; Walker & Leakey, 1988).
Recent cladistic studies have differed markedly concerning the phylogenetic relationships of
A. africanus. It has been reconstructed as being the sister of ‘‘robust’’ species (Chamberlain &
Wood, 1987), the sister of Homo (Wood, 1991, 1992a), the sister of a Homo+‘‘robust’’ clade
(Wood, 1988; Skelton & McHenry, 1992), or a species nested within the Homo clade
(Lieberman et al., 1996).
In response to the growing consensus that the H. habilis sensu lato sample may represent more
than one species (see Wood, 1992b for review), three cladistic studies have addressed the
relationships of the early Homo sample. Chamberlain & Wood (1987) divided H. habilis sensu lato
into two groups that correspond to geographic areas (Olduvai Gorge vs. Koobi Fora), and
found evidence that Homo might be paraphyletic. However, this geographic division is not
generally accepted. Subsequently, Wood (1991, 1992a) recognized two morphologically
distinct species within H. habilis sensu lato, H. habilis sensu stricto and H. rudolfensis, and found them
to be sisters. Recently, Lieberman et al. (1996) concluded that they were not sisters.
The present study offers a reappraisal of early hominid phylogeny. It differs from prior
cladistic studies in four important respects: (1) the recognition of taxa and their
hypodigms—i.e., the construction of operational taxonomic units (OTUs), (2) the use of
functional and structural inferences, (3) the choice of characters and the assignment of
states—i.e., the character analysis, and (4) the configuration of the parsimony analysis.
Alpha taxonomy
Hominid (ingroup) taxa recognized and employed here include A. afarensis, A. africanus,
A. aethiopicus, A. robustus, A. boisei, H. habilis sensu stricto, H. rudolfensis, Homo ergaster, and Homo
sapiens (Table 1). The generic nomenclature employed in this study is conservative in that only
two taxa are recognized, viz. Australopithecus and Homo. This is necessary so as not to presuppose
evolutionary relationships among australopithecine taxa. Although we have recognized the
validity of, and have employed the nomen Paranthropus elsewhere (Grine, 1986, 1988a,b;
Jungers & Grine, 1986; Grine et al., 1990; Grine & Susman, 1991; Grine & Daegling, 1993;
Grine & Strait, 1994), the studies by Wood (1988), Skelton & McHenry (1992) and Lieberman
et al. (1996) have suggested that the species usually attributed to this genus are not
monophyletic. If true, then Paranthropus carries with it only a grade (as opposed to a
phylogenetically meaningful) connotation. Thus, the approach employed by Tobias (1967) will
be adopted here, in that all non-Homo early hominid species are attributed to Australopithecus.
However, the fundamental principle of cladistic classification is that taxonomic names should
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19
represent monophyletic groups, and it is possible that this genus is paraphyletic. Although
there has yet to be any serious doubt cast upon the monophyletic nature of the genus Homo,
it too is possibly paraphyletic. Therefore, the matter of taxonomic nomenclature will be
revisited following the analysis of phylogenetic relationships.
There are very few instances in the African Plio-Pleistocene fossil record where postcranial
remains may be assigned with confidence to a specific taxon, the holotype of which invariably
is comprised by cranial, mandibular and/or dental remains. Even in those instances where
isolated skeletal elements may be attributed with reasonable assurity to a particular species, it
is exceedingly rare to be able to document homologous elements in more than perhaps one or
two other taxa. As a result, the inclusion of postcranial skeletal features would have resulted in
a data matrix with an inordinate amount of missing information. Because of this, the present
analysis does not incorporate postcranial characters, other than to recognize that bipedalism,
and the features associated with this mode of locomotion ultimately define the Hominidae (as
traditionally recognized). Thus, in this study, only craniodental characters are examined.
Similarly, it was not deemed possible to include the recently described species Ardipithecus
ramidus (White et al., 1994, 1995), Australopithecus anamensis (Leakey et al., 1995), and
Australopithecus bahrelghazali (Brunet et al., 1996) because they lack many of the cranial and
dental elements employed in this study.
The A. afarensis hypodigm employed here includes all of the cranial, mandibular and dental
remains from the Laetolil Beds, Tanzania (White, 1977, 1980), and all of the fossils from the
Sidi Hakoma, Denan Dora and Kada Hadar Members of the Hadar Formation (Kimbel et al.,
1982, 1984, 1994; White & Johanson, 1982; Johanson et al., 1982). Also included are the
frontal fragment from Behlodelie (White, 1984; Asfaw, 1987), the mandible from Maka (White
et al., 1993), and the teeth from Fejej (Fleagle et al., 1991). The fragmentary cranial vault and
face from the lower part of the Tulu Bor Member of the Koobi Fora Formation (KNM-ER
2602) is also accepted as representative of this species (Kimbel, 1988). Although Wood (1991)
has argued that this specimen fits best within A. boisei or A. aethiopicus, he also has noted that it
lacks an occipital–marginal sinus, which appears to be characteristic of A. boisei and, more
importantly, that the possibly associated dental remains have relatively thin enamel (Beynon &
Wood, 1986). Both A. aethiopicus and A. boisei have very thick tooth enamel (Beynon & Wood,
1986; Grine & Martin, 1988). Moreover, the presence of deciduous teeth in possible
association with this fragmentary cranium suggests it to have been a subadult individual, which
would not exclude it from A. afarensis simply because the temporal lines and nuchal ridges fail
to merge.
The A. africanus hypodigm comprises the specimens from Taung, Members 3 and 4 of the
Makapansgat Formation, and Member 4 of the Sterkfontein Formation (Wood, 1985).
Specimens that have been recovered since the late 1960s at Sterkfontein by the Witwatersrand
University excavations of calcified Member 4 breccia from the Type Site, as well as some of
the specimens recovered from decalcified breccia in the Extension Site (=West Pit) that is of
presumed Member 4 equivalence have been included in this analysis. Skelton & McHenry
(1992) included Sts 25, a very poorly preserved neurocranium with heavily etched fragments
of the cranial base, in their sample of A. africanus. It was omitted from the present study,
however, because it lacks diagnostic morphology that permits its secure attribution to any of
the taxa recognized here.
The A. robustus hypodigm includes specimens recovered from Kromdraai B East by Broom
and Brain, and by Vrba through her excavation of in situ Member 3 breccia (Grine, 1988a).
Although Howell (1978), Grine (1982, 1985, 1988b) and Jungers & Grine (1986) have cited
. . 
20
Table 1
ET AL.
Cranial and mandibular specimens included here in the hypodigms of early hominid
species
A. afarensis:
Crania:
AL
Garusi
KNM-ER
Mandibles:
AL
LH
MAK
A. africanus:
Crania:
Sts
Stw
TM
Taung
MLD
Mandibles:
Sts
Stw
MLD
A. aethiopicus:
Crania:
KNM-WT
L
Mandibles:
KNM-WT
L
Omo
A. robustus:
Crania:
SK
SKW
SKX
TM
Mandibles:
SK
SKW
SKX
TM
A. boisei:
Crania:
OH
KNM-ER
KNM-WT
KNM-CH
Omo
Mandibles:
KNM-ER
KNM-WT
L
Natron
33-125, 58-22, 162-28, 199-1, 200-1, 288-1, 333-1, 333-2, 333-45, 333-105, 417-1, 444-2
1
2602
128-23, 145-35, 188-1, 198-1, 207-13, 266-1, 277-1, 288-1, 311-1, 333w-1, 333w-12, 333w-60,
400-1a, 417-1
4
VP-1/12
5, 17, 20, 26, 67, 71, 52a
13, 73, 252, 505
1511, 1512
1
1, 6, 9, 37/38
7, 36, 52b
384, 404, 498, 513
2, 12, 22, 29, 34, 40, 45
17000
338-y-6
16005
55-s-33, 860-2
18-1967-18, 44-1970-2466, 57-4-1968-41
12, 13/14, 46, 47, 48, 49, 52, 55, 65, 79, 83, 848
8, 11, 29, 2581
265
1517
6, 12, 23, 34, 1586
5
4446, 5013
1517
5
405, 406, 407, 732, 733, 13750, 23000
17400
1
323-896
403, 404, 725, 727, 728, 729, 801, 805, 810, 818, 1468, 1469, 1483, 1803, 1806, 3229, 3230, 3729,
3954, 5429, 5877, 15930
16841
7a-125, 74a-21
Table 1 continued on next page
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Table 1 Continued from previous page
H. habilis:
Crania:
OH
KNM-ER
Sts
Stw
SK
L
Mandibles:
OH
KNM-ER
SK
H. rudolfensis:
Crania:
KNM-ER
Mandibles:
KNM-ER
UR
H. ergaster:
Crania:
KNM-ER
KNM-WT
Mandibles:
KNM-ER
KNM-WT
7, 13, 24, 62
1805, 1813, 1478, 3735
19
53
27, 847
894-1
7, 13
1501, 1502, 1805
15, 45
1470, 1590, 3732, 3891
819, 1482, 1483, 1801, 1802
501
3733, 3883
15000
730, 820, 992, 1507
15000
Isolated teeth and specimens from which only dental measurements are taken were not included.
features—primarily subtle dental differences—that may support a specific distinction between
the ‘‘robust’’ australopith samples from Kromdraai B East and the Member 1 ‘‘Hanging
Remnant’’ of the Swartkrans Formation, this is certainly a minority opinion. Most authorities
continue to regard these fossils as comprising a single species. This view is adopted here,
principally because specimens from these two sites do not differ in the characters that were
employed in the present analysis. Thus, fossils from Members 1, 2 and 3 of the Swartkrans
Formation are included in the A. robustus hypodigm (Grine, 1988b; Grine & Daegling, 1993;
Grine & Strait, 1994).
The A. boisei hypodigm includes specimens from Olduvai Gorge Beds I and II, the Humba
Formation at Lake Natron, the Chemoigut Formation at Chesowanja, the Upper Burgi, KBS
and Okote Members of the Koobi Fora Formation, Members G through L of the Shungura
Formation, and the Kaitio Member of the Nachukui Formation (Grine, 1981; Walker &
Leakey, 1988; Wood, 1991; Brown et al. 1993; Wood et al., 1994). This cranial, dental and
mandibular sample conforms to that attributed by Wood et al. (1994) to Paranthropus boisei sensu
stricto. In particular, the Omo 323-896 cranium from Member G (G6-7) of the Shungura is
here considered to belong to that taxon.
The A. aethiopicus hypodigm includes the KNM-WT 17000 cranium and KNM-WT 16005
mandible from the Lokalalei Member of the Nachukui Formation, and the Omo 44-19702466 mandible, the L 338-y-6 partial cranium, and the Omo 57-4-1968-41 mandible from
Member E (Units E-1, E-3 and E-4 respectively) of the Shungura Formation. It also includes
the L 860-2 mandible from Member F (F-1) and the L 55-s-33 and Omo 18-1967-18
mandibles from Member C (C-6 and C-8) among the more complete specimens. Additionally,
22
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ET AL.
isolated teeth—principally molars and premolars—that have been identified by Suwa (1988) as
a ‘‘robust’’ australopithecine less derived than A. boisei, and by Wood et al. (1994) as Paranthropus
aff. P. boisei are included here in the hypodigm of A. aethiopicus. Among the isolated teeth so
attributed are the deciduous molars (L 64-2 and L 704-2) from Member D that were initially
attributed to A. boisei by Grine (1985). The A. aethiopicus hypodigm comprises ‘‘robust’’
australopithecine specimens that derive from strata in the Turkana Basin that predate
Member G of the Shungura Formation. This attribution follows upon and is consistent with
the studies by Rak & Howell (1978), Walker et al. (1986), Holloway (1988), Suwa (1988), and
Wood et al. (1994). In addition, Suwa (1988) has identified ‘‘robust’’ australopithecine teeth
that differ from those of A. boisei within the lower units of Member G.
The specific attribution of the East African early Homo specimens follows that of Wood
(1992a). Thus, three species (H. habilis, H. rudolfensis and H. ergaster) are recognized for the Late
Pliocene and Early Pleistocene ‘‘gracile’’ hominid fossils from Ethiopia, Tanzania and Kenya.
Because of the controversy surrounding the attribution of the KNM-BC 1 temporal from the
Chemeron Formation (Hill et al., 1992; Falk & Baker, 1992; Tobias, 1993) we have refrained
from assigning it to any specific hypodigm and it is not considered here. The Stw 53 cranium
from Member 5 of the Sterkfontein Formation is tentatively attributed to H. habilis following
suggestions by Tobias (1978, 1991), although recent analyses indicate that it may represent a
separate species (Grine et al. 1993, 1996). Unlike Skelton & McHenry (1992), who recognized
the Sts 19 basicranium from Sterkfontein as a specimen of A. africanus, we assign it to Homo
following the analysis of Kimbel & Rak (1993). More specifically, it is referred here to
H. habilis, because that is the species to which all other Sterkfontein specimens of Homo have
been attributed (Tobias, 1991). This specific attribution may also require revision. Furthermore, although the SK 847 cranium from the Member 1 ‘‘Hanging Remnant’’ of the
Swartkrans Formation has been seen to represent early H. erectus (=H. ergaster) by some workers
(e.g., Walker, 1981; Clarke, 1985), it is here tentatively attributed to H. habilis following upon
the studies by Howell (1978), Chamberlain (1987, 1989) and Grine et al. (1993, 1996).
The outgroup taxa employed in the present analysis are Pan troglodytes and Gorilla gorilla.
These species are appropriate because it is (almost universally) accepted that one or both are
the closest extant relatives of hominids. Ten males and ten females of each species were
sampled from the Mammalogy and Anthropology collections of the American Museum of
Natural History. Although the phylogenetic relationships of Pan and Gorilla are the subject of
considerable debate (e.g., Andrews & Martin, 1987; Begun, 1994; Goodman et al., 1994;
Marks, 1994; Ruvolo, 1994), a preponderance of genetic studies indicate that Pan is the sister
taxon of humans and early hominids, and this topology is assumed here.
Functional morphology in phylogenetic analysis
The role of functional morphology in phylogenetic analysis has been the subject of
considerable debate (e.g., Fischer, 1981; Cracraft, 1981; Szalay, 1981a,b, 1982; Skelton et al.,
1986; Andrews & Martin, 1987; Begun, 1992; Skelton & McHenry, 1992). Within the
framework of numerical cladistics, functional morphology is relevant because functionally
related characters violate the assumption of character independence that is implicit in a
cladistic analysis (Farris, 1983; Kluge, 1989). The bias that results from such character
redundance is removed when functionally related traits are treated as a unit (i.e., as a single
‘‘character’’ or complex). Although several studies of hominoid phylogeny have appealed to
this logic (Skelton et al., 1986; Skelton & McHenry, 1992; Begun, 1992), not all of the
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23
functional hypotheses to which they ascribed have been rigorously tested. Such an omission is
risky, because the refutation of those hypotheses may undermine the validity of any phylogeny
based upon them. A prudent approach, therefore, would be to test functional hypotheses prior
to a phylogenetic analysis. Until this is accomplished, however, characters alleged to be
functionally or structurally related should be treated as independent traits.
Basicranial characters provide an excellent example. It has been suggested that A. boisei,
A. robustus and members of the genus Homo share a number of presumably derived basicranial
character states. These include a flexed cranial base, coronally oriented petrous bones, a
horizontal foramen magnum, a deep glenoid fossa with a steep articular eminence, and a small
post-glenoid process that is fused to the tympanic (e.g., DuBrul, 1977; Dean & Wood, 1981,
1982; White et al., 1981; Kimbel et al., 1984; Dean, 1986, 1988a). These characters may
indicate a sister group relationship between these species, as has been suggested by Skelton
et al. (1986) and Skelton & McHenry (1992), or that the cranial base was characterized by
considerable homoplasy during the course of hominid evolution. However, it has also been
claimed that some or all of these characters are either functionally or structurally related to one
of several factors, including brain or cerebellar size (e.g., Biegert, 1963; Gould, 1977; Dean,
1986, 1988a,b; Ross & Ravosa, 1993), degree of facial prognathism (e.g., Scott, 1958; Kimbel
et al., 1984), facial orientation (e.g., Cameron, 1924; Enlow, 1975), posture (e.g., Dart, 1925;
Schultz, 1942, 1955; DuBrul, 1950; Ashton & Zuckerman, 1951, 1952, 1956; DuBrul &
Laskin, 1961), the size of the masticatory apparatus (Biegert, 1963), and vocalization (e.g.,
Laitman et al., 1978, 1979; Laitman & Heimbuch, 1982; Lieberman, 1984). If any of these
hypotheses can be supported, then arguably a number of basicranial features should be treated
as a single character. Support for a Homo+A. boisei+A. robustus clade would therefore be
weakened. To date, few studies have tested competing hypotheses (Ross & Ravosa, 1993; Ross
& Henneberg, 1995; Strait, 1994). However, studies by Ross and colleagues refer to only a
single character, namely basicranial flexion, and although the study by Strait (1994) examined
a broader range of basicranial characters, it was only of a preliminary nature. Thus, at present
it is unclear which of the factors listed above (if any) is a primary influence on basicranial form,
and which basicranial characters (if any) might be so highly related to such a factor as to evolve
as a unit. Until these relationships are established, subjective attempts to group such characters
into complexes for the purposes of phylogenetic analysis are likely to be either incorrect or, at
the very least, the subject of considerable disagreement.
Recently, Lieberman (1995) and Lieberman et al. (1996) have proposed another approach
by which functional morphology may be incorporated into a cladistic analysis. According to
this method, the only characters that should be employed in phylogeny reconstruction are
those that are unlikely to be influenced by epigenetic factors, and that also produce
developmentally homologous states in different taxa. These criteria, however, can be applied
only when the developmental biology of a character has been established experimentally.
Because this condition has not been met for many of the characters examined in this study,
these two criteria were not used in character selection.
Thus, because the functional, structural and developmental relationships among many
cranial characters are poorly understood, this study does not attempt to conflate characters or
construct complexes using functional inferences, with one exception. That exception relates to
masticatory features. Skelton et al. (1986) and Skelton & McHenry (1992) have argued that
characters related to mastication contribute disproportionately to, and thus bias reconstructions of hominid phylogeny (all of which are based upon a consideration of craniodental
characters). In particular, they concluded that masticatory features unfairly link A. africanus
24
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ET AL.
with the ‘‘robust’’ australopithecines (Skelton et al., 1986), and unfairly supported ‘‘robust’’
australopithecine monophyly (Skelton & McHenry, 1992). We are reluctant to conflate
characters or construct complexes using untested functional inferences, but because of their
assertions, we have undertaken an analysis in which features plausibly related to trophic
adaptation were eliminated from consideration (see below). This was necessary for two
reasons. In the first instance, had these characters not been omitted, we would have failed to
address one of the more important conclusions reached by Skelton et al. (1986) and Skelton
& McHenry (1992). Second, one of us (Grine, 1988a) has argued in favor of ‘‘robust’’
australopithecine monophyly, and the removal of presumed masticatory features provides a
conservative test of this hypothesis.
Character analysis
Coded character states are the raw data of any parsimony analysis. For this reason, the
character analysis is arguably the most important stage in a cladistic study. Skelton &
McHenry’s (1992) character analysis presents the most comprehensive summary to date of
characters traditionally cited in studies of early hominid phylogeny. Because the characters
that they employed, and many of the states that they recognized were drawn extensively from
the literature, Skelton & McHenry’s (1992) character analysis served as a sensible starting
point for the present study [as it has for other studies, e.g., Lieberman et al. (1996)]. Each
character employed by them, and a number drawn from other sources (e.g., Clarke, 1977;
Walker et al., 1986; Kimbel et al., 1984, 1988; Wood, 1991, 1992a; Wood et al., 1994), as well
as our own observations, were examined here. One hundred and one craniodental characters
were examined, of which 60 were selected for use in the present study (Table 2, Appendices 1
and 2).
The present character analysis differs from that of Skelton & McHenry (1992) in several
ways. A number of characters were added that were omitted from that study. In addition,
several characters that were employed by them were rejected because they failed to
discriminate among taxa, or were considered to be invalid. In other instances, features
employed by them were replaced by another character that described the same morphology in
a very different manner. Frequently, their characters were accepted, but modified such that
one or several states were changed.
Furthermore, a concerted effort was made to eliminate characters that redundantly describe
the same morphological feature. Such characters, which are abundant in the literature, affect
a phylogenetic analysis in the same manner as functionally related characters: they unfairly
increase the weight of what should be only a single trait. For instance, Kimbel et al. (1984)
described two features of the zygomatic (the height of the masseter origin and the shape of the
zygomaticoalveolar crest) that they recognized as being necessary correlates of each other: the
origin cannot be high unless the zygomatic rises superolaterally. Accordingly, the masseter
origin in ‘‘robust’’ australopithecines and A. africanus is high, and these species also have a
straight zygomaticoalveolar crest. In contrast, A. afarensis and Homo have a low masseter origin,
which is made possible by a strongly arched crest. Kimbel et al. (1984) treated these two
features as a single character, because they essentially describe the same trait. Similarly, in the
present study, any such group of features was expressed as a single character (Appendix 1).
The elimination of descriptively redundant characters is fundamentally different from the
conflation of functionally related ones. For instance, cranial base flexion and the position of the
foramen magnum may be functionally or structurally related to each other (e.g., DuBrul, 1950;
9. Height of the
masseter origin
10. M-L thickness of
zygomatic arch at
root of frontal
process
11. Anterior projection of
zygomatic bone
relative to piriform
aperture (dishing)
12. Anterior palatal
depth
8. Palate thickness
5. Nasoalveolar clivus
contour in coronal
plane
6. Protrusion of incisor
alveoli beyond
bicanine line (basal
view)
7. Nasal cavity entrance
3. Infraorbital foramen
location
4. Anterior pillars
1. Projection of nasal
bones above
frontomaxillary
suture
2. Inferior orbital
margin rounded
laterally
0
Shallow
0
Shallow
0
Stepped
0
Stepped
0
Posterior
0
Yes
0
Yes
0
Posterior
0
High
0
Absent
0
Convex
0
Thin
0
Low
0
Thin
0
No
Pan: 0
No
Gorilla: 1
Variable
0
High
0
Absent
0
Convex
0
Thin
0
Low
0
Thin
1
Projected,
expanded
0
Projected,
tapered
A. afarensis
0
Shallow
3
Anterior
(dished)
2
Smooth,
overlap
1
Thick
1
High
1
Thick
2
Low
0
Absent
2
Concave
(gutter)
1
No
2
Yes
1
Projected,
expanded
A. aethiopicus
Character
No.
Pan/Gorilla
Characters and the distribution of their states
Table 2
1
Variable
posteriorinterior
2
Deep
(shelved)
0
Thin
1
High
0
Thin
0
Stepped
0
Yes
1
Variable
1
Variable
1
Straight
0
No
3
Variable
A. africanus
0
Shallow
3
Anterior
(dished)
2
Smooth,
overlap
1
Thick
1
High
1
Thick
2
Low
2
Present
2
Concave
(gutter)
1
No
2
Yes
1
Projected,
expanded
A. robustus
2
Deep
(shelved)
3
Anterior
(dished)
2
Smooth,
overlap
1
Thick
1
High
1
Thick
2
Low
0
Absent
2
Concave
(gutter)
1
No
0
No
1
Projected,
expanded
A. boisei
1
Variable
0
Posterior
0
Thin
0
Low
0
Thin
1
Variable
0
Yes
0
High
1
Variable
1
Straight
0
No
2
Not
projected
H. habilis
2
Deep
(shelved)
2
Intermediate
0
Thin
0
Low
?
0
Stepped
1
No
0
Absent
1
Straight
?
0
No
2
Not
projected
H. rudolfensis
2
Deep
(shelved)
0
Posterior
0
Thin
0
Low
0
Thin
0
Stepped
0
Yes
0
High
0
Absent
1
Straight
0
No
2
Not
projected
H. ergaster
2
Deep
(shelved)
0
Posterior
3
Smooth, no
overlap
0
Thin
0
Low
0
Thin
0
Yes
0
High
0
Absent
0
Convex
0
No
2
Not
projected
H. sapiens
  
25
24.
23.
22.
21.
20.
1
At or
anterior
0
Absent
0
At or
posterior
0
Absent
1
At or
anterior
1
Present
0
Prognathic
A. robustus
A. boisei
H. habilis
H. rudolfensis
H. ergaster
H. sapiens
0
Not extensive
2
Absent
0
No
2
Absent
0
Yes
1
428–550 cm3
0
Lateral flare,
posterior
protrusion
1
Intermediate
2
Moderate
?
2
Absent
0
No
?
0
Yes
2
Yes
3
Strong
1
2530 cm3
1
Tucked
1
Extensive
2
Absent
1
Yes
1
Partial
0
Yes
2
Yes
3
Strong
2
Absent
0
No
2
Absent
1
No
0
No
0
Weak
2
Absent
0
No
2
Absent
1
No
0
Weak
?
2
Absent
0
No
2
Absent
1
No
1
Intermediate
0
Weak
4
21400
1
Tucked
0
0
0
0
Not extensive Not extensive Not extensive Not extensive
1
Variable
0
No
1
Partial
0
No
1
Variable
moderate–
weak
0
Yes
1
2
3
3
475–530 cm3 509–675 cm3 750–875 cm3 750–875 cm3
1
1
1
1
Tucked
Tucked
Tucked
Tucked
1
2
2
2
2
3
3
Variable
Mesognathic Mesognathic Mesognathic Mesognathic Orthognathic Orthognathic
prognathic–
mesognathic
0
1
1
0
?
0
0
At or
At or
At or
At or
At or
At or
posterior
anterior
anterior
posterior
posterior
posterior
0
1
0
0
0
0
0
Absent
Present
Absent
Absent
Absent
Absent
Absent
A. africanus
. . 
19.
18.
17.
0
Prognathic
0
Prognathic
A. aethiopicus
0
0
0
<500 cm3 400–500 cm3
419 cm3
Cerebellar
0
0
0
morphology
Lateral flare, Lateral flare, Lateral flare,
posterior
posterior
posterior
protrusion
protrusion
protrusion
O–M sinus present in
0
2
0
high frequency
No
Yes
No
Anteromedial
2
2
3
incursion of the
Moderate
Moderate
Strong
superior temporal
lines
Sagittal crest present,
Pan: 1
0
0
at least in
No
Yes
Yes
presumptive males
Gorilla: 0
Yes
Compound T/N
0
0
0
crest, at least in
Extensive
Extensive
Extensive
presumptive males
Asterionic notch
0
0
0
Present
Present
Present
Parietal overlap of
0
0
1
occipital at asterion,
No
No
Yes
at least in males
Squamosal suture
0
0
1
overlap extensive, at Not extensive Not extensive Extensive
least in males
15. Maxillary trigon
(zygomaticomaxillary
step)
16. Cranial capacity
13. Index of palate
protrusion anterior to
sellion (facial
prognathism)
14. Masseteric position
relative to sellion
A. afarensis
Character
No.
Pan/Gorilla
Continued from previous page
Table 2
26
ET AL.
35. Medio–lateral
position of external
auditory meatus
34. Configuration of
tympanic
33. Postglenoid process
size and position
30. External cranial base
flexion
31. Horizontal distance
between TMJ and
M2/M3
32. Relative depth of
mandibular fossa
29. Supraglenoid gutter
width
27. Pneumatization of
temporal squama
28. Facial hafting
25. Lateral inflation of
mastoid process
relative to
supramastoid crest
26. Postorbital
constriction
0
Tubular (or
weak crest)
Pan: 0
Medial
Gorilla: 2
Lateral
Pan: 0
Shallow
Gorilla: 2
Intermediate
0
Large and
anterior
1
Moderate
Pan: 1
Moderate
Gorilla: 0
Marked
0
Extensive
0
Low
Pan: 0
Narrow
Gorilla: 1
Wide
0
Flat
0
Long
0
Shallow
0
Flat
0
Long
0
Extensive
1
High
1
Wide
0
Marked
2
Inflated
A. aethiopicus
2
Flexed
0
Long
2
Reduced
1
High
1
Wide
0
Marked
2
Inflated
A. robustus
2
2
Intermediate Intermediate
1
Moderate
0
Long
0
Extensive
0
Low
0
Narrow
1
Moderate
0
Not inflated
A. africanus
3
Deep
2
Flexed
0
Long
1
Variable
1
High
1
Wide
0
Marked
2
Inflated
A. boisei
0
Long
?
2
Reduced
0
Low
0
Narrow
1
Moderate
0
Not inflated
H. rudolfensis
2
2
Intermediate Intermediate
2
Flexed
1
Short
2
Reduced
0
Low
0
Narrow
1
Moderate
1
Variable
H. habilis
2
Flexed
1
Short
2
Reduced
0
Low
0
Narrow
1
Moderate
0
Not inflated
H. ergaster
2
Flexed
1
Short
2
Reduced
0
Low
0
Narrow
2
Slight
0
Not inflated
H. sapiens
1
3
Variable
Deep
shallow–
intermediate
0
1
1
3
3
2
1
3
3
Large and Intermediate Intermediate Small and
Small and
Variable
Intermediate Small and
Small and
anterior
fused to
fused to intermediate–
fused to
fused to
tympanic
tympanic
small
tympanic
tympanic
0
1
1
1
2
1
?
1
1
Tubular (or Crest with
Crest with
Crest with
Crest with
Crest with
Crest with
Crest with
weak crest) vertical plate vertical plate vertical plate inclined plate vertical plate
vertical plate vertical plate
0
0
0
2
2
1
0
0
0
Medial
Medial
Medial
Lateral
Lateral
Variable
Medial
Medial
Medial
0
Shallow
0
Long
?
0
Extensive
0
Low
0
Narrow
0
Not inflated
0
Not inflated
A. afarensis
Character
No.
Pan/Gorilla
Continued from previous page
Table 2
  
27
47.
46.
45.
44.
A. africanus
A. robustus
A. boisei
H. habilis
1
Variable
2
Lateral
2
Vertical
1
Variable
?
?
2
Weakly
inclined
?
2
Coronal
?
?
?
H. rudolfensis
H. sapiens
2
Absent
2
Lateral
2
Vertical
2
Absent
3
Posterior
2
Vertical
2
2
Moderate to Moderate to
large
large
1
1
Absent or
Absent or
slight
slight
2
2
Coronal
Coronal
1
0
Variable
Absent
2
2
Weakly
Weakly
inclined
inclined
1
1
At
At
bi-tympanic bi-tympanic
line
line
2
1
Strongly
Roughly
inclined
horizontal
(anterior)
1
1
Deep,
Deep,
narrow notch narrow notch
0
0
Small
Small
H. ergaster
. . 
43.
42.
41.
40.
39.
38.
37.
A. aethiopicus
0
0
0
0
2
2
1
Small or
Small or
Small or
Small or
Moderate to Moderate to
Variable
absent
absent
absent
absent
large
large
Eustacian process of
0
1
1
0
0
1
1
tympanic
Present and Absent or
Absent or Present and Present and Absent or
Absent or
prominent
slight
slight
prominent
prominent
slight
slight
Petrous orientation
0
1
2
1
2
2
2
Sagittal
Intermediate
Coronal
Intermediate
Coronal
Coronal
Coronal
Heart shaped
0
0
2
0
0
2
0
foramen magnum
Absent
Absent
Present
Absent
Absent
Present
Absent
Inclination nuchal
0
1
2
2
2
2
2
plane
Steeply
Intermediate
Weakly
Weakly
Weakly
Weakly
Weakly
inclined
inclined
inclined
inclined
inclined
inclined
Position of foramen
0
1
1
1
3
3
2
magnum relative to
Well
At
At
At
Well anterior Well anterior Variable at
bi-tympanic line
posterior
bi-tympanic bi-tympanic bi-tympanic
or anterior
line
line
line
Inclination of
0
?
?
0
1
1
1
foramen magnum
Strongly
Strongly
Roughly
Roughly
Roughly
inclined
inclined
horizontal
horizontal
horizontal
(posterior)
(posterior)
Origin of digastric
0
0
?
0
1
0
1
muscle
Broad,
Broad,
Broad,
Deep,
Broad,
Deep,
shallow fossa shallow fossa
shallow fossa narrow notch shallow fossa narrow notch
Mandibular
Pan: 0
0
2
0
2
2
0
cross-sectional area at
Small
Small
Large
Small
Large
Large
Small
M1
Gorilla: 1
Variable
Orientation of
0
1
2
1
2
2
2
mandibular
Receding Intermediate
Vertical
Intermediate
Vertical
Vertical
Vertical
symphysis
Direction of mental
Pan: 0
1
2
1
2
2
2
foramen opening
Anterior
Variable
Lateral
Variable
Lateral
Lateral
Lateral
Gorilla: 1
Variable
Hollowing above and
0
0
2
1
2
2
2
behind mental
Present
Present
Absent
Variable
Absent
Absent
Absent
foramen
36. Vaginal process
A. afarensis
Character
No.
Pan/Gorilla
Continued from previous page
Table 2
28
ET AL.
60.
59.
58.
57.
56.
55.
54.
53.
52.
51.
50.
49.
Pan: 0
0
Narrow
Narrow
Gorilla: 2
Wide
Mandibular
0
0
deciduous canine
Apex central, Apex central,
shape
mesial
mesial
convexity,
convexity,
low
low
Incisal reduction
0
1
No
Moderate
Canines reduced
0
1
No
Somewhat
Prominence of
0
0
median lingual ridge
Prominent
Prominent
of mandibular canine
Premolar crown area
Pan: 0
1
Smallest
Gorilla: 3
Molar crown area
Pan: 0
1
Smallest
Gorilla: 2
dM1 mesial crown
0
1
profile
MMR
MMR slight,
absent,
protoconid
protoconid
anterior,
anterior,
fovea open
fovea open
Distal marginal ridge
0
0
of d M2
Low
Low
Separation of molar
0
0
and premolar cusp
Wide
Wide
apices
Frequency of well
0
1
developed P3
Absent
Infrequent
metaconid
Relative enamel
0
1
thickness
Thin
Thick
Dental development
0
0
rate
Delayed
Delayed
48. Width of mandibular
extramolar sulcus
A. afarensis
Character
No.
Pan/Gorilla
Continued from previous page
Table 2
2
Wide
A. robustus
2
Wide
A. boisei
2
2
2
3
3
Largest
5
Largest
0
1
1
Apex central, Apex mesial, Apex mesial,
mesial
mesial
mesial
convexity,
convexity,
convexity,
low
high
high
1
2
2
Moderate
Yes
Yes
1
2
2
Somewhat
Very
Very
1
2
2
Variable
Weak
Weak
1
Variable
A. africanus
2
Hyperthick
?
2
Frequent
1
Thick
0
Delayed
2
Frequent
2
Hyperthick
2
Accelerated
2
Frequent
2
Hyperthick
2
Accelerated
2
Frequent
2
1
2
2
MMR thick, MMR slight, MMR thick, MMR thick,
protoconid protoconid protoconid protoconid
even with
anterior,
even with
even with
metaconid, fovea open metaconid, metaconid,
fovea closed
fovea closed fovea closed
?
0
1
1
Low
High
High
2
1
2
2
Narrow
Intermediate
Narrow
Narrow
3
Largest
4
1
Moderate
2
Very
?
?
2
Wide
A. aethiopicus
1
Thick
?
2
Frequent
0
Low
0
Wide
?
1
1
1
Moderate
2
Very
2
Weak
?
1
Variable
H. habilis
1
Thick
0
Delayed
2
Frequent
0
Low
0
Wide
?
2
2
1
Moderate
2
Very
2
Weak
?
0
Narrow
H. rudolfensis
0
Narrow
H. sapiens
0
Smallest
0
Smallest
2
Frequent
0
Low
0
Wide
1
1
Thick
Thick
1
1
Intermediate Intermediate
2
Frequent
0
Wide
?
1
1
MMR slight, MMR slight,
protoconid protoconid
anterior,
anterior,
fovea open fovea open
0
Smallest
1
0
0
Apex central, Apex central,
mesial
mesial
convexity,
convexity,
low
low
1
2
Moderate
Yes
2
2
Very
Very
2
2
Weak
Weak
0
Narrow
H. ergaster
  
29
30
. . 
ET AL.
DuBrul & Laskin, 1961; Kimbel et al., 1984), but they do not describe the same feature. As
noted above, such characters should be considered independent until relevant functional or
structural hypotheses can be tested. In contrast, the identification of descriptively redundant
traits does not depend upon the validity of such hypotheses.
In addition, this analysis differs from those of prior studies in the procedures used to assign
states to characters. With respect to qualitative characters (comprising 39 of the 60 traits
employed in this study), a fossil species was here characterized as exhibiting a particular
morphology only if it was present in every relevant specimen in the hypodigm. If two (or more)
morphological variants were observed within a species, then it was coded as being variable for
that character. This stringent definition was used so that the manner in which variable states
were treated could be manipulated (see below, Analyses 1–4). However, this criterion did not
apply to characters that are known to be highly sexually dimorphic, at least in extant
hominoids. In such cases, the character was restricted to a consideration of the morphology of
only one sex (e.g., Table 2, character 20: ‘‘Sagittal crest present, at least in males’’). Because
larger samples were available for extant species, and therefore morphological outliers were
more likely to be observed, P. troglodytes, G. gorilla and H. sapiens were considered variable only
if two or more morphologies were present in sizable proportions (i.e., if a second variant was
present in more than 15% of the sample).
The 21 quantitative characters were coded using the method of Almeida & Bisby (1984).
This ‘‘common-sense’’ approach assigns different states to taxa when their observed ranges are
discontinuous or exhibit minimal overlap. In the event that the range of a species spanned the
ranges of two relatively discontinuous groups of taxa, the former was coded as being variable.
There has been considerable debate concerning methods of coding quantitative characters
(Mickevich & Johnson, 1976; Simon, 1983; Thorpe, 1984; Archie, 1985; Chappill, 1989;
Farris, 1990; Thiele, 1993; Strait et al., 1996), but all such methods work best with reasonable
sample sizes. These are rarely obtained for fossil hominids. With respect to other cladistic
analyses of early hominids, Chamberlain & Wood (1987) and Wood (1991) employed segment
coding, Skelton & McHenry (1992) assigned codes according to the rank order of taxon means
(which tends to produce many states), and Lieberman et al. (1996) did not state how they coded
quantitative characters. The method of Almeida & Bisby (1984) was used here for three
reasons. First, many of the more rigorous methods (gap coding, generalized gap coding,
segment coding and gap weighting) rely on arbitrary decisions made by the researcher.
Second, the potential benefits of using non-arbitrary methods (homogenous subset coding and
finite mixture coding) were limited by the small sample sizes present in many of the species
(often a sample of one). Finally, the method of Almeida & Bisby (1984) essentially treats
quantitative characters in a qualitative fashion, meaning that all 60 of the characters examined
here were coded in a similar manner.
Of the 21 quantitative characters, ten are represented by indices or angles (i.e., they are
scale free), ten take the form of linear measurements, and one (cranial capacity) is volumetric.
In general, it is desirable to avoid measurements with scale because they may merely reflect
variation in body size. However, it is apparent that the 11 linear and volumetric measurements
employed here do not simply vary according to body size, because, among the taxa examined,
the largest do not always exhibit the largest character state. Furthermore, australopithecine
species and some species of early Homo are quite similar in average body mass (Jungers, 1988;
McHenry, 1988, 1992) and, even though they may display considerable intraspecific body size
variation, on an interspecific level there is (with respect to many species) an approximation of
narrow allometry.
  
31
State assignments were based on observations of original fossils, casts and descriptions in the
literature. Measurements by the authors were supplemented with those recorded by other
workers (e.g., White, 1977, 1980; Johanson et al., 1982; Chamberlain, 1987; Wood, 1991).
Parsimony analyses
The parsimony analyses in this study were conducted with PAUP 3.0s (Swofford, 1991). Eight
separate analyses were undertaken in order to determine whether tree topologies varied
according to alterations in methodology. These analyses differed in their treatment of variable
character states, the reversibility and ordering of characters, the presence or absence of missing
data, and characters related to mastication. The eight analyses are referred to as: (1)
VARIABLE=INTERMEDIATE, (2) NON-VARIABLE, (3) VARIABLE=MAJORITY, (4)
VARIABLE=MISSING DATA, (5) IRREVERSIBLE, (6) NON-MASTICATORY, (7) NO
MISSING DATA, and (8) UNORDERED. Analyses two through eight represent alterations
of the VARIABLE=INTERMEDIATE analysis.
In the first seven analyses, the most parsimonious tree was constructed using Wagner
parsimony, which allows characters with ordered states to reverse freely (the exception being
analysis number 5, in which some characters were held to be irreversible). Ordered characters
are weighted such that a state change between morphological extremes is treated as if the
character has passed through all intermediate states (e.g., a change from state 0 to state 3 is
weighted to represent three steps). In general, ordering encourages characters to change
incrementally. All characters were ordered except for nasal bone projection, nasal cavity
entrance, the configuration of the tympanic bone, and the direction of the mental foramen
opening (Table 2: characters 1, 7, 34, 46). These four characters were unordered.
In all eight analyses, each change between adjacent states (e.g., between 0 and 1, between
1 and 2, etc.) was counted as a single step in a tree. Skelton & McHenry (1992) stated that this
approach biases an analysis in favor of those characters that have many states. In an attempt
to weight characters equally, they scaled their traits according to the number of states that each
possessed. In other words, state changes in different characters were not weighted equally. For
instance, when using scaled characters, a state change in a character with two states is
weighted twice as much as a state change in a character with four states. As noted by Farris
(1990: p. 92), however,
‘‘since in parsimony calculations the weight of a character is the numerical effect of a step, applying the
idea of equal weighting in phylogenetic analyses would lead simply to attributing the same effect to
steps in different characters.’’
Consequently, equally weighted state changes are used throughout this study.
Character polarity was determined by rooting the outgroup. The most parsimonious tree
was obtained using the ‘‘branch and bound’’ search option. The most parsimonious tree is
presented along with its length and its consistency, retention, and rescaled consistency indices.
These indices are measures of the amount of homoplasy present. The consistency index (CI)
is calculated as the minimum possible tree-length divided by the observed tree-length (Kluge
& Farris, 1969; Farris, 1989). If there is no homoplasy in a tree, then its observed length equals
the minimum tree-length, and the CI equals one. If homoplasy is present, then the CI is less
than one. The retention index (RI) is calculated by subtracting the observed tree-length from
the maximum possible tree-length, and then dividing that value by the difference between the
maximum and minimum lengths (Archie, 1989; Farris, 1989). The rescaled consistency index
32
. . 
ET AL.
(RC) is calculated by multiplying the CI by the RI (Farris, 1989). Both the RI and RC are
similar in principle to the CI in that they will equal one if homoplasy is absent, and decrease
in value as homoplasy increases. Although the CI is the more traditional measurement, the RI
and RC have been claimed to be less sensitive to variations in maximum and minimum
tree-length (Archie, 1989; Farris, 1989).
In addition, patterns of character evolution are documented. The most parsimonious tree is
presented with a reconstruction of the unambiguous character state transformations required
at each node. Some state changes are ambiguous, because it is often equally parsimonious to
attribute homoplasy to either parallelism or reversal. Thus, the reconstruction is not a
comprehensive list of all character state transformations in the cladogram. The reconstruction
of character states at nodes was performed with MacClade 3.04 (Maddison & Maddison,
1992).
Finally, in each analysis, a 50% majority-rule consensus tree was constructed based on the
topologies of all trees within three steps of the most parsimonious cladogram. In such a figure,
branching events are presented only if they occur in more than half of the trees under
consideration. Thus, if the most parsimonious tree has a length of 50 steps, all trees of length
53 or less will be used to construct the consensus tree. If there are ten such trees, then a given
branching event will be depicted in the consensus tree only if it is present in five or more of
those ten. Each branching event is labeled to indicate the proportion of trees in which it
occurred. In this way, it is possible to summarize the topologies of many trees, and to evaluate
the consequences of accepting a tree that is marginally less parsimonious than the favored one.
Analysis 1 (VARIABLE=INTERMEDIATE)
In this analysis, a species that was variable for a given character was assigned an intermediate
character state. This is reflected in the numerical codes that correspond to character states in
Table 2. A ‘‘variable intermediate’’ state differs from a ‘‘true intermediate’’ state, in which all
specimens that comprise a hypodigm share a distinct morphology. An implicit assumption of
using variable intermediate states is that characters will pass through a variable phase as they
change from one state to another. This assumption may not be valid, in which case the use of
variable states inflates the number of steps that are required to change between morphologies.
Regrettably, there are few suitable alternatives to this procedure (see Analyses 2–4). Although
PAUP allows the assignment of polymorphic character states, these are most appropriately
applied to supraspecific taxa (Maddison & Maddison, 1992). Because the OTUs in this study
represent species, this option was not adopted. Variable character states are common,
particularly within A. africanus (nine characters) and H. habilis (11 characters).
Analysis 2 (NON-VARIABLE)
As noted above, variable intermediate states require assumptions concerning character state
transformation that may not be valid. Since variable character states are frequently encountered in A. africanus and H. habilis, the inferred phylogenetic relationships of these taxa could be
biased. Consequently, a parsimony analysis was undertaken in which characters that exhibited
variable states were excluded. A total of 22 characters were thus excluded (Table 2, characters
1–4, 7, 11–13, 19, 22, 25, 32, 33, 35, 36, 39, 41, 44, 46–48, 52).
Analysis 3 (VARIABLE=MAJORITY)
Another method of addressing the problem of variable states is to assign to a species that
character state displayed by the majority of the specimens in its hypodigm. This procedure
  
33
assumes that normal patterns of intraspecific variation are such that a species can be
characterized as having a particular morphology even if it is not present in all specimens. The
risk inherent in this methodology is that the variation present in a species may be dramatically
oversimplified. This is particularly true in reference to the fossil record, where even a single
individual may represent a large proportion of the species-sample. Variable states were
eliminated in all characters in which the majority of the specimens in an OTU possessed a
common morphology (characters 2, 3, 11, 12, 13, 22, 25, 32, 33, 35, 36, 39, 41, 46–48, 52).
This necessitated a renumbering of the codes corresponding to the states of a given character
(the codes presented in Table 2 correspond to the conditions described for the
VARIABLE=INTERMEDIATE analysis). If a majority state was not present (i.e., if different
morphologies were present in equal numbers of specimens), then a variable intermediate state
was retained. This applies to six characters (1, 4, 7, 19, 44, 46).
Analysis 4 (VARIABLE=MISSING DATA)
Finally, a variable taxon can be assigned a code indicating that a state for the given character
is unknown. This means that PAUP will assign such a species a state so that a minimum
number of steps are added to the tree. The danger of this method is that the assigned state may
not necessarily be one actually observed in the taxon, or it may be a state that is present in only
a minority of the specimens in the hypodigm.
Analysis 5 (IRREVERSIBLE)
In the absence of strong evidence to the contrary, it should be assumed that all morphological
characters are free to reverse their states. It might be argued, however, that evolutionary
reversals are likely to occur infrequently in some of the characters examined in this study. Such
characters relate to either large-scale reorganizations of cranial form, or highly complex organ
systems. It seems plausible that such characters are relatively conservative, and are unlikely to
reverse as frequently. Five characters were considered potentially irreversible: index of palate
protrusion (a measure of facial prognathism), cranial capacity, cerebellar morphology, cranial
base flexion, and petrous orientation (characters 13, 16, 17, 30, 38). The choice of these
characters is subjective. Other researchers might select a different list of characters, and the
results might differ accordingly.
Analysis 6 (NON-MASTICATORY)
Skelton et al. (1986) and Skelton & McHenry (1992) noted that masticatory features
contribute disproportionately to the trait lists used to construct hominid phylogenies. They
(Skelton & McHenry, 1992) presented evidence that trophic features support ‘‘robust’’
australopithecine monophyly, and a sister group relationship between A. africanus and a
‘‘robust’’ clade. Because that topology was inconsistent with the cladogram generated from
their entire trait list, and from several other functional and anatomical character complexes,
they concluded that
‘‘hominid evolution was characterized by a large amount of homoplasy, especially in traits related to
heavy chewing’’
and that therefore
‘‘traits relating to heavy chewing are not reliable for reconstructing hominid phylogeny’’ (1992:
p. 345).
34
. . 
ET AL.
Of the 60 characters considered in the present study, 30 plausibly have at least a remote
functional relationship with mastication (characters 4–6, 8–11, 13–15, 19–24, 29, 31–33, 44,
48, 50, 51, 53–55, 57–59). Although we have argued above that untested functional
inferences should not be used to conflate characters or construct complexes, Analysis 6 was
undertaken in order to determine whether masticatory features may have biased the results
of this study. This was necessary, because if these characters had not been omitted, we
would have failed to address one of the more important conclusions reached by Skelton et
al. (1986) and Skelton & McHenry (1992). Although the decision about which characters
are related to mastication carries a certain degree of subjectivity (indeed, our list differs from
that enumerated by Skelton & McHenry, 1992), the 30 features chosen here represent those
that are traditionally and most reasonably associated with chewing. Disagreement concerning the identification of masticatory features merely underscores the need for rigorous
testing of functional hypotheses.
Analysis 7 (NO MISSING DATA)
Of the 60 characters considered here, 20 are missing data with respect to at least one species
(Table 2: characters 3, 9, 10, 14, 18, 21, 24, 30, 34, 36, 37, 39–43, 49, 52, 55, 56, 60). When
PAUP is presented with a character for which state information is unavailable for an OTU, its
state is reconstructed such that a minimum number of steps (ideally, none) is added to the tree.
This is problematic, because there is no guarantee that these reconstructions are correct. In
order to assess the effect that such reconstructions have on tree topologies, a separate analysis
was undertaken in which characters with missing state information were excluded.
Analysis 8 (UNORDERED)
In each of the prior analyses, it has been assumed that most of the characters have ordered
states. For many characters, ordered states seem intuitively reasonable. For instance, it is
certainly more plausible to suggest an incremental increase in brain size through a series of
hominid taxa rather than a single leap from 400 cm3 to 1400 cm3. Nonetheless, ordered
character states impose considerable constraints on a parsimony analysis. In particular,
ordered states increase tree length, because a change from state 0 to state 3 represents three
state changes (0 to 1, 1 to 2, and 2 to 3). It is possible that many of the characters employed
here should be unordered. That is, characters may change between morphological extremes
without penalty, so that a change from state 0 to state 3 represents but a single step. Analysis
8 treats all characters as unordered (i.e., trees are constructed using Fitch parsimony).
Results
The trees favored by the eight parsimony analyses are depicted in Figures 1 through 8. Their
tree-lengths and index statistics are presented in Table 3. The index values are fairly consistent
among the eight analyses. CI values in the seven analyses that employ ordered states range
from 0·58 to 0·65. The CI in the UNORDERED analysis is noticeably higher (0·72). This is
not unexpected, because the CI is known to increase with decreasing tree-length (Archie, 1989;
Meier et al., 1991), and unordered characters reduce tree-length. This is because unordered
characters can change between morphological extremes in a single step, thereby eliminating
steps that would have been treated as homoplasies had the characters been ordered. RI values
for all eight analyses range between 0·67 and 0·73. RC values vary between 0·39 and 0·47 in
the ordered analyses; it is 0·51 in the UNORDERED analysis.
199
0.59
0.68
0.40
Variable=
intermediate
102
0.64
0.72
0.46
Non-variable
163
0.63
0.71
0.44
Variable=
majority
147
0.64
0.72
0.47
202
0.58
0.70
0.41
Analyses
Variable=
missing data
Irreversible
Treelengths and indices of the cladograms favored by the eight parsimony analyses
Treelength
Consistency index
Retention index
Rescaled consistency index
Statistic
Table 3
93
0.65
0.73
0.47
Nonmasticatory
146
0.58
0.67
0.39
No missing
data
163
0.72
0.70
0.51
Unordered
  
35
(a)
(b)
G
G
.g
o
P. rill
tr a
o
A. glo
a f dy
A. are tes
a f ns i
A. rica s
r o nu
b
A. us s
bo tus
A. ise
a i
H eth
. h io
H ab pic
. r ili u
s
H ud s
. e ol
rg fen
as s i
H
s
. s ter
ap
ie
ns
ET AL.
.g
o
P. rill
tr a
A. ogl
a f o dy
A. are tes
a f ns
A. rica is
r o nu
A. bus s
bo tu
A. ise s
a i
H eth
. h io
p
H abi icu
. r lis s
ud
H
. e olf
r e
H g a ns i
. s st s
ap e r
ie
ns
. . 
36
78
100
56
100
56
78
100
Most parsimonious
tree
100
Majority rule
consensus tree
Figure 1. (a) Most parsimonious cladogram favored by the VARIABLE=INTERMEDIATE analysis. (b)
50% majority rule consensus of the nine trees whose length is less than or equal to 202. Numbers next to
branching events represent the proportion of trees in which that branch is found.
These index values are similar to, or lower than those of most other cladistic analyses of
early hominid taxa. With the exception of the study by Lieberman et al. (1996), all others have
employed ordered characters, and are therefore, most comparable with Analyses 1–7.
Chamberlain & Wood (1987) presented trees with CI values between 0·69 and 0·71, and
Wood’s (1991, 1992a) most parsimonious tree had a CI of 0·65. However, neither study
included A. aethiopicus. This species has dramatically increased the amount of homoplasy in
reconstructions of early hominid phylogeny (Walker et al., 1986; Kimbel et al., 1988). Thus, it
is not unexpected that index values in the studies by Chamberlain & Wood (1987) and Wood
(1991, 1992a) are comparatively high. Wood’s (1988) analysis of Walker et al.’s (1986) data set
produced a tree with a CI of 0·68, which is somewhat higher than those obtained in our
Analyses 1–7. The most parsimonious tree produced by Skelton & McHenry’s (1992) entire
data set has a CI of 0·72 and a RC of 0·52. These values are much higher than those of
Analyses 1–7. A likely explanation for this discrepancy is that the present study has conflated
many of Skelton & McHenry’s (1992) characters (see Appendix 1). However, Skelton &
McHenry’s RI value (0·71) is similar to those presented here. The most parsimonious tree
found by Lieberman et al. (1996), who used unordered characters, had a CI of 0·68, which is
lower than that of our UNORDERED analysis (0·72).
Index values should not be used to select which analysis provides the most appropriate
results. To do so would amount to circular reasoning, because the results of an analysis would
be used to test the assumptions of that analysis. Selecting among analyses should be based on
an a priori consideration of the configuration of each analysis. We prefer the VARIABLE=
INTERMEDIATE and IRREVERSIBLE analyses for four reasons. First, both employ
ordered characters, which encourage incremental rather than quantum-level changes in
morphology. Second, both include all 60 characters in the trait list. Third, both assign variable
character states intermediate codes (of the four treatments of variable states presented here,
this is the only one that does not ignore some or all of the expressions of variable characters
in taxa). Finally, they differ only with respect to their treatment of character irreversibility,
which is an unproven but (to some researchers) intuitively reasonable possibility (but see
Farris, 1983).
Analysis 1 (VARIABLE=INTERMEDIATE)
The most parsimonious tree produced by this analysis [Figure 1(a)] recognizes a ‘‘robust’’
australopithecine clade, within which A. boisei is the sister of A. robustus. It also recognizes
  
37
a Homo clade. The ‘‘robust’’ clade is the sister of Homo. Within Homo, H. rudolfensis is more
closely related to an H. sapiens+H. ergaster clade than is H. habilis. A. africanus is the sister of
the Homo+‘‘robust’’ australopithecine clade. A. afarensis is the sister of all other hominids.
Australopithecus is paraphyletic.
Unambiguous apomorphic state changes required by this tree are summarized in Table 4.
A hominid clade is supported by ten apomorphies. Several of these reflect well-known trends
in australopithecine evolution (e.g., increased post-canine tooth size and enamel thickness,
reduced anterior dentition, and basicranial reorganization). An A. africanus+Homo+‘‘robust’’
clade is supported by 11 apomorphies, one of which is an increase in cranial capacity. Most of
the others are related to facial architecture or the cranial base. A Homo+‘‘robust’’ australopithecine clade is defined by seven derived states from the cranial base, mandible and canine.
A ‘‘robust’’ clade is defined by 20 apomorphies, 15 of which are plausibly related to
mastication. A. aethiopicus is characterized by reversals in six basicranial, facial and neurocranial
features (accounting for between seven to ten steps, depending on how the characters are
reconstructed at other nodes), including a slight decrease in cranial capacity. A Homo clade is
defined by only four derived states, one of which is increased cranial capacity. Six characters
(including a deep palate, a narrow extramolar sulcus, a larger brain, and three related to
muscle origins) suggest that H. rudolfensis is more closely related to H. ergaster and H. sapiens than
is H. habilis.
The most parsimonious tree has a length of 199 steps. Eight additional trees (for a total of
nine) were found whose length was less than or equal to 202 steps. The 50% majority-rule
consensus of these [Figure 1(b)] indicates that marginally less parsimonious trees generally
support the topology of the most parsimonious one. All nine agree that H. sapiens and H. ergaster
are sister taxa, that ‘‘robust’’ australopithecines and hominids as a whole are monophyletic,
and that A. africanus, Homo, and the ‘‘robust’’ species form a clade. In two of the nine trees,
A. africanus is the sister of the ‘‘robust’’ australopithecines. The nine trees generally disagree
concerning the placement of H. habilis and H. rudolfensis, with four trees making either species
the sister of the ‘‘robust’’ clade.
Analysis 2 (NON-VARIABLE)
When characters with intraspecifically variable states are excluded from the analysis, three
equally parsimonious trees are supported that disagree concerning the placement of
H. rudolfensis. One of these trees is identical to that produced by the VARIABLE=
INTERMEDIATE analysis [Figure 1(a)], except that A. boisei is recognized as the sister of
A. aethiopicus. The other two maintain this topology except that H. rudolfensis is the sister of
either the three remaining Homo species, or the three ‘‘robust’’ australopithecines. The strict
consensus tree of these topologies (the most highly resolved tree that is compatible with
all of them) is presented in Figure 2(a). The unresolved polytomy at the base of the
Homo+‘‘robust’’ clade indicates the uncertainty concerning the placement of H. rudolfensis.
Patterns of character evolution are not described, since they vary considerably in the three
trees.
Forty-two trees are within three steps of the most parsimonious three. All 45 include
hominid and ‘‘robust’’ clades, and a clade that includes all hominid species except A. afarensis
[Figure 2(b)]. Thirty-nine trees (87%) make Homo and the ‘‘robust’’ species monophyletic; in
the remaining six, A. africanus and the ‘‘robust’’ species form a clade. There is poor agreement
concerning all other relationships.
38
Table 4
. . 
ET AL.
Unambiguous apomorphies required by the VARIABLE=INTERMEDIATE analysis
Synapomorphies of the hominid clade:
1. Nasal bones projected and expanded above frontomaxillary suture.
38. Petrous orientation intermediate.
40. Inclination of nuchal plane intermediate.
41. Foramen magnum positoned at bi-tympanic line.
45. Mandibular symphysis orientation intermediate.
50. Incisors moderately reduced.
51. Canines somewhat reduced.
55. dM1 mesial marginal ridge slight, with open anterior fovea, and protoconid set mesial to metaconid.
58. Well-developed P3 metaconid infrequent.
59. Tooth enamel thick.
Synapomorphies of the A. africanus+Homo+‘‘robust’’ clade:
5. Nasoalveolar clivus straight in coronal plane.
13. Index of palate protrusion is variably prognathic and mesognathic.
16. Cranial capacity increased to state 1 (approximately 500 cm3).
21. Partial compound T/N crest.
22. Asterionic notch either variable or absent.
33. Postglenoid process size and position intermediate.
34. Tympanic crest with vertical plate.
40. Nuchal plane weakly inclined.
47. Hollowing above and behind mental foramen variable.
52. Prominence of median lingual ridge of mandibular canine is variable.
58. Well developed P3 metaconid is frequent.
Synapomorphies of the Homo+‘‘robust’’ clade:
38. Petrous orientation coronal.
42. Foramen magnum roughly horizontal.
45. Vertically oriented mandibular symphysis.
46. Mental foramen opens laterally.
47. No hollowing above and behind mental foramen.
51. Canines very reduced.
52. Weak median lingual ridge of mandibular canine.
Synapomorphies of the ‘‘robust’’ clade:
3. Infraorbital foramen low.
5. Nasoalveolar clivus concave in coronal plane (guttered clivus).
6. Incisor alveoli do not project beyond bicanine line (parallel with H. rudolfensis).
7. Smooth entrance to nasal cavity, with overlapping clivus and palate.
8. Thick palate.
10. Thick zygomatic arch.
11. Zygomatic projects anterior to piriform aperture (dished face).
14. Masseteric tubercle at or anterior to sellion (parallel with A. afarensis).
19. Strong anteromedial incursion of the temporal lines.
24. Extensive overlap of squamosal suture.
25. Mastoid process inflated lateral to the supramastoid crest.
26. Marked postorbital constriction.
28. Face hafted high.
29. Wide supraglenoid gutter.
44. Large mandibular cross sectional area at M1 (parallel with H. rudolfensis).
48. Wide mandibular extramolar sulcus.
53. Premolar area increases to either state 3 or state 4.
55. dM1 mesial marginal ridge thick, enclosing fovea anterior; protoconid even with metaconid.
57. Molar and premolar cusp apices narrowly separated.
59. Hyperthick enamel.
Synapomorphies of A. robustus and A. boisei:
18. O–M sinus frequently present (parallel with A. afarensis).
33. Postglenoid process small and fused to tympanic (parallel with H. ergaster+H. sapiens clade).
35. External auditory meatus laterally placed.
36. Vaginal process moderate to large.
41. Foramen magnum well anterior to bi-tympanic line.
50. Incisors reduced (parallel with H. sapiens).
Synapomorphies of the Homo clade:
1. Nasal bones do not project above frontomaxillary suture.
16. Cranial capacity increased to state 2 (510–675 cm3).
19. Variable (moderate to weak) anteromedial incursion of the temporal lines (reversal).
27. Reduced pneumatization of temporal squama (parallel with A. robustus).
Table 4 continued on next page
  
Table 4 Continued from previous page
Synapomorphies of H. rudolfensis+H. ergaster+H. sapiens:
12. Palate deep anteriorly (shelved; parallel with A. africanus and P. boisei).
16. Cranial capacity increased to state 3 (750–875 cm3).
19. Weak anteromedial incursion of the temporal lines.
20. Sagittal crest absent in presumptive males.
21. Compound T/N crest absent (parallel with A. africanus and A. boisei).
48. Mandibular extramolar sulcus narrow (reversal).
Synapomorphies of H. ergaster+H. sapiens:
13. Index of palate protrusion is orthognathic.
33. Postglenoid process small and fused to tympanic (parallel with A. robustus+A. boisei clade).
54. Molar area reduced to state 0 (smallest; reversal).
60. Dental development rate intermediate.
Apomorphies of A. afarensis:
14. Masseteric tubercle at or anterior to sellion (parallel with ‘‘robust’’ clade).
18. O–M sinus frequently present (parallel with A. robustus+A. boisei clade).
Apomorphies of A. africanus:
1. Projection of nasal bones variable.
4. Anterior pillars variable (parallel with H. habilis).
12. Palate deep anteriorly (shelved; parallel with A. boisei and H. rudolfensis+H. ergaster+H. sapiens clade).
21. Compound T/N crest absent (parallel with H. rudolfensis+H. ergaster+H. sapiens clade).
Apomorphies of A. aethiopicus:
13. Index of palate protrusion prognathic (reversal).
16. Cranial capacity reduced to state 0 (less than 500 cm3; reversal).
21. Compound T/N crest extensive (reversal).
22. Asterionic notch present (reversal).
30. Flat cranial base (reversal).
32. Shallow mandibular fossa (reversal).
Apomorphies of A. robustus:
4. Anterior pillars present.
27. Reduced pneumatization of temporal squama (parallel with Homo clade).
37. Eustacian process present and prominent (reversal).
Apomorphies of A. boisei:
12. Palate deep anteriorly (parallel with A. africanus and the H. rudolfensis+H. ergaster+H. sapiens clade).
32. Deep mandibular fossa (parallel with H. sapiens).
34. Tympanic crest with inclined plate.
53. Premolar crown area increased to state 5 (largest).
Apomorphies of H. habilis:
4. Anterior pillars variable (parallel with A. africanus).
7. Variable entrance to nasal cavity.
35. M–L position of external auditory meatus variable.
41. Foramen magnum variably at or anterior to bi-tympanic line.
Apomorphies of H. rudolfensis:
6. Nasoalveolar contour does not protrude beyond bicanine line (parallel with ‘‘robust’’ clade).
11. Intermediate projection of zygomatic bone relative to piriform aperture.
44. Mandibular cross-sectional area at M1 variable.
47. Variable hollowing above and behind mental foramen (reversal).
Apomorphies of H. ergaster:
32. Mandibular fossa variably shallow and intermediate (reversal).
39. Heart shaped foramen magnum variable.
42. Foramen magnum strongly inclined anteriorly.
Apomorphies of H. sapiens:
5. Nasoalveolar clivus convex in coronal plane (reversal).
7. Smooth entrance to nasal cavity, without overlapping clivus and palate.
16. Cranial capacity increased to state 4 (21400 cm3).
26. Postorbital constriction slight.
32. Mandibular fossa deep (parallel with A. boisei).
46. Mental foramen opens posteriorly.
50. Incisors reduced (parallel with A. robustus+A. boisei).
53. Premolar crown area reduced to state 0 (smallest).
The character number is followed by a description of the apomorphic state.
39
(a)
(b)
G
G
.g
o
P. rill
tr a
o
A. glo
a f dy
A. are tes
a f ns i
A. ric s
b o a nu
i
A. sei s
ae
A. thi
r o
H obu picu
.h s
t s
H a b i us
. r li
s
H udo
. e lf
rg en
H ast sis
e
.s
ap r
ie
ns
ET AL.
.g
o
P. rill
tr a
A. ogl
a f o dy
A. are tes
a f ns
i
A. rica s
b o nu
A. ise s
a i
A. eth
ro iop
H bu icu
. h st s
u
H abi s
. r lis
ud
H
. e olf
r e
H g a ns i
. s st s
ap e r
ie
ns
. . 
40
64
100
53
87
100
100
Strict consensus
tree
Majority rule
consensus tree
(b)
G
.g
o
P. rill
tr a
A. ogl
af ody
A. are tes
af ns
A. rica is
ro nu
A. bus s
bo tu
A. ise s
a i
H eth
. h io
p
H abi icu
. r lis s
ud
H
. e olf
r e
H ga nsi
. s st s
ap er
ie
ns
(a)
G
.g
o
P. rill
tr a
o
A. glo
af dy
A. are tes
af nsi
A. rica s
ro nu
b
A. us s
bo tus
A. ise
a i
H eth
. h io
H abi pic
. r li us
s
H udo
. e lf
rg en
H ast sis
e
.s
ap r
ie
ns
Figure 2. (a) Strict consensus of the three most parsimonious cladograms favored by the NON-VARIABLE
analysis. Polytomies represent uncertain phylogenetic relationships rather than polytomous branching events.
(b) 50% majority rule consensus of the 45 trees whose length is less than or equal to 105. Numbers next to
branching events represent the proportion of trees in which that branch is found.
57
81
100
57
95
100
Strict consensus
tree
100
Majority rule
consensus tree
Figure 3. (a) Strict consensus of the two most parsimonious cladograms favored by the VARIABLE=
MAJORITY analysis. Polytomies represent uncertain phylogenetic relationships rather than polytomous
branching events. (b) 50% majority rule consensus of the 21 trees whose length is less than or equal to 166.
Numbers next to branching events represent the proportion of trees in which that branch is found.
Analysis 3 (VARIABLE=MAJORITY)
When a species with a variable morphology is assigned the state possessed by a majority
of the specimens comprising its hypodigm, two equally parsimonious trees are favored.
They differ only with respect to whether H. habilis or H. rudolfensis is the sister of
the H. ergaster+H. sapiens clade. The strict consensus tree of these topologies [Figure 3(a)] is
similar to those favored by the VARIABLE=INTERMEDIATE and NON-VARIABLE
analyses [Figures 1(a), 2(a)] in that the ‘‘robust’’ australopithecines are monophyletic,
they form a clade with Homo, A. africanus is the sister of that clade, and A. afarensis is
the sister of all other hominids. Patterns of character evolution are very similar in the two
trees (Table 5), and to the pattern observed in the VARIABLE= INTERMEDIATE
analysis.
Nineteen trees are within three steps of the most parsimonious two [Figure 3(b)]. The
majority rule consensus of these 21 supports the basic topology just described. All include
hominid and ‘‘robust’’ clades, and a clade that includes all hominid species except A. afarensis.
All but one (95%) identify Homo and the ‘‘robust’’ species as sister taxa; in the remaining tree,
A. africanus is the sister of the ‘‘robust’’ clade. In a large majority of the trees (81%), H. ergaster
and H. sapiens are sister taxa.
  
41
Unambiguous apomorphies required by the VARIABLE=
MAJORITY analysis, and found in both of the most parsimonious
trees
Table 5
Synapomorphies of the hominid clade:
46. Mental foramen opens laterally.
As in Table 4: 1, 38, 40, 41, 45, 50, 51, 55, 58, 59.
Synapomorphies of the A. africanus+Homo+‘‘robust’’ clade:
13. Index of palate protrusion is mesognathic.
As in Table 4: 5, 16, 21, 33, 34, 40, 58.
Synapomorphies of the Homo+‘‘robust’’ clade:
As in Table 4: 38, 42, 45, 47, 51, 52.
Synapomorphies of A. robustus and A. boisei:
As in Table 4: 18, 35, 36, 41, 50.
Synapomorphies of H. rudolfensis+H. ergaster+H. sapiens:
None.
Synapomorphies of H. ergaster+H. sapiens:
As in Table 4: 13, 54, 60.
Apomorphies of A. africanus:
As in Table 4: 1, 4, 21.
Apomorphies of H. habilis:
As in Table 4: 4, 7.
Apomorphies of H. rudolfensis:
As in Table 4: 6, 44.
Apomorphies of H. ergaster:
32. Mandibular fossa shallow (reversal).
As in Table 4: 42.
(b)
G
.g
o
P. rill
tr a
A. ogl
af ody
A. are tes
af ns
A. rica is
ro nu
A. bus s
bo tu
A. ise s
a i
H eth
. h io
p
H abi icu
. r lis s
ud
H
. e olf
r e
H ga nsi
. s st s
ap er
ie
ns
(a)
G
.g
o
P. rill
tr a
o
A. glo
af dy
a t
A. re es
ro nsi
s
b
A. us
bo tus
A. ise
a i
A. eth
a io
H fric pic
. h a us
n
H abi us
. r li
s
H udo
. e lf
rg en
H ast sis
e
.s
ap r
ie
ns
Only nodes that differ from those of Table 4 are presented. The character
number is followed by a description of the apomorphic state.
54
86
100
61
82
100
Strict consensus
tree
100
Majority rule
consensus tree
Figure 4. (a) Strict consensus of the three most parsimonious cladograms favored by the VARIABLE=
MISSING DATA analysis. Polytomies represent uncertain phylogenetic relationships rather than polytomous branching events. (b) 50% majority rule consensus of the 28 trees whose length is less than or equal to
150. Numbers next to branching events represent the proportion of trees in which that branch is found.
Analysis 4 (VARIABLE=MISSING DATA)
When variable character states are coded as missing data, three equally parsimonious trees are
supported. One of these is identical to Figure 1(a). In another, H. habilis and H. rudolfensis
exchange positions. In the third, A. africanus is the sister of the ‘‘robust’’ clade. In a strict
consensus tree [Figure 4(a)], this ambiguity is reflected by the polytomy at the base of the
A. africanus+Homo+‘‘robust’’ clade. Patterns of character evolution differ greatly in the three
trees, so they are not described. As a cautionary note, however, a consequence of equating
variability with missing data is that the resulting topologies require states to be assigned to
42
Table 6
. . 
ET AL.
Unambiguous apomorphies required by the IRREVERSIBLE analysis
Synapomorphies of the hominid clade:
38. Petrous orientation intermediate.
Synapomorphies of the Homo+‘‘robust’’ clade:
38. Petrous orientation coronal.
Synapomorphies of A. robustus and A. boisei:
13. Index of palate protrusion is mesognathic (parallel with Homo clade).
16. Cranial capacity increased to state 1 (approximately 500 cm3; parallel with Homo and A. africanus).
17. Cerebellum is tucked under cerebrum (parallel with Homo clade).
30. Flexed cranial base (parallel with Homo clade and A. africanus).
Synapomorphies of the Homo clade:
13. Index of palate protrusion is mesognathic (parallel with A. boisei+A. robustus clade).
16. Cranial capacity increased to state 2 (509–675 cm3; parallel with A. africanus and A. boisei+A. robustus clades).
17. Cerebellum is tucked under cerebrum (parallel with A. boisei+A. robustus clade).
30. Flexed cranial base (parallel with A. boisei+A. robustus clade and A. africanus).
Synapomorphies of H. rudolfensis+H. ergaster+H. sapiens:
16. Cranial capacity increased to state 3 (750–875 cm3).
Synapomorphies of H. ergaster+H. sapiens:
13. Index of palate protrusion is orthognathic.
Apomorphies of A. africanus:
13. Index of palate protrusion is variable (mesognathic/prognathic).
16. Cranial capacity is increased to state 1 (approximately 500 cm3; parallel with Homo and A. boisei+A. robustus
clades).
30. Intermediate basicranial flexion (parallel with Homo and A. boisei+A. robustus clades).
Apomorphies of H. sapiens:
16. Cranial capacity increased to state 4 (21400 cm3).
Only the state changes in the putatively irreversible characters are listed. With respect to all other characters, state
assignments are the same as those presented in Table 4. The character number is followed by a description of the
apomorphic state. Note that in some cases, parallel evolution occurs in separate clades even though those clades do
not share the same state (i.e., in one clade, a character changes from state 0 to state 1, while in another clade it changes
from 0 to 2).
many of the variable taxa. A large proportion of those assignments (respectively 32, 26 and
44% in the three trees) represent the state exhibited by only a minority of the specimens in the
species-sample. Such state assignments must be viewed with skepticism.
Twenty-five trees are within three steps of the most parsimonious three. All 28 include a
hominid, a ‘‘robust’’, and an A. africanus+Homo+‘‘robust’’ clade [Figure 4(b)]. In twenty-three
trees (82%), Homo is the sister of the ‘‘robust’’ taxa. A. africanus is the sister of Homo in two trees,
and of the ‘‘robust’’ species in three. H. ergaster and H. sapiens are sister taxa in 24 of the 28
trees.
Analysis 5 (IRREVERSIBLE)
When the five characters noted above (13, 16, 17, 30, 38) are considered to be irreversible,
ingroup relationships [Figure 5(a)] are identical to those favored by the VARIABLE=
INTERMEDIATE analysis. Patterns of character evolution are therefore also identical, except
with respect to those characters that are held to be irreversible (Table 6). By making these
traits irreversible, parallelisms are required in several taxa, whereas in the VARIABLE=
INTERMEDIATE analysis, reversals are necessary in only one species (A. aethiopicus).
However, unlike the VARIABLE=INTERMEDIATE analysis, marginally less parsimonious trees are in wide disagreement with each other [Figure 5(b)]. Thirty-three trees are within
three steps of the most parsimonious one, and of these 34, only a bare majority (53%) support
43
(b)
G
.g
o
P. rill
tr a
A. ogl
a od
A. far yte
ro en s
A. bu sis
bo stu
A. is s
ae e i
t
A. hio
af pic
r
H ica us
.h n
u
H abi s
. r lis
ud
H
. e olf
r e
H g a ns i
. s st s
ap e r
ie
ns
(a)
G
.g
o
P. rill
tr a
o
A. glo
a f dy
A. are tes
a f ns i
A. rica s
r o nu
b
A. us s
bo tus
A. ise
a i
H eth
. h io
H abi pic
. r li us
s
H udo
. e lf
rg en
H ast sis
e
.s
ap r
ie
ns
  
100
100
100
65
100
76
53
Most parsimonious
tree
Majority rule
consensus tree
(b)
G
.g
o
P. rill
tr a
A. ogl
af ody
A. are tes
af ns
A. rica is
ro nu
A. bus s
bo tu
A. ise s
a i
H eth
. h io
p
H abi icu
. r lis s
ud
H
. e olf
r e
H ga nsi
. s st s
ap er
ie
ns
(a)
G
.g
o
P. rill
tr a
o
A. glo
af dy
A. are tes
af nsi
A. rica s
ro nu
b
A. us s
bo tus
A. ise
a i
H eth
. h io
H abi pic
. r li us
s
H udo
. e lf
rg en
H ast sis
e
.s
ap r
ie
ns
Figure 5. (a) Most parsimonious cladograms favored by the IRREVERSIBLE analysis. (b) 50% majority rule
consensus of the 34 trees whose length is less than or equal to 205. Numbers next to branching events
represent the proportion of trees in which that branch is found.
77
67
86
100
Most parsimonious
tree
100
Majority rule
consensus tree
Figure 6. (a) Most parsimonious cladograms favored by the NON-MASTICATORY analysis. (b) 50%
majority rule consensus of the 43 trees whose length is less than or equal to 96. Numbers next to branching
events represent the proportion of trees in which that branch is found.
a monophyletic hominid clade. Furthermore, although there is moderate support (76%) for a
clade that includes A. africanus, Homo and the ‘‘robust’’ species, there is no agreement as to how
these three groups are related to each other. The only areas of universal agreement concern
the presence of Homo and ‘‘robust’’ clades, and the relationships within those clades. Thus,
even though the IRREVERSIBLE and VARIABLE=INTERMEDIATE analyses both
support the same tree, this support is tenuous in the former, because a change in only one or
a few character states could dramatically alter the favored topology.
Analysis 6 (NON-MASTICATORY)
When the 30 features that are likely to be related to mastication are excluded from the
analysis, the most parsimonious tree [Figure 6(a)] is identical to that found by the
VARIABLE=INTERMEDIATE analysis. Patterns of character evolution are therefore also
identical in the characters common to both studies.
Forty-two trees are within three steps of the most parsimonious one, and all 43 support a
hominid clade, and A. afarensis as the sister of all other hominid taxa [Figure 6(b)]. Most (86%)
support a clade that includes Homo and ‘‘robust’’ species, but in six trees, A. aethiopicus is the
sister of a clade that includes A. africanus, Homo and the remaining ‘‘robust’’ species.
Nonetheless, a majority (67%) of trees support a ‘‘robust’’ clade. This support is weaker than
in other analyses, but this is not surprising given that masticatory features (accounting for 50%
of all characters considered here) have been removed.
(a)
(b)
G
G
.g
o
P. rill
tr a
o
A. glo
a f dy
A. are tes
a f ns i
A. rica s
r o nu
b
A. us s
bo tus
A. ise
a i
H eth
. h io
H abi pic
. r li us
s
H udo
. e lf
rg en
H ast sis
e
.s
ap r
ie
ns
ET AL.
.g
o
P. rill
tr a
A. ogl
a f o dy
A. are tes
a f ns
A. rica is
r o nu
A. bu s
bo stu
A. ise s
a i
H eth
. h io
p
H abi icu
. r lis s
ud
H
. e olf
r e
H g a ns i
. s st s
ap e r
ie
ns
. . 
44
84
100
92
100
84
Strict consensus
tree
Majority rule
consensus tree
(b)
G
.g
o
P. rill
tr a
A. ogl
af ody
A. are tes
af ns
A. rica is
ro nu
A. bus s
bo tu
A. ise s
a i
H eth
. h io
p
H abi icu
. r lis s
ud
H
. e olf
r e
H ga nsi
. s st s
ap er
ie
ns
(a)
G
.g
o
P. rill
tr a
o
A. glo
af dy
A. are tes
af nsi
A. rica s
bo nu
A. isei s
a
A. eth
ro iop
H bu icu
. h st s
H abi us
. r li
s
H udo
. e lf
rg en
H ast sis
e
.s
ap r
ie
ns
Figure 7. (a) Strict consensus of the four most parsimonious cladograms favored by the NO MISSING
DATA analysis. Polytomies represent uncertain phylogenetic relationships rather than polytomous branching events. (b) 50% majority rule consensus of the 38 trees whose length is less than or equal to 149. Numbers
next to branching events represent the proportion of trees in which that branch is found.
55
70
100
92
96
Strict consensus
tree
100
Majority rule
consensus tree
Figure 8. (a) Strict consensus of the three most parsimonious cladograms favored by the UNORDERED
analysis. Polytomies represent uncertain phylogenetic relationships rather than polytomous branching events.
(b) 50% majority rule consensus of the 53 trees whose length is less than or equal to 166. Numbers next to
branching events represent the proportion of trees in which that branch is found.
Analysis 7 (NO MISSING DATA)
When characters that lack state information for a taxon are excluded, four equally
parsimonious trees are supported. One of these is identical to that preferred by the
VARIABLE=INTERMEDIATE analysis. The remaining three represent slight alterations
of this topology. In one, H. habilis is the sister of the H. ergaster+H. sapiens clade. In another,
A. robustus and A. aethiopicus are sister taxa, and in the third, both of the preceding alterations
are observed. The polytomies in the strict consensus tree [Figure 7(a)] indicate these uncertainties. Patterns of character evolution are not described, since they vary in the four trees.
Thirty-four trees are within three steps of the most parsimonious four. In general, these
support the broad framework of the topologies just described. All 38 support a ‘‘robust’’ clade,
and a clade that includes it, A. africanus and species of the genus Homo. Most (84%) support a
hominid clade and a H. ergaster+H. sapiens clade. All but three include A. africanus as a sister of
a clade comprising Homo and ‘‘robust’’ species; in two of these, A. africanus is the sister of the
‘‘robust’’ species, and in one it is the sister of Homo.
Analysis 8 (UNORDERED)
When all 60 characters are used to construct a tree under Fitch parsimony, three equally
parsimonious trees are favored. One of these is identical to the tree favored by the
VARIABLE=INTERMEDIATE analysis. The other two differ only concerning the placement of H. habilis, which is reconstructed as being the sister of either the ‘‘robust’’ clade or of
  
45
H. sapiens+H. ergaster. Thus, the strict consensus of these three [Figure 8(a)] includes an
unresolved polytomy at the base of the Homo+‘‘robust’’ clade. Patterns of character evolution
vary depending on the placement of H. habilis, and therefore are not reported.
Fifty trees are within three steps of the most parsimonious three. All 53 support a hominid
clade and a ‘‘robust’’ clade. All but two (96%) support a clade that includes A. africanus, Homo
and the ‘‘robust’’ species; in those two, A. africanus is the sister of all other hominids. All but
four (92%) support a clade that comprises the Homo and ‘‘robust’’ species; in those four,
A. africanus is the sister of Homo.
Discussion
Comparisons among the eight analyses
The analyses conducted here consistently support certain phylogenetic relationships. These
commonalties were obtained despite variations in the eight analyses concerning the nature of
the character matrix, and the configuration of the PAUP program. In all eight, A. afarensis is
recognized as the sister of all other hominids, the ‘‘robust’’ australopithecines are monophyletic, and H. ergaster and H. sapiens are sister taxa. In seven of the eight analyses, the
‘‘robust’’ species form a clade with Homo, and A. africanus is the sister of that clade. These
results were also observed in two of the three trees supported by the VARIABLE=MISSING
DATA analysis; in the third, A. africanus was the sister of the ‘‘robust’’ species. However, this
analysis requires the assignment of states to taxa even when those states are present in only a
minority of the specimens in that species. For instance, the VARIABLE=MISSING DATA
analysis reconstructs A. africanus as lacking anterior pillars, even though pillars are present in all
but one specimen (Sts 52a). Thus, the results of this analysis must be viewed cautiously. On
balance there is good support for a Homo+‘‘robust’’ clade.
The constancy of these relationships is particularly important in light of claims that an
over-representation of masticatory features may bias reconstructions of hominid phylogeny
(Skelton et al., 1986; Skelton & McHenry, 1992). Although the 30 masticatory features
recognized here do favor ‘‘robust’’ monophyly and an A. africanus+‘‘robust’’ clade, their
exclusion (Analysis 6) produces a most parsimonious tree that is identical or broadly similar to
those of analyses in which such traits are included. Even when marginally less parsimonious
trees are considered, a majority (67%) still support a ‘‘robust’’ clade [Figure 6(b)]. Moreover,
Analysis 6 is conservative in that presumed masticatory features have not merely been
conflated, but rather eliminated entirely. Thus, the results of Analysis 6 do not support claims
for such a bias. Admittedly, other authors might identify a different set of masticatory features
(e.g., Skelton & McHenry, 1992), and results might differ accordingly. This underscores the
need for rigorous testing of functional hypotheses.
A survey of marginally less parsimonious trees indicates that the relationships described
above are relatively stable, in the sense that the addition of one or a few steps is unlikely to
dramatically alter the results. With the exception of the IRREVERSIBLE analysis, the
majority-rule consensus trees reveal broad agreement that A. afarensis is the sister of all other
hominids, and the ‘‘robust’’ species are monophyletic. There is also strong (but slightly weaker)
support for A. africanus being the sister of a Homo+‘‘robust’’ clade. Alternative phylogenies
reveal this species to be the sister of either group. The IRREVERSIBLE analysis differs from
the other analyses in that early branches in the tree are very unstable, but later branches (in
the Homo and ‘‘robust’’ clades) are universally agreed upon. This pattern is due to the fact that
some irreversible characters evolve in parallel, meaning that their apomorphic state changes
46
. . 
ET AL.
occur ‘‘high’’ in the tree. In contrast, in other analyses, their apomorphies occur ‘‘low’’ in the
tree, but reverse in some lineages. When these apomorphies shift from low to high, the early
branches lose stability in favor of the later ones.
Despite the instability of the IRREVERSIBLE analysis, it is one of the two preferred here
for a priori reasons having to do with the treatment of characters. As noted above, the
VARIABLE=INTERMEDIATE and IRREVERSIBLE analyses are preferred because both
employ ordered characters, include all 60 characters, and assign variable character states
intermediate codes. Moreover, given that irreversible characters tranfer apomorphies from low
to high in a tree, it is not unexpected that the IRREVERSIBLE analysis exhibits instability at
its basal nodes.
The eight analyses presented here are in disagreement concerning relationships within the
‘‘robust’’ australopithecine clade and within the Homo clade. In particular, the positions of
H. habilis and H. rudolfensis are highly variable. In fact, in two analyses (UNORDERED and
NON-VARIABLE), it is possible that one of these species may be the sister of the ‘‘robust’’
clade, thereby making Homo paraphyletic. A consideration of marginally less parsimonious
trees further indicates that the relationships of these two species are highly unstable. With
respect to the relationships of H. habilis and H. rudolfensis, the topologies of the VARIABLE=
INTERMEDIATE and IRREVERSIBLE analyses are preferred here, though it is recognized
that these relationships are tenuously supported.
Comparisons with other phylogenetic studies
Since the description of KNM-WT 17000 (Walker et al., 1986), five cladistic studies of early
hominids have been undertaken (Chamberlain & Wood, 1987; Wood, 1988, 1991; Skelton &
McHenry, 1992; Lieberman et al., 1996). For the most part, these studies agree that A. afarensis
is the sister of other hominids, A. boisei and A. robustus are sister taxa, and that Homo is a
monophyletic clade within which H. sapiens and H. ergaster are sisters. These relationships are
all supported by the VARIABLE=INTERMEDIATE and IRREVERSIBLE analyses.
Principal areas of disagreement concern the relationships of A. africanus, A. aethiopicus, H. habilis
and H. rudolfensis. Thus, it is the relationships of these species that provide the context for
comparisons between the present and prior cladistic studies.
It is possible, using MacClade 3.04, to compare the principal results of prior studies to those
preferred here. The topology favored by the VARIABLE=INTERMEDIATE and
IRREVERSIBLE analyses [Figure 9(a)] was altered to fit the trees proposed by other authors,
and then, using the data set of the present study, differences in tree-lengths were observed.
Because the OTUs of the other studies differed from those recognized here, it was not possible
to duplicate exactly the trees of other authors. Hence, cladograms were constructed that reflect
the major points raised by those hypotheses. Comparisons between these cladograms [Figures
9(b)–9(h)] and Figure 9(a) permit the quantification of differences among alternative phylogenies.
Chamberlain & Wood (1987) undertook the first computer-assisted cladistic analysis of early
hominids. They explored the consequences of recognizing either one or two species within the
hypodigm of H. habilis. When only a single taxon was recognized for that sample, A. africanus
and ‘‘robust’’ australopithecines formed a clade that was the sister of Homo, and
A. afarensis was recognized as the sister of all other hominids. When the early Homo sample was
subdivided into two species (one from Olduvai Gorge, and another from East Turkana), two
most parsimonious trees were found that differed with respect to the placement of the East
Turkana Homo sample. In one of these, that sample is nested within a Homo clade. In the other,
it is the sister of an A. boisei+A. robustus clade.
.g
o
P. rill
tr a
o
A. glo
a f dy
a t
A. re es
r o ns i
s
b
A. us
bo tus
i
A. se
a i
A. eth
a io
H fric pic
. h a us
H a b nus
. r il
i
H ud s
. e ol
rg fen
as s i
H
s
. s ter
ap
ie
ns
G
(b)
Lengths:
VAR = INTERMEDIATE: 199
IRREVERSIBLE:
202
Lengths:
VAR = INTERMEDIATE: 200
IRREVERSIBLE:
203
G
(d)
Lengths:
VAR = INTERMEDIATE: 217
IRREVERSIBLE:
220
Lengths:
VAR = INTERMEDIATE: 205
IRREVERSIBLE:
205
G
(f)
Lengths:
VAR = INTERMEDIATE: 212
IRREVERSIBLE:
212
Lengths:
VAR = INTERMEDIATE: 220
IRREVERSIBLE:
224
G
G
.g
o
P. rill
tr a
o
A. glo
af dy
a t
A. re es
ro nsi
s
b
A. us
bo tus
i
A. se
a i
A. eth
a iop
H fric icu
. h an s
H ab us
. r il
i
H ud s
. e ol
rg fen
as si
H
s
. s ter
ap
ie
ns
(e)
.g
o
P. rill
tr a
o
A. glo
af dy
A. are tes
ae nsi
s
t
A. hio
bo pi
c
A. is
us
af ei
A. rica
ro nu
H bu s
. h st
H ab us
. r ili
H ud s
. e ol
rg fen
as si
H
s
. s ter
ap
ie
ns
G
.g
o
P. rill
tr a
o
A. glo
af dy
t
A. are es
ae nsi
A. thi s
ro opi
A. bus cus
b tu
H ois s
. r ei
A. ud
af olf
H ric ens
. h an is
H ab us
. e ili
rg s
as
H
. s ter
ap
ie
ns
(c)
.g
o
P. rill
tr a
o
A. glo
af dy
A. are tes
a ns
A. eth is
af iop
A. ric icu
ro anu s
A. bus s
bo tu
H ise s
.h i
H ab
. r ili
H ud s
. e ol
rg fen
as si
H
s
. s ter
ap
ie
ns
G
.g
o
P. rill
tr a
o
A. glo
af dy
t
A. are es
ro nsi
s
b
A. us
bo tus
i
A. se
a i
A. eth
af iop
H rica icu
.h n s
H ab us
. r il
i
H ud s
. e ol
rg fen
as si
H
s
. s ter
ap
ie
ns
(a)
.g
o
P. rill
tr a
o
A. glo
af dy
a t
A. ren es
ro sis
A. bus
ae tu
A. thi s
b op
A. ois icu
s
af ei
H rica
.h n
H abi us
. r li
s
H udo
. e lf
rg en
H ast sis
.s
e
ap r
ie
ns
G
.g
o
P. rill
tr a
o
A. glo
a f dy
A. are tes
a f ns i
A. rica s
r o nu
b
A. us s
bo tus
A. ise
a i
H eth
. h io
H ab pic
. r ili u
s
H ud s
. e ol
rg fen
as s i
H
s
. s ter
ap
ie
ns
  
(g)
(h)
Lengths:
VAR = INTERMEDIATE: 238
IRREVERSIBLE:
240
Lengths:
VAR = INTERMEDIATE: 211
IRREVERSIBLE:
211
Figure 9. Alternative tree topologies. Trees are presented with the tree lengths required under the conditions
set by VARIABLE=INTERMEDIATE and IRREVERSIBLE analyses. (a) The topology favored by the
present study. This topology was altered to produce Figures 9(b)–9(h). Although these altered topologies
correspond to the phylogenies proposed by other authors, they do not represent them exactly because of
differences concerning the recognition of OTUs. (b) Tree after Chamberlain & Wood (1987). (c) Tree after
Wood (1991, 1992a). (d) Tree further modified after Wood (1991, 1992a). (e) Tree after Wood (1988) and
Skelton & McHenry (1992). (f) Tree after Lieberman et al. (1996). (g) Tree after Walker et al. (1986) and
Walker & Leakey (1988). (h) Tree after Olson (1981, 1985) and Falk (1988).
47
48
. . 
ET AL.
Chamberlain & Wood’s (1987) results (at least when recognizing H. habilis sensu lato) were
broadly consistent with a prevalent phylogenetic hypothesis of the time, which stated that
A. africanus was the ancestor of the ‘‘robust’’ australopithecines (Johanson & White, 1979;
White et al., 1981; Rak, 1983; Kimbel et al., 1984). For the purposes of discussion, our
preferred tree was altered such that A. africanus was made the sister of a ‘‘robust’’ clade [Figure
9(b)]. This tree is not identical to Chamberlain & Wood’s, but it reflects their major
phylogenetic conclusions. It is marginally less parsimonious than the one preferred here
[Figure 9(a)], requiring only one additional step, regardless of whether or not certain
characters are constrained from reversing.
Wood’s (1991, 1992a) cladistic analysis suggested that ‘‘robust’’ australopithecines are the
sister group of an A. africanus+Homo clade. Although he did not include A. aethiopicus in a
cladogram, it is clear from his phyletic tree that he considered KNM-WT 17000 to represent
a plausible ancestor of other ‘‘robust’’ australopithecines. Furthermore, he notes in the text
(1991: p. 278; 1992a: pp. 354–355) that if A. aethiopicus is included, it is the sister of A. boisei. In
addition, he refers to A. aethiopicus as ‘‘Australopithecus aff. A. boisei’’ (see also Wood et al., 1994).
He also considered H. habilis and H. rudolfensis to be sisters. In order to enable comparison with
our preferred tree [Figure 9(a)], Wood’s (1991, 1992a) is modified to include A. aethiopicus as
the sister of A. boisei [Figure 9(c)]. Compared with Figure 9(a), this tree requires 18 extra steps
regardless of whether irreversibility is allowed. Thus, it is highly unparsimonious given the data
set of the present study. Many of these extra steps were obtained because of the sister group
relationship between H. habilis and H. rudolfensis, and that between A. boisei and A. aethiopicus.
If those relationships are disallowed, then a tree in which A. africanus is the sister of Homo
[Figure 9(d)] requires 205 steps (i.e., six extra steps if characters can reverse, three if they
cannot).
Skelton & McHenry (1992) proposed that A. boisei and A. robustus are the sisters of Homo, and
that A. aethiopicus is the sister of all other hominids except A. afarensis. According to that study,
A. africanus is the sister of the A. boisei+A. robustus+Homo clade. In an earlier analysis, Wood
(1988) had found an unrooted topology which, if rooted conventionally, would result in an
identical tree. Neither study recognized separate species of Homo. For purposes of comparison,
their hypothesis was represented by a cladogram that included our reconstruction of
relationships within that genus [Figure 9(e)]. Compared with Figure 9(a), this tree requires an
additional 13 steps if characters are free to reverse, and ten if they are not.
Lieberman et al. (1996) used cladistic methodology to examine the phylogenetic relationships
of H. habilis and H. rudolfensis. They concluded that these species were unlikely to be sister taxa,
that H. habilis was the likely sister of H. ergaster (and, presumably, later members of the Homo
clade), and that H. rudolfensis shared a number of derived character states with australopithecines. In contrast, the present study found that the relationships of H. habilis and
H. rudolfensis were quite unstable, but that H. rudolfensis was the taxon most closely related
to H. ergaster and H. sapiens. A possible explanation for this discrepancy is that the present
study assigned South African early Homo specimens to H. habilis, thereby making that taxon
quite variable. With respect to the phylogenetic relationships of other hominid species,
Lieberman et al. (1996) did not endorse a tree because their most parsimonious cladogram
was barely shorter than several other trees. In fact, however, their four shortest trees all
support certain relationships. Namely, A. afarensis is the sister of all other hominids, A. africanus
is either nested within or is the sister of the Homo clade, H. ergaster and H. habilis are sister
taxa, and the ‘‘robust’’ australopithecines are paraphyletic. For purposes of comparison,
Figure 9(a) is compared with their most parsimonious tree, which has been modified
  
49
[Figure 9(f)] to make H. sapiens the sister of H. ergaster. This tree is highly unparsimonious given
the data set of the present study. It requires 21 extra steps if characters can reverse, and 22 if
some same cannot. Most of these steps are the result of removing A. aethiopicus from the
‘‘robust’’ clade.
Since the description of A. aethiopicus, there have been many non-cladistic phylogenetic
analyses of early hominids. Most of these (Kimbel et al., 1986; Delson, 1986; Skelton et al.,
1986; Kimbel et al., 1988; Grine, 1988a) are generally consistent with one or several of the
cladistics studies described above. However, two hypotheses are considerably different, and
warrant comparison with the analyses presented here. It is difficult to make quantitative
comparisons, of course, because these studies do not present their hypotheses in the form
of cladograms. For the purposes of discussion, cladograms have been constructed that
are consistent with their proposed phylogenies, but because this is a reversal of the logical
process of inference (Tattersall & Eldredge, 1977), these should be viewed as only crude
approximations.
In the first of these two phylogenies, Walker et al. (1986) and Walker & Leakey (1988)
expressed their belief that KNM-WT 17000 represented a basal member of the A. boisei
lineage. The fact that it was demonstrably more primitive than A. robustus indicated to them
that many of the features shared by A. boisei and A. robustus must have evolved in parallel. Thus,
they suggested that A. robustus may have evolved from A. africanus. Using the data set of the
present study, a cladogram that is consistent with this phylogeny [Figure 9(g)] is enormously
less parsimonious than the topology preferred here [Figure 9(a)], requiring 39 additional steps
if characters can reverse and 38 if some cannot.
The second of these two phylogenies is based on patterns of cranial venous drainage (Falk
& Conroy, 1983; Falk, 1986, 1988, 1990). It is similar to a phylogeny proposed earlier by
Olson (1981, 1985), who suggested that two species were represented at Hadar and Laetoli.
One of these was held to be ancestral to the ‘‘robust’’ australopithecines, while the other was
ancestral to a Homo lineage that included A. africanus. Olson’s morphological analysis has been
refuted (Kimbel et al., 1985), but Falk & Conroy (1983) and Falk (1986, 1988, 1990) state that
A. afarensis must be considered the ancestor of the ‘‘robust’’ australopithecines due to the
presence of an enlarged occipito-marginal sinus system. A cladogram that is consistent with
this phylogeny [Figure 9(h)] is noticeably less parsimonious than Figure 9(a), requiring 211
steps regardless of whether or not certain characters can reverse.
In summary, the tree preferred here [Figure 9(a)] is only marginally to moderately more
parsimonious than those in which A. africanus is the sister of either the ‘‘robust’ australopithecines [Figure 9(b)], or of Homo [Figure 9(d)]. These two trees are, therefore, plausible
(though less parsimonious) alternatives to the one favored by the present analysis. Clearly, any
reconstruction of A. africanus relationships must be accepted cautiously. In contrast, Figure 9(a)
is substantially more parsimonious than trees in which the ‘‘robust’’ australopithecines are
paraphyletic [Figures 9(e), (f), (g)]. Thus, there is a considerable cost in terms of tree length of
accepting ‘‘robust’’ australopithecine paraphyly. This undermines phylogenetic hypotheses
that do not reconstruct these species as being monophyletic.
It is interesting to note that among recent phylogenetic analyses, two of the areas of major
controversy pertain to the relationships of H. habilis and of A. africanus. As noted above, the
hypodigms of these two taxa are notably the most variable among early hominid species.
These factors suggest that the H. habilis hypodigm accepted here, which includes both South
and East African fossils, may warrant revision. It also suggests that the traditionally recognized
hypodigm of A. africanus may require re-evaluation.
50
. . 
ET AL.
Taxonomic implications
In order that biological classification, and hence zoological nomenclature, does not simply
reflect convenient, ad hoc groupings, it should convey information about the hypothesized
genealogical history of the organisms under consideration (Hennig, 1966; Brundin, 1966).
Although Hennigian systematics cannot claim proprietary rights to genealogical
classifications—or, at least, to classifications that purport to reflect such history—taxonomies
that are rooted in phenetic (Sneath & Sokal, 1973) and evolutionary (Simpson, 1961; Mayr,
1969) ontologies recognize paraphyletic groups either implicitly or explicitly. In this regard, a
paraphyletic taxon is considered to be one that comprises a common ancestor and some, but
not all of its descendants (Farris, 1974). While taxonomists who subscribe to phenetic and
evolutionary paradigms may recognize that taxa should be monophyletic, their definitions of
monophyly are such that nearly any taxon can be so regarded (Wiley, 1981). Simpson (1961),
for example, was of the opinion that a genus may be considered to be monophyletic even if it
is polyphyletic at the species level. As Wiley (1981) has observed, however, any taxon that is
monophyletic at one level and polyphyletic at another is a historically unnatural group. The
acknowledgement of paraphyletic taxa by phenetic and evolutionary taxonomies is firmly
rooted in their recognition of evolutionary grades as historical entities. An evolutionary grade,
which is characterized by a particular organizational level (Huxley, 1958), is a class (sensu
Ghiselin, 1980) comprised of species that have achieved a certain perceived adaptive plateau
through the parallel or convergent evolution of two or more independent lineages.
Because neither evolutionary grades nor paraphyletic groups reflect the phylogenetic history
of the organisms that comprise them, they have no place in any taxonomic classification that
purports to reflect genealogical history (i.e., a taxonomy that is phylogenetically natural).
All eight parsimony analyses conducted in the present study support a monophyletic clade
comprised by A. aethiopicus, A. robustus and A. boisei. Indeed, this is one of the most strongly
supported and consistent conclusions to emerge from this study. Accordingly, the genus name
Paranthropus may be applied legitimately to this species group. Although the genus name
Paraustralopithecus Arambourg & Coppens, 1967 is available for specimens here now referred to
P. aethiopicus (Table 1), this species can be (in our opinion) considered a congener of P. robustus
and P. boisei. In support of the present argument for Paranthropus monophyly, other
phylogenetic analyses have also concluded that ‘‘robust’’ australopithecines comprise a
monophyletic group (Wood & Chamberlain, 1986; Chamberlain & Wood, 1987; Kimbel et al.,
1988; Wood, 1991, 1992a). There is a strong body of evidence that reveals Paranthropus
to be characterized by a host of morphological specializations, many (but not all) of
which are probably related to trophic parameters (Clarke, 1977; Wood & Stack, 1980;
Grine, 1981, 1986, 1988a; Wood & Ellis, 1986; Wood & Chamberlain, 1987; Turner
& Wood, 1993a). Moreover, Turner & Wood (1993b) have argued that there is no good
paleobiogeographic evidence on which to base a rejection of the hypothesis of Paranthropus
monophyly.
Thus, just as the monophyletic nature of the genus Homo seems to be widely accepted by
students of hominid evolution, the evidence presented here and elsewhere overwhelmingly
indicates that Paranthropus, too, should be accorded recognition as a holophyletic taxon.
However, even with the removal of the species Paranthropus aethiopicus, P. robustus and P. boisei
from Australopithecus, the latter remains paraphyletic because of the hypothesized relationship of
A. afarensis and A. africanus. These two species do not represent sisters; rather, A. afarensis is the
sister of A. africanus plus all other hominids. This relationship, or a similar relationship that
renders Australopithecus paraphyletic, is the conclusion attained by all cladistic analyses that
  
51
have included A. africanus and A. afarensis (Chamberlain & Wood, 1987; Wood, 1988, 1991,
1992a; Skelton & McHenry, 1992; Lieberman et al., 1996). Accordingly, Australopithecus afarensis
requires transfer to another genus. This creates a rather confusing situation, because the
species name A. afarensis Johanson et al., 1978 is a widely used replacement name for
Meganthropus africanus (Weinert, 1950) and Praeanthropus africanus (Weinert, 1950).
The Garusi maxillary fragment (Garusi 1), which was recovered by Kohl-Larsen in 1939
from the surface of the Laetolil Beds, was first examined by Abel, who regarded it to have
belonged to a novel form of anthropoid ape (Kohl-Larsen, 1943). Abel’s suggestion was taken
up by Hennig (1948), who proposed that the genus name Präanthropus (=Praeanthropus) be
applied to the specimen. Hennig (1948), however, failed to designate a type-species for
Praeanthropus. Thus, as noted by Day et al. (1980), the name Praeanthropus was not available as
of Hennig’s work and was, therefore, a nomen nudum. Johanson et al. (1978) and Clarke (1977)
are incorrect in stating that because of Hennig’s failure to designate a type-species, the name
Praeanthropus is invalid.
Weinert (1950) proposed that Garusi 1 be referred to the genus Meganthropus, which had
been erected by von Koenigswald in reference to the Sangiran 6 mandible from Indonesia as
M. palaeojavanicus (Weidenreich, 1945). However, not only was Weinert’s (1950) designation
clearly provisional (although permitted before 1961), he failed to provide any diagnosis for the
new species, M. africanus. Indeed, the only apparent similarity between the Sangiran 6 and
Garusi 1 specimens noted by Weinert (1950) was the observation that the two ‘‘fit together’’!
Nevertheless, the name M. africanus was employed by Remane (1951) in his detailed
description of Garusi 1, although Robinson (1953) was quick to note that it could not be
readily distinguished from Australopithecus africanus homologues. He concluded that Garusi 1
was simply an East African representative of that species. In any case, it is now widely accepted
that this specimen does not share any special affinity with Sangiran 6, and thus should not be
considered a congener of Meganthropus palaeojavanacus.
Praeanthropus was made available by Senüyrek (1955) as the replacement genus name for
M. africanus Weinert, 1950. Thus, as of Senyürek’s work, Praeanthropus is an available name for
a genus, the type-species of which is M. africanus Weinert, 1950.
Because Johanson et al. (1978) attributed the Laetoli and Hadar hominid samples to a novel
species of Australopithecus, and particularly because they considered the Garusi 1 maxilla to
represent a species that cannot be distinguished at the generic level from Australopithecus, they
provided A. afarensis as a replacement name for M. africanus Weinert, 1950. This was necessary
because the latter would have become a junior homonym of A. africanus Dart, 1925.
Olson (1981) proposed that the Laetoli fossils and some of the Hadar specimens represent
the most primitive species of Paranthropus, and that this material is therefore attributable to the
species Paranthropus africanus (Weinert, 1950). Olson’s (1981) recognition of Paranthropus
elements within the Laetoli and Hadar samples has not survived serious scrutiny (Kimbel et al.,
1985) and, in any event, there is no clear sister relationship between the taxon that is
represented by these samples and the other species that comprise the Paranthropus clade. Thus,
the genus name Paranthropus should not be applied to the Hadar and Laetoli fossils.
The nomen Praeanthropus africanus (Weinert, 1950) is available for the fossils from the Hadar
Formation, the Laetolil Beds, and other sites in eastern Africa that have been attributed to
Australopithecus afarensis. Because these specimens appear to represent a single species that can
be diagnosed as being clearly distinct from A. africanus, and because its attribution to the genus
Australopithecus results in the latter’s paraphyly, it is necessary to employ P. africanus (Weinert,
1950) in priority usage for this species (Day et al., 1980; Harrison, 1993).
. . 
nu
s
af
ri
op
ca
us
Pa
nu
ro
ra
s
bu
nt
hr
st
us
Pa
op
us
ra
nt
bo
hr
is
H
o
ei
om
pu
s
o
ae
ha
th
bi
H
io
lis
om
pi
cu
o
ru
s
do
H
lfe
om
ns
o
is
er
ga
s
te
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r
om
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cu
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hr
Pa
ra
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Au
ET AL.
ith
e
op
us
yt
es
Pr
ae
an
th
r
gl
od
tr
o
n
Pa
G
or
ill
a
go
ri
lla
52
Figure 10. The cladogram preferred by the present study after the adoption of nomenclatural revisions.
The transfer of the Hadar and Laetoli hominids from A. afarensis to P. africanus is analogous
to other recent changes in primate nomenclature. For instance, it is now well established that
Pan, Gorilla and Pongo are paraphyletic (e.g., Andrews & Martin, 1987; Begun, 1994; Goodman
et al., 1994). Consequently, the family name Pongidae is now rarely used in reference to all of
these taxa, despite the fact that they are gradistically very similar.
It is, therefore, proposed that P. africanus (Weinert, 1950) be recognized as the appropriate
nomen for the accommodation of the East African Pliocene remains that have been attributed
to A. afarensis Johanson et al., 1978. The obvious result of the recognition of P. africanus is that
there remains but a single species of Australopithecus, viz. A. africanus. A less obvious, but
nonetheless influential effect of this is the loss of any meaning to the term australopithecine
that would extend beyond a strict reference to A. africanus. Even then, the term is meaningless
in a strict zoological context because of its inference that A. africanus is attributable to a distinct
subfamily from other early hominid taxa (Grine, 1981).
Given these taxonomic conclusions, it is necessary to re-label the cladogram preferred by
the present study to reflect the revised nomenclature (Figure 10).
Patterns of phyletic evolution
The generation of a cladogram is only the first stage of a phylogenetic reconstruction
(Tattersall & Eldredge, 1977). The second stage is to use the cladogram as a guide for
suggesting a phyletic tree, which indicates possible ancestor–descendant relationships. Such
trees are necessarily speculative, and many phyletic trees can be generated from a single
cladogram. However, an accepted principle of building phyletic trees is that a taxon (i.e.,
OTU) can be considered as an ancestor only if it resembles the hypothetical ancestor (HTU)
present in the cladogram. Such OTUs lack autapomorphies or other derived states that would
differentiate it from the HTU. Because it is rare for OTUs to be identical to HTUs, the
designation of a recognized fossil species as an ancestor often requires that certain character
states of an OTU which are not strictly compatible with the HTU be ignored. Without this
degree of imprecision, it is doubtful that any ancestor could be identified in the paleontological
  
53
Homo sapiens
Time (mya)
1
2
?
Paranthropus
boisei
Paranthropus
robustus
Homo
ergaster
Paranthropus
aethiopicus
Homo
habilis
?
3
Homo
rudolfensis
?
Australopithecus
africanus
4
Praeanthropus
africanus
Figure 11. Phyletic tree consistent with the VARIABLE=INTERMEDIATE analysis. Solid vertical bars
indicate known temporal ranges of species. Dashed vertical bars indicate suspected temporal ranges of
species. Question marks indicate hypothetical ancestors (see text). Dashed lines indicate oversimplified
evolutionary relationships among species.
record, or that it would be possible to construct any phyletic tree that was not identical to the
cladogram on which it was based.
Although the VARIABLE=INTERMEDIATE and IRREVERSIBLE analyses support
the same cladogram, they imply subtly different patterns of phyletic evolution. A phyletic
tree based on the VARIABLE=INTERMEDIATE analysis (Figure 11) has P. africanus giving
rise to A. africanus, which in turn gives rise to a hypothetical A. africanus-like species. This
species lacks certain A. africanus apomorphies, but shares derived features of the Homo+
Paranthropus clade (Table 4). It subsequently gives rise to the Homo and Paranthropus
lineages. The basal member of the Homo lineage is plausibly H. habilis. H. ergaster is posited as
an ancestor of the lineage leading to H. sapiens. The immediate ancestor of the Paranthropus
lineage is a generalized Paranthropus species. Paranthropus aethiopicus could not have been the
ancestor of this lineage because it possesses too many derived reversals that discriminate
it from the hypothetical ancestor reconstructed in the VARIABLE=INTERMEDIATE
cladogram (Table 4). Thus, P. aethiopicus is best considered a side-branch of the Paranthropus
radiation.
The phyletic position of P. aethiopicus is admittedly non-intuitive. This species predates
P. boisei and P. robustus, and lacks a number of derived features shared by P. robustus
and P. boisei. These qualities have led many researchers (Delson, 1986; Grine, 1988; Kimbel
54
. . 
ET AL.
et al., 1988; Wood, 1991, 1992a) to suggest that P. aethiopicus is a basal species of the
Paranthropus lineage. Nonetheless, if Paranthropus is descended from an A. africanus-like
ancestor, then the supposedly primitive features of P. aethiopicus must be considered derived
reversals.
The position of A. africanus directly contradicts the principle conclusion reached by many
researchers soon after the discovery of KNM-WT 17000 (Delson, 1986; Walker et al., 1986;
Walker and Leakey, 1988; Kimbel et al., 1988). Kimbel et al. (1988: p. 263) stated that:
‘‘With the discovery of KNM-WT 17000, and the consequent legitimization of A. aethiopicus,
hypotheses that describe A. africanus as either the ancestor of ‘robust’ Australopithecus species . . . or the
last common ancestor of the Homo and ‘robust’ clades . . . are, in our judgment, no longer tenable and
should be considered effectively refuted. The basis for this refutation is the large number of primitive
characters . . . shared by A. afarensis and KNM-WT 17000, implying extensive character reversal in a
transition from A. afarensis through A. africanus to A. aethiopicus.’’
The implication of this statement is that reversals are rare, and that a large number of reversals
are so unlikely as to doom any phylogeny that requires them. Yet, although the cladogram
favored by the VARIABLE=INTERMEDIATE analysis [Figure 1(a)] requires many reversals
in P. aethiopicus, it is still more parsimonious than ones that require fewer reversals [e.g., Figure
9(d)–(h)]. Thus, if one accepts the principle that all characters can reverse, the number of
reversals that occur should not be of singular importance.
The alternative, of course, is to reject the principle that all characters are equally likely to
reverse. A phyletic tree based on the IRREVERSIBLE analysis (Figure 12) shows Praeanthropus
africanus giving rise to a primitive A. africanus-like species. This species possesses the synapomorphies of A. africanus, Paranthropus, and Homo (Table 6), but retains primitive states in three
of the irreversible characters (facial prognathism, cranial capacity, and basicranial flexion).
This species subsequently gives rise to A. africanus and a species that represents the common
ancestor of the Homo and Paranthropus lineages. The latter possesses synapomorphies of
the Homo+Paranthropus clade, but retains the primitive traits noted above. A. africanus cannot
be this ancestor because such a relationship would require reversals in several characters that
are posited to be irreversible. Relationships within the Homo lineage are equivalent to those
stated before.
A consequence of accepting irreversibility in several characters is that there are relatively
few features that discriminate P. aethiopicus from the hypothetical Paranthropus ancestor
reconstructed in the IRREVERSIBLE cladogram (Table 6). It is, therefore, plausible to
consider P. aethiopicus as the basal member of the Paranthropus lineage. Such a relationship is
more intuitively satisfying than one in which this species is an evolutionary dead-end (Figure
11). A consequence, however, is that multiple parallelisms (as opposed to reversals) are
required in several features. For instance, cranial capacity must increase independently in
three lineages (A. africanus, Paranthropus and Homo).
Predictions of the phyletic trees
Each of the two phyletic trees proposed here predict the existence of two hypothetical hominid
species. Future fossil discoveries will test these predictions and, thus, the trees themselves. The
tree consistent with the VARIABLE=INTERMEDIATE analysis (Figure 11) predicts the
presence of a derived A. africanus-like species and a generalized Paranthropus species. Both
hypothetical ancestors should be present in the fossil record between 3·0 and 2·6 Ma. Given
the extreme variability of the A. africanus and early Homo samples, it is possible that
  
55
Homo sapiens
?
1
Paranthropus
boisei
Time (mya)
Homo
rudolfensis
2
Paranthropus
robustus
Homo
ergaster
Homo
habilis
?
Paranthropus
aethiopicus
3
Australopithecus
africanus
4
?
Praeanthropus
africanus
Figure 12. Phyletic tree consistent with the IRREVERSIBLE analysis. Solid vertical bars indicate known
temporal ranges of species. Dashed vertical bars indicate suspected temporal ranges of species. Question
marks indicate hypothetical ancestors (see text). Dashed lines indicate oversimplified evolutionary relationships among species.
representatives of the A. africanus-like species have already been discovered but have gone
unrecognized. Anatomically, this ancestor may resemble Stw 53, because this specimen recalls
A. africanus with respect to aspects of its morphology (e.g., anterior pillars, moderate incursion
of the temporal lines, small vaginal process) and yet exhibits synapomorphies of the
Homo+Paranthropus clade (e.g., coronally oriented petrous bones, horizontal foramen magnum).
However, this specimen has other features (e.g., enlarged cranial capacity, reduced pneumatization in the temporal squama) that may preclude it from membership in this hypothetical
species. Representatives of the generalized Paranthropus species are probably not currently
known. That species possesses many of the features shared by P. boisei and P. robustus, but lacks
their autapomorphies and the seemingly primitive features of P. aethiopicus. Such morphology
is not consistent with that of Stw 252—particularly with regard to anterior tooth size—which
Clarke (1988) has posited as an ancestor of Paranthropus.
The phyletic tree consistent with the IRREVERSIBLE analysis (Figure 12) also predicts the
presence of two hypothetical ancestral species. One of these is A. africanus-like, but retains
several primitive features seen in A. afarensis (strong facial prognathism, small cranial capacity,
flat cranial base). That species should exist before 3·0 Ma. The second hypothetical species
mixes derived aspects of Homo and Paranthropus with the same primitive features. Such a
description superficially resembles the condition seen in KNM-ER 1805. Like A. afarensis, this
56
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ET AL.
early Homo specimen exhibits a sagittal crest and asterionic notch. Furthermore, like
Paranthropus, it has laterally inflated mastoid processes, partial compound temporonuchal
crests, and laterally extended tympanics. However, this specimen exhibits morphology
(e.g., intermediate alveolar prognathism, enlarged cranial capacity) that probably excludes it
from membership in this hypothetical species. KNM-BC 1 also combines Homo-like features
(Hill et al., 1992) with Paranthropus morphology (laterally extended tympanic, large mandibular
fossa, external auditory meatus large and circular). This unique combination may be at the
root of its uncertain taxonomic assignation.
Scenarios of hominid evolution
According to Tattersall & Eldredge (1977), the final (and most speculative) stage of a
phylogenetic reconstruction is the proposal of an evolutionary scenario that explains the
observed phylogenetic pattern. In this regard, the present study is consistent with much that
has been surmised already. It is possible that bipedalism (and hence, the earliest hominids)
evolved in response to changing ecological conditions in Africa during the late Miocene and
Early Pliocene. It is also possible that between 2·5 and 1·5 Ma, hominid diversity is associated
with environmental desiccation (e.g., Vrba, 1988). After 2·5 Ma, hominid diversity is
represented primarily by two distinct lineages, Paranthropus and Homo, which probably reacted
to such desiccation by following different evolutionary trajectories (i.e., hypermastication vs.
hyperencephalization).
In addition to these generalizations, the two phyletic trees proposed by this study offer
insights into the evolution of certain frequently discussed characters. Both trees suggest that the
masticatory apparatus increased moderately in the early stages of human evolution, and that
subsequently it decreased in Homo and increased markedly in Paranthropus. This dichotomy
almost certainly represents a divergence in trophic adaptations.
The two trees differ regarding the evolution of facial prognathism, cranial capacity and
basicranial flexion. A scenario consistent with the VARIABLE=INTERMEDIATE analysis,
which does not require parallelisms in these features, suggests that the selective pressures
responsible for their evolution produced trends that characterize hominids in general. In
contrast, a scenario consistent with the IRREVERSIBLE analysis implies that such pressures
created independent trends in different hominid lineages, because the features evolved in
parallel. Detailed functional and ecological analyses are required to better infer the nature of
these selective pressures.
Finally, the two phyletic trees proposed here suggest different scenarios pertaining to the
evolution of P. aethiopicus. In both the IRREVERSIBLE and VARIABLE=INTERMEDIATE
analyses, this taxon undergoes reverse evolution in some characters. However, in the
IRREVERSIBLE analysis, these characters are few in number and are restricted to features
that may be related to mastication (compound temporal–nuchal crest, asterionic notch,
shallow mandibular fossa). Thus, it possible that these features reflect the trophic adaptations
of the basal Paranthropus lineage, and that they evolved along with other ‘‘robust’’ traits.
Subsequent species in this lineage modified their morphology, reflecting either an increased
level of dietary specialization or perhaps a subtle shift in diet. Under the conditions of the
VARIABLE=INTERMEDIATE analysis, however, such a scenario does not adequately
explain reversals in Paranthropus aethiopicus. Not only does this analysis require (elusive)
explanations for reversals in brain size, facial prognathism and cranial base flexion, but
reversals in masticatory features must now be seen as specific alterations of a generalized
‘‘robust’’ form. It is difficult to specify why such alterations would be necessary. Cresting
  
57
patterns and the asterionic region have been related to the configuration of the temporalis
muscle (Robinson, 1958; Tobias, 1967; Jolly, 1970; Wolpoff, 1974; Kimbel et al., 1984;
Kimbel & Rak, 1985), and thus, might indicate dietary habits. However, they could just as
easily reflect brain size, because the size of the braincase affects the origin of the temporalis
(e.g., Robinson, 1958; Wolpoff, 1974). Furthermore, despite the fact that it is part of the
masticatory apparatus, functional interpretations of mandibular fossa depth are confounded by
the presence of similar morphologies in P. boisei and H. sapiens, two species that undeniably had
different dietary regimens.
Inevitably, certain aspects of hominid evolution remain enigmatic.
Conclusion
Eight parsimony analyses were performed on all or some of 60 cranial, dental and mandibular
characters. These analyses shared several results. There is robust evidence for a Paranthropus
clade. Australopithecus, as conventionally defined, is paraphyletic. Consequently, specimens
attributed to A. afarensis should be referred to Praeanthropus africanus. That species is the sister of
all later hominids. Seven analyses found Paranthropus to be the sister of Homo, and A. africanus
to be the sister of the Homo+Paranthropus clade. In one analysis, the relationships of A. africanus,
Homo and Paranthropus cannot be resolved unambiguously. The eight analyses disagreed
concerning species-level relationships within Paranthropus and within Homo. With respect to
these relationships, we favor the topology produced by the VARIABLE=INTERMEDIATE
and IRREVERSIBLE analyses (Figure 10).
This phylogenetic reconstruction can be tested by re-examining its character analysis, by
the discovery of new fossils, and by comparisons with independent data sets. In particular,
Wood’s (1991, 1992a) analysis should be repeated, but with P. aethiopicus as a member of the
ingroup. His data are essentially independent from ours, because most of his characters are
quantitative.
The results of this study differ from those of prior analyses, particularly concerning the
relationships of A. aethiopicus, A. africanus, H. habilis and H. rudolfensis. This is due principally to
differences of opinion concerning early hominid morphology (as described in Appendices 1
and 2). In turn, we welcome reconsideration of our morphological characterizations, because
phylogenetic hypotheses should be supported or rejected according to the strength of their
character analyses. Detailed discussion of morphological characters can only enhance our
understanding of early hominid phylogeny.
Acknowledgements
We would like to thank W. Kimbel, H. McHenry, B. Wood, T. Rae, D. Lieberman,
C. Lockwood, and R. Asher for providing valuable comments on versions of this study.
Thanks also to the directors, curators and staffs of the Transvaal Museum, Kenya National
Museum, University of the Witwatersrand Department of Anatomy, and the American
Museum of Natural History for providing access to neontological and fossil specimens in their
care. D. Swofford generously allowed us to use a borrowed version of PAUP because that
program is not currently available for purchase (pending the development of Version 4.0). This
research was supported by a National Science Foundation grant (BNS number 9120117)
to F.E.G.
58
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Appendix 1: Description of Characters
The following is a description and discussion of the characters listed in Table 2. Many of the
characters listed below were also used by Skelton & McHenry (1992), so Appendix 1 is
extensively cross-referenced with their Table 1 (1992: pp. 314–324). To avoid confusion,
numbered characters from Skelton & McHenry (1992) are referred to as ‘‘S&M #’’.
Numbered characters from the present study are referred to simply as ‘‘character #’’. Unless
specimens are specifically mentioned, a morphological characterization of a species refers to all
relevant published specimens comprising its hypodigm.
1. Projection of nasal bones relative to the frontomaxillary suture
Hominids vary in the pattern by which the nasal bones project above the frontomaxillary
suture (Olson, 1985; Eckhardt, 1987; Walker & Leakey, 1988). Four states are recognized: (0)
nasal bones extend superior to the frontomaxillary suture by means of a narrow projection
(i.e., the nasal bones taper above the suture line); (1) nasal bones project above the
frontomaxillary suture, widening laterally as they pass the suture line; (2) nasal bones do not
project above the frontomaxillary suture; and (3) variable. The final state is observed in
A. africanus. The nasals project and are expanded in Taung but they project and are superiorly
tapered in Stw 505 and Sts 5. Whereas Olson (1985) suggested that the nasal bones do not
project in Sts 5, Broom & Robinson (1950) have noted that they do in fact project above the
suture. Pan and Gorilla are coded as having nasals that project and taper. Although Eckhardt
(1987) observed morphological variants in 8% of ape crania, the dominant pattern in both
genera is state 0. Our own observations confirm this. This character is treated as unordered
because there seems to be an equal probability of deriving each state from any other.
This character combines S&M 1 (nasion approaches glabella) and S&M 2 (location of
greatest width of nasal bones). When nasion approaches glabella, the nasal bones project, but
it is necessary to combine S&M 1 and S&M 2 because it is uncertain whether projecting nasal
bones are homologous when the manner in which they project differs (i.e., tapered vs.
expanded). Furthermore, the frontomaxillary suture is a better reference point than glabella
for assessing nasal bone projection because nasion may approach glabella because of either the
superior movement of nasion or the inferior migration of glabella (Clarke, 1977).
2. Inferior orbital margin rounded laterally
Olson (1985) describes the morphology of the inferior margin of the orbit as being either
rounded laterally or not rounded. Rounded lateral margins are present in A. aethiopicus and
A. robustus. They are also present in a large proportion of male G. gorilla, and thus this species
is coded as being variable. This character is equivalent to S&M 3.
3. Infraorbital foramen location
This character is equivalent to S&M 4. However, the character is here quantified as a ratio of
the distances between orbitale and the foramen, and orbitale and the root of the zygoma. A
high foramen has a value of <0·5 (i.e., it is located within the upper half of the malar). A low
foramen has a value of >0·5 (within the lower half). State assignments produced by this ratio
corespond to those of S&M 4 except with respect to A. africanus. This is because Sts 52a
possesses a low foramen whereas the foramina in all other A. africanus specimens are high.
Accordingly, A. africanus is here coded as being variable. Although Skelton & McHenry (1992)
noted that Sts 52a has a low foramen, they coded the species as ‘‘high’’.
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4. Anterior pillars
This character is equivalent to S&M 6, but state assignments here differ considerably from
theirs. Skelton & McHenry (1992) followed Rak (1983) in accepting that pillars in A. boisei (and,
by extension, A. aethiopicus) had been ‘‘obscured by infilling’’. This state assignment is based on
a functional interpretation of australopithecine facial evolution (Rak, 1983), rather than upon
strict observation. Consequently, pillars are considered to be absent in A. boisei and A. aethiopicus.
Furthermore, pillars are variable in A. africanus rather than present (contra S&M 6) because they
are absent in Sts 52a but present in all other relevant specimens. Finally, contrary to S&M 6,
pillars are considered to be variable in H. habilis (present in Stw 53, OH 62, KNM-ER 1805;
absent in KNM-ER 1813, SK 847), and absent in H. rudolfensis (KNM-ER 1470).
5. Nasoalveolar clivus contour in coronal plane
This character is equivalent to S&M 12. Rak (1983) describes three forms of the nasoalveolar
clivus: convex, straight, and concave (=nasoalveolar ‘‘gutter’’). In our definition, ‘‘straight’’
includes both slight concavity and slight convexity. Skelton & McHenry (1992) considered the
clivus of A. aethiopicus (KNM-WT 17000) to be more concave than that in A. robustus and
A. boisei. All three of these species are coded here as being concave. Furthermore (contra S&M
12), H. habilis possesses a straight clivus.
6. Protrusion of incisor alveoli beyond bi-canine line
When the nasoalveolar contour protrudes, the incisors are anterior to the canines, and when
there is no protrusion, the incisors are in the same coronal plane as the canines. Skelton &
McHenry (1992) stated that Stw 73 lacks protrusion, and thus coded A. africanus as variable.
We disagree with this assessment, and consider protrusion to be present in this and all other
A. africanus specimens.
7. Nasal cavity entrance
This character seems to be equivalent to S&M 10 (‘‘distinct subnasal and intranasal
components of nasoalveolar clivus’’). McCollum et al. (1993) described differences in the form
of the entrance to the nasal cavity in fossil hominids. A stepped entrance (state 0) is one in
which there is a drop posterior to the nasoalveolar clivus. A smooth entrance is one in which
the clivus grades into the nasal floor. However, a smooth entrance can be formed in two
(presumably non-homologous) ways. In H. sapiens, the smooth entrance is formed by flexions
in the clivus and hard palate, which do not overlap (state 3). In other hominid species that
exhibit a smooth entrance, the clivus and palate ovelap considerably (state 2). State
assignments follow McCollum et al. (1993: p. 97, Table 7), who differ from Skelton &
McHenry (1992) regarding A. africanus, A. robustus, and A. boisei. McCollum et al. (1993) also
differ from Walker et al. (1986), who characterized A. africanus as being intermediate.
Furthermore, McCollum et al. (1993) characterized KNM-ER 1805 as having a smooth nasal
cavity entrance, whereas Skelton and McHenry (1992) stated that it is stepped. According to
McCollum et al. (1993), H. habilis is variable (state 1; smooth in Stw 53, OH 62, KNM-ER
1805; stepped in KNM-ER 1813, OH 24, SK 847).
8. Palate thickness
McCollum et al. (1993: Tables 4 and 5) recorded palatal thickness posterior to the incisive fossa
for a variety of extant and extinct hominoids. A palate thicker than 7 mm is found only in
‘‘robust’’ australopithecines. All other hominids have a thin palate (<7 mm).
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9. Height of the masseter origin
Kimbel et al. (1984) measured this character as an index of zygomaticoalveolar height relative
to orbitoalveolar height. With the exception of OH 5, all specimens of A. africanus, A. robustus
and A. boisei had index values in excess of 56·4. All specimens of A. afarensis, H. habilis,
H. rudolfensis and H. sapiens fall below 56·6. All Pan specimens were found to fall below 60, as
did all but one Gorilla specimen (Kimbel, pers. comm.). Excluding the single Gorilla outlier, the
range for that species is 42·2–58·5. Thus, two states are recognized with slightly overlapping
ranges: state 0, a high masseter origin, with an index value in excess of 56, and state 1, a low
origin, with an index value below 60. OH 5 has an index value of 47·9. Kimbel et al. (1984:
footnote to Table 5) attribute this to its extraordinary facial height, and thus still consider
A. boisei to have a high masseter origin.
This character is redundant with S&M 15 (zygomaticoalveolar crest weakly arched in
frontal view). This feature was first described by Rak (1983), but Kimbel et al. (1984: p. 349)
poined out that the form of the crest is a necessary correlate of masseter height, and they
(1984: p. 375) recognized these features as a single character.
10. Mediolateral thickness of the zygomatic arch at the root of the frontal process
Two states are recognized: (0) thin, with a thickness of less than 8 mm; and (1) thick (>8 mm).
Zygomatic thickness does not simply reflect body size, because the zygomae are thin in both
male and female Gorilla.
This character replaces S&M 13 (‘‘robusticity of zygomatic bone’’). Robusticity is usually
measured as an index of breadth to height, but this did not reveal any clear taxonomic
groupings among fossil hominids or African apes.
11. Anterior projection of zygomatic bone relative to piriform aperture
This character describes the ‘‘dishing’’ of the face and is equivalent to S&M 16, although they
did not indicate the point of reference for their characterizations. By using the piriform
aperture as a reference, three distinct morphologies are recognized: the infra-orbial plate of the
zygoma is posterior to the aperture (state 0), the zygoma is at the level of the aperture (state 2),
and the zygoma is anterior to the aperture. State 3 is equivalent to having a dished face. A.
africanus is variable (state 1) because Sts 71 exhibits the intermediate morphology (state 2),
whereas all other specimens of that species have posteriorly positioned zygomatics.
12. Anterior palatal depth
This character is equivalent to S&M 43. Two morphologies are recognized: shallow, in which
the palate grades posteriorly from the base of the incisors as a flat surface or in which there is
a slight depression anterior to the incisive foramen (state 0), and deep (i.e., shelved), in which
the palate rises sharply vertically before turning posteriorly at the incisive foramen (state 2).
H. habilis is variable (state 1; shallow in KNM-ER 1813 and OH 24; deep in OH 62, Stw 53,
SK 847). S&M 43 considered both of these taxa to be deep. A preponderance of humans have
deep palates (DeVilliers, 1968).
13. Index of palate protrusion anterior to sellion (facial prognathism)
Rak (1983) used this measurement to describe the proportion of the palate that is found
anterior to sellion. It is essentially a measure of facial prognathism. Based on Rak (1983: Table
3), Kimbel & White (1988: p. 185), and measurements by the authors, three morphologies are
recognized: prognathic (index >57; state 0), mesognathic (index between 57 and 30; state 2),
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and orthognathic (index <30; state 3). A. africanus is variable (state 1) with Sts 5 being
prognathic and Sts 71 and Sts 52a being mesognathic. The measurment of KNM-ER 1470
reflects the reconstruction employed by Wood (1991) and Grine et al. (1996). This measurement is equivalent to S&M 44, but because they coded taxa by ranking means, they assigned
different states to taxa whose means were very similar (e.g., 50% vs. 51%).
This character is redundant with S&M 46 (‘‘angle between sellion-prosthion and Frankfurt
Horizontal’’), because as the palate protrudes, this angle will decrease. Consequently, S&M 46
is not used. In addition, S&M 17 (‘‘position of anterior edge of zygomatic process origin’’) is
redundant with both this character and character 11 (anterior projection of zygomatic).
Although the position of the zygomatic relative to the tooth row is frequently used in
phylogenetic analyses (e.g., Kimbel et al., 1984; Walker et al., 1986; Skelton et al., 1986; Kimbel
et al., 1988; Wood et al., 1994), it is clearly a function of zygomatic projection and palate
protrusion, because an orthognathic face and a projecting zygomatic would require the
zygomatic root to arise relatively anteriorly on the tooth row, and vice-versa.
14. Masseteric position relative to sellion
Rak’s (1983, 1988) index of masseteric position describes the anterior migration of the
masseter muscle. An index value of 100 indicates that the masseteric tubercle is in the same
coronal plane as sellion. Two states are recognized: (0) the tubercle is at or posterior to sellion
(index values ¦105), and (1) it is at or anterior to sellion (index values §95). The ranges of
these two states overlap partially.
This character is not redundant with character 11 (anterior projection of zygomatic), since
the former describes the placement of a muscle attachment whereas the latter describes a bony
structure. However, S&M 45 (‘‘index of overlap’’) and S&M 47 (‘‘index of palate protrusion
anterior to masseter’’) are measurements developed by Rak (1983) that incorporate the same
information presented in this character and character 13 (index of palate protrusion).
Consequently, S&M 45 and 47 are not used.
15. Maxillary trigon
According to Rak (1983: pp. 32–33), this is a gutter-like triangular depression in the
infraorbital region. He identified it in A. robustus, and it is also present in KNM-WT 17000.
16. Cranial capacity
The cube root of cranial capacity was used so as to counter-act the exponential growth of this
volume in relation to the linear dimensions of the braincase. However, equivalent groupings
were found using the volumetric measurement, and this is presented here because readers are
more familiar with the manner in which this measurement is distributed in hominids. Five
states are recognized (data from Aiello & Dean, 1990; Tobias, 1991; Brown et al., 1993), some
of which overlap considerably: (0) below 500 cm3, (1) approximately 500 cm3, (2) 509–
680 cm3, (3) 750–875 cm3, (4) approximately 1400 cm3. State 0 includes P. troglodytes, G. gorilla,
A. afarensis and A. aethiopicus. Although some gorillas may exceed 500 cm3, their body sizes are
also much greater, and thus their brains are relatively quite small. State 1 includes A. africanus,
A. robustus, and A. boisei. Stw 505 has considerably extended the range of A. africanus (Clarke,
pers. comm.). State 2 includes H. habilis. Although the range of H. habilis overlaps considerably
with those of australopithecines, the smallest H. habilis cranial capacity (KNM-ER 1813:
509 cm3) is still much larger than the smallest representatives of the australopithecine species.
Holloway (1973) estimated the cranial capacity of Sts 19 to be 436 cm3, but reconstructions
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that include the calotte Sts 58 indicate cranial capacities between 530 and 570 cm3 (Broom &
Robinson, 1948; Tobias, 1975). State 3 includes H. rudolfensis and H. ergaster. State 4 includes
H. sapiens. In general, S&M 65 recognized finer distinctions between taxa than are recognized
here.
17. Cerebellar morphology
Holloway (1988) stated that in Homo, A. boisei and A. robustus, the cerebellum is tucked
underneath the cerebrum. In most other hominids, the cerebellum flares laterally and
protrudes posteriorly. Holloway did not discuss A. afarensis, but our observations suggest that it
too flares laterally with posterior protrusion. This character is equivalent to S&M 66.
18. Frequency of occipito-marginal sinus
Although Kimbel (1984) argued that this character is sufficiently variable in humans to
preclude its use in phylogenetic studies, it nonetheless appears to discriminate between
hominid taxa. An O–M sinus is present either uni- or bilaterally in high frequency (state 2) in
A. afarensis, A. boisei, and A. robustus (Falk, 1986, 1988, 1990). It is absent or found in low
frequency (state 0) in African apes (Kimbel, 1984), H. habilis (Tobias, 1991), H. rudolfensis
(Kimbel, 1984; Tobias, 1991). It is absent in the juvenile A. aethiopicus L. 338 y-6 (Rak &
Howell, 1978; Walker & Leakey, 1988), and while Walker et al. (1986) inferred that it is present
in KNM-WT 17000 because of a reduced transverse sinus, Holloway (1988) has disputed this.
An O–M sinus is present in 25% of A. africanus specimens. It is absent in MLD 37/38 (Conroy
et al., 1990), MLD 1, Sts 5, Sts 26, TM 1511, and Sts 71 (Kimbel, 1984), and present in Taung
(Tobias & Falk, 1988) and one Stw specimen (Kimbel, pers. comm.). Between 8 and 46% of
modern humans exhibit and O–M sinus (Kimbel, 1984). Accordingly, A. africanus and Homo
sapiens are coded as having an intermediate frequency (state 1).
19. Anteromedial incursion of the superior temporal lines
This character, as defined by Kimbel (1988; see also Clarke, 1977; Rak, 1983), describes the
course of the superior temporal line between frontomalaretemporale and the point of
maximum inflection on the line as it turns from being medially directed to posteriorly directed.
Incursion is best seen in superior view. Incursion is strong in the ‘‘robust’’ australopithecines,
where the temporal line penetrates far medially and is parallel to the orbital margin. Incursion
is weak in H. ergaster, H. rudolfensis and H. sapiens, where the line barely penetrates medially and
is strongly oblique to the orbital margin. H. habilis is variable, because incursion is weak in OH
24 and KNM-ER 1813, but moderate in SK 847 and Stw 53. Data for A. afarensis comes from
KNM-ER 2602 and the Belohdeli frontal (Kimbel, 1988; Asfaw, 1987).
Incursion is related to, but not entirely redundant with the presence/absence of a sagittal
crest (character 20). For example, both G. gorilla and ‘‘robust’’ australopithecines have a sagittal
crest, but these taxa differ in their degree of incursion. However, S&M 69 (‘‘supraorbital tori
in form of costa supraorbitalis’’) and S&M 70 (‘‘temporal lines converge anterior to bregma’’)
are redundant with this character. With respect to S&M 69, Clarke (1977) describes the
supraorbital tori of Pongo, A. boisei and A. robustus as ‘‘rib-like’’ because they are flattened
superiorly and have a sharp posterior margin. He notes, however, that the development of the
rib is related to postorbital constriction (character 26) and the incursion of the temporal lines,
both of which are used here. S&M 70 (‘‘temporal lines converge anterior to bregma’’) is also
redundant with incursion and the presence/absence of a sagittal crest.
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20. Sagittal crest present, at least in presumptive males
A sagittal crest is present in KNM-ER 1805 and Stw 505 (Clarke, pers. comm.). Consequently,
a sagittal crest is present in all hominid species except H. rudolfensis, H. ergaster and H. sapiens.
S&M 71 recognized that a crest is present in KNM-ER 1805, but coded Homo as lacking this
feature.
21. Compound temporonuchal crest, at least in presumptive males
Following Kimbel (1988), three states are recognized: (0) an extensive crest extending almost
the entire distance between inion and the lateral margin of the supramastoid crest, (1) a partial
crest confined to the lateral third of the bi-asterioric breadth, and (2) a crest is absent.
A. robustus is coded as missing data for this character. Although Robinson (1958) reported that
crests were absent in SK 48, SK 49 and SK 83, the nuchal region is missing in SK 48 and SK
83, and SK 49 is not a definitive male. Even though crests are absent in SK 47, this specimen
is a juvenile, and may not be ontogenetically old enough to exhibit a crest. The H. habilis state
assignment is based on KNM-ER 1805.
S&M 72 (‘‘size of posterior relative to anterior part of temporalis muscle’’) is rejected
because it represents an interpretation of morphology rather than an observation, and because
that interpretation is based on patterns of cranial cresting, which have already been accepted
as characters in this analysis. Thus, S&M 72 is redundant with this character and character 20
(presence of sagittal crest). For instance, a sagittal crest combined with a partial temporal
nuchal crest would indicate a large anterior temporalis.
22. Asterionic notch
Kimbel & Rak (1985) note that an asterionic notch is present in apes, A. afarensis and KNM-ER
1805. H. habilis is correspondingly coded as being variable. Skelton & McHenry (1992) noted
the condition of KNM-ER 1805, but considered a notch to be absent in Homo. Walker et al.
(1986) and Kimbel et al. (1988) state that a notch is probably present in KNM-WT 17000.
Although a notch is absent in L. 338 y-6, a putative juvenile A. aethiopicus, Kimbel & Rak (1985)
note that the character is absent in juvenile apes. All other taxa lack an asterionic notch.
23. Parietal overlap of occipital at asterion, at least in males
Kimbel & Rak (1985) describe a morphology present in male A. boisei in which the external
tables of the temporal, parietal and occipital bones overlap in a complex ‘‘sandwich’’ pattern
at asterion. This pattern is also present in KNM-WT 17000 (Kimbel et al., 1988), and
incipiently in L. 338 y-6. S&M 73 describes this character as ‘‘temporoparietal overlap of
occipital’’. The name is shortened here to ‘‘parietal overlap of occipital’’ because the
relationship between the temporal and parietal bones is subsumed within character 24
(squamosal suture overlap extensive). Thus, this character refers only to the flange of parietal
bone that extends posteroinferiorly over the occipital.
24. Squamosal suture overlap extensive, at least in males
This character refers in particular to the superior and posterosuperior aspects of the squamosal
suture. Rak (1978; see also 1983; Kimbel & Rak, 1985) noted that the squamosal suture of
A. boisei is extensive, tapered with a bevel (sutura limbosa), and possesses numerous interdigitations. L.338y-6 also has an extensive squamosal suture (Rak & Howell, 1978). KNM-WT
17000 has an extensive suture, though there is disagreement concerning its morphology. Rak
& Kimbel (1991, 1993) consider the specimen to have a unique posterolateral extension of the
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suture, which is disputed by Walker et al. (1993). We follow the latter in the interpretation of
KNM-WT 17000. Squamosal suture morphology is poorly preserved in adult A. robustus, so
that species cannot be characterized. All other species lack an extensive suture.
25. Lateral inflation of mastoid process relative to supramastoid crest
Olson (1981) stated that this character discriminates between Paranthropus (including A. afarensis)
and Homo (including A. africanus). Two morphologies are recognized here: the mastoid is not
inflated (i.e., it extends up to but not beyond the supramastoid crest; state 0), and the mastoid
is inflated lateral to the supramastoid crest (state 2). H. habilis is variable (state 1), since the
mastoids in KNM-ER 1805 are lateral to the crests, and those in Stw 53 and KNM-ER 1813
are level with the crests. Skelton & McHenry (1992) stated that mastoids in Homo are
uninflated (S&M 77). Furthermore, A. africanus is here considered uninflated whereas they
referred to this species as being variable. The mastoid of Sts 5 is medial to its crest, while those
of Sts 71 and MLD 37/38 are level with their crests. Contra Olson (1981), A. afarensis is
considered to lack an inflated mastoid.
26. Postorbital constriction
This character is measured as an index of minimum frontal breadth to superior facial breadth
(bi-frontomalaretemporale). Although there is slight overlap between the ranges of living taxa,
three states are recognized: (0) marked constriction (index values below 65%), (1) moderate
constriction (between 65 and 77%), and (2) slight constriction (>77%). Data from Asfaw (1987)
and Chamberlain (1987) were supplemented with our own measurements.
This character is partially related to character 19 (‘‘anteromedial incursion of the superior
temporal lines’’). However, the size and configuration of the temporalis muscle is independent
of constriction, and Gorilla has marked constriction but only moderate incursion. Thus the
characters are not considered to be redundant.
27. Pneumatization of temporal squama
Two states are recognized based on Sherwood (1994). Extensive pneumatization is observed
when pneumatic tracts extend to the squamosal suture, thereby thickening the squamous
temporal. Sherwood describes this as a squamous antrum. Pneumatization is not extensive
when the antrum is absent. A squamous antrum is present in apes, A. afarensis, A. aethiopicus and
A. africanus. Sherwood (1994) states that an antrum is absent in A. boisei, but Walker & Leakey
(1988) observed extensive pneumatization in KNM-WT 17400. Consequently, A. boisei is
considered variable. S&M 76 considered A. africanus to be weakly pneumatized.
28. Facial hafting
This character, as described by Tobias (1967) and Howell (1978), expresses the position of the
face relative to the neurocranium. The face is hafted high (state 1) in A. robustus, A. boisei and
A. aethiopicus. The face is low (state 0) in all other species. Facial hafting and the incursion of
the temporal lines (character 19) are related to the presence or absence of a frontal trigon (Rak,
1983), and therefore the latter is not considered an independent character.
29. Supraglenoid gutter width
Data from Wood (1991) were supplemented by our own measurements. Wood (1991) refers to
this character as temporal gutter width. A wide gutter is one that is broader than 25 mm, while
a narrow gutter is less than 25 mm. KNM-ER 1470 is inferred to have had a narrow gutter.
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30. External cranial base flexion
Flexion is characterized as the angle between Frankfurt Horizontal and the basion–hormion
chord (i.e., the inclination of the basioccipital and basisphenoid measured externally). Flexion
is traditionally measured internally, but an external measurement allows the inclusion of many
more specimens. The states in S&M 55 are accepted here except for A. afarensis, because the
relevant morphology is preserved only in an infant (AL 333-105). Furthermore, contra S&M 55,
the morphology of P. robustus is probably better expressed in SK 47 than in SK 48. Although
SK 47 is a juvenile, it is much older ontogenetically than AL 333-105. KNM-WT 15000 has
a flexed base to judge from the descriptions and figures in Walker & Leakey (1993). The bases
of other H. ergaser specimens (KNM-ER 3733, KNM-ER 3883) appear flat, but may have been
partially distorted.
31. Horizontal distance between TMJ and M2/M3
This character replaces S&M 56 (distance between M3 and the TMJ). By measuring to the
distal margin of M2, it is possible to include KNM-ER 1470. Furthermore, this measurement
approximates the load arm of molar bite force during mastication (Grine et al., 1993). Two
states are recognized: (0) long (>258 mm), and (1) short (<258 mm). There is slight overlap
between taxa across the 58 mm boundary. Regardless, the principal point is that the distance
is long in A. boisei and A. robustus, contrary to S&M 56. As reconstructed by Grine et al. (1996),
KNM-ER 1470 is on the boundary between long and short, but is considered long because
other reconstructions (e.g., Bromage, 1993) would suggest a considerably greater distance.
This character was also measured as a ratio of the distance between the TMJ and M2/M3
relative to bi-foramen ovale breadth, and comparable results were obtained.
32. Relative depth of mandibular fossa
This character was measured as an index of depth perpendicular to Frankfurt Horizontal
(depth from the base of the articular eminence to the apex of the fossa, divided by the breadth
of the eminence from the articular tubercle to the entoglenoid process). Three states are
recognized: shallow (<15%), deep (>25%), and intermediate (15–25%). The state assignments
of S&M 57 represent the conventional wisdom concerning this character. In contrast,
A. robustus, H. habilis and H. rudolfensis are here assigned to the intermediate category. H. ergaster
is variable, because the fossa of KNM-ER 3883 is of intermediate depth while those of
KNM-ER 3773 and KNM-WT 15000 are shallow. These taxa are usually all considered to
have deep fossae, but index values do not support such state assignments. Furthermore, of all
the species examined, only H. sapiens and A. boisei possess an articular eminence whose
posterior face is so steeply inclined as to be nearly vertical. This undoubtedly accounts for the
fact that they have larger index values than any other species. Wood (1993) states that H. habilis
has a deeper fossa than H. rudolfensis, but we characterize both as intermediate. The
discrepancy between our state assignments and those of others may rest in the fact that our
measurement is one of relative depth. The fossae of A. robustus and early Homo may be as
absolutely deep as in H. sapiens, but only those of A. boisei are relatively as deep. The fossae of
A. boisei are absolutely the deepest and widest of any hominid.
33. Size and position of postglenoid process
Size and position are sometimes considered separate characters (e.g., Kimbel & Rak, 1993;
Tobias, 1991), but size probably influences the spatial relationship between the process and the
tympanic plate. This character differs from S&M 58 in that they do not consider size. Three
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morphologies are recognized: a large process that is well anterior and not fused to the
tympanic (state 0), a medium-size process that is variably fused or unfused (state1), and a small
process that is fused to the tympanic (state 3). Presumably, it is not possible for a small process
to be unfused and anteriorly placed, or for a large process to be fused, as these states have not
been observed. The position of a process of intermediate size is not constrained. For example,
the processes of MLD 37/38 and Sts 5 are intermediate in size and anteriorly placed, while
that of Sts 71 is the same size, but fused to the tympanic. H. habilis is variable (state 2) because
Sts 19 has a process of intermediate size (it is also fused) while all other specimens of that
species have small processes.
34. Configuration of tympanic
This character combines two features, the shape of the tympanic canal and the orientation of
the anterior tympanic plate (S&M 59, 60). The two characters are redundant because a
tubular tympanic will always have a horizontal plate, and a vertical plate cannot exist without
a crest (the crista petrosa of Weidenreich, 1943). Although it is possible to have an inclined
plate and a crest (e.g., A. boisei), such a plate is presumably not homologous to the horizontal
plate of a tubular tympanic. Three states are recognized: (0) tubular tympanic, (1) crista
petrosa with a vertically inclined plate, and (2) crista petrosa with a posteroinferiorly inclined
plate where the crest is nearly apposed to the mastoid process. We differ from Wood
et al. (1994), who state that all hominids except A. afarensis have a vertically inclined plate. This
character is treated as unordered because there seems to be an equal probability of deriving
each state from any other.
35. Mediolateral position of the external auditory meatus
This character describes the M–L position of the inferolateral tip of the tympanic relative to
porion. There are two morphologies: the edge of the tympanic is medial to porion, which
results in a suprameatal roof (state 0), and the edge of the tympanic approximates porion
without a suprameatal roof (state 2). The meatus is laterally positioned in A. boisei, A. robustus
and Gorilla (Grine & Strait, 1993; Clarke, 1977). H. habilis is variable (state 1) because the
meatus is lateral in KNM-ER 1805, medial in Sts 19 and (presumably) SK 847, and nearly
lateral in Stw 53. H. rudolfensis is inferred to have had a medial meatus based on the presence
of small fragments of the tympanic of KNM-ER 1470. KNM-BC 1, a putative early Homo
specimen, has a laterally placed meatus (Grine & Strait, 1993).
36. Vaginal process of tympanic
Two morphologies are recognized: absent to small (state 0), and moderate to large (state 2).
Although the process is said to be absent in apes (Kimbel & Rak, 1993; Clarke, 1977), small
processes have been observed in some P. troglodytes and G. gorilla specimens (Strait, pers. obs.).
H. habilis is variable (state 1) because the process is moderate in Sts 19, and small in Stw 53,
KNM-ER 1813, KNM-ER 1805, and SK 847. KNM-BC 1 has a large process.
37. Eustacian process of tympanic
Broom & Robinson (1950) and Clarke (1977) noted that a strong process is present in
A. africanus. Aiello & Dean (1990) add that is is also present in A. robustus and apes. Kimbel &
Rak (1993) state that it is absent in apes. Our observations are consistent with those of Aiello
& Dean (1990). Two states are recognized: (0) prominent, and (1) absent or only slightly
developed.
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38. Petrous orientation
Dean & Wood (1981, 1982) and Dean (1986, 1988a) note that A. boisei, A. robustus and Homo
share coronally oriented petrous bones, while the petrous orientation of all other taxa is more
sagittal. Walker et al. (1986) state that KNM-WT 17000 has coronally oriented petrous bones,
but S&M 61 disagreed, stating that the condition in KNM-WT 17000 is sagittal. Measurements recorded by us on original specimens and casts resulted in the recognition of three
states: (0) coronally oriented petrous bones (i.e., angled at less than 50) to the bitympanic line),
(2) sagittally orientated bones (greater than 60)), and (1) intermediate (between 60) and 50)).
With respect to KNM-WT 17000, we concur with Walker et al. (1986). A. africanus is
considered intermediate. Although Dean & Wood (1982) stated that petrous orientation in
A. africanus is sagittal, they qualified this by confirming that it is less sagittal than apes. Kimbel
(pers. comm.) states that A. afarensis has an intermediate petrous orientation.
39. Heart shaped foramen magnum
Tobias (1967) noted that A. boisei has a heart shaped foramen that is flattened and widest
anteriorly. This condition is best seen in KNM-ER 406, which possesses a tubercle on its
anterior border that accentuates the heart shape. Walker et al. (1986) observed a flattened
anterior border in KNM-WT 17000. This character is equivalent to S&M 62. However,
H. ergaster is considered variable because KNM-WT 15000 has a heart shaped foramen with
a distinct anterior tubercle, while KNM-ER 3733 and KNM-ER 3883 have an oval foramen.
40. Inclination of the nuchal plane
Kimbel et al. (1984) measured this character as an angle between the inion–opisthion chord
and Frankfurt Horizontal. Their measurements were supplemented with our own. Three
states are recognized, namely steeply inclined (angle >60)), intermediate (60–45)), and weakly
inclined (angle <45)). The state assignment of A. robustus (weakly inclined) is based on an
estimated value for SK 47. This character was also estimated in KNM-ER 1470, since the
orientation of Frankfurt Horizontal depends on how the neurocranium joins the face. The
preserved portion of the nuchal plane indicates weak inclination. Finally, although S&M 64
characterize KNM-ER 1805 as having a steep plane, our measurement indicates that the
plane is weakly inclined. This specimen exhibits a sharp angulation in the nuchal plane such
that its superior portion is steep and inferior portion is weakly inclined. In this regard, it is
similar to KNM-ER 406. With respect to these two specimens, the measurement employed
here represents an average of these two inclinations.
41. Position of the foramen magnum relative to the bitympanic line
Dean & Wood (1981, 1982; see also Wood, 1991; Wood et al., 1994) define this character as
the position of basion relative to a line connecting the most inferiolateral points on the
tympanics. Three states are recognized: (0) basion is well posterior to the line, (1) basion
approximates the line, (2) basion is well anterior to the line. H. habilis is variable (basion is
anterior in KNM-ER 1805, while it approximates the line in Sts 19, KNM-ER 1813, and Stw
53). Unlike Dean & Wood (1982), we refrain from assigning states to SK 847 and KNM-ER
1470 because basion is missing.
42. Inclination of the foramen magnum
This character is measured as the angle between the basion–opisthion chord and Frankfurt
Horizontal. Measurements by us are comparable to those recorded by Wood (1991), but
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differ somewhat from those of Kimbel et al. (1984). All three data sets, however, group
species into similar clusters. The foramen magnum is strongly inclined posteriorly (state 0) if it
faces backwards at an angle of greater than 17·5). It is strongly inclined anteriorly (state 2) if
it faces forward at an angle of greater than 13). An intermediate inclination (state 1)
approximates the horizontal plane. Basion is missing in A. afarensis specimens that can be
reliably oriented in Frankfurt Horizontal, so a state was not assigned to that species. Similarly,
states could not be assigned to H. rudolfensis and A. aethiopicus. H. ergaster is coded as having
state 2 based on the reconstruction of KNM-WT 15000 presented in Walker & Leakey (1993;
A. Walker, pers. comm.); the cranial bases of KNM-ER 3733 and KNM-ER 3883 are
distorted.
43. Origin of the digastric muscle
When the cranium is viewed in norma occipitalis, the digastric origin may be seen to take
the form of either a broad, shallow fossa (state 0) or a deep, restricted furrow (state 1). Kimbel
et al. (1985) demonstrated that Olson’s (1981, 1985) characterization of the AL 333-45 cranium
was mistaken, and that this specimen has an ape-like, shallow fossa. The AL 444-2 cranium
also exhibits a broad, shallow fossa (Kimbel, pers. comm.). MLD 37/38 exhibits a fossa that
appears to be deep and narrow (Olson, 1978, 1981). However, Kimbel et al. (1985: pp.
128–129) assert that the morphology of this specimen is not homologous to that of modern
humans; rather it is comparable to that of African apes. Thus A. africanus is coded as having
state 0. Olson (1978) observed that SK 47 has a deep, narrow fossa, so this state is present in
A. robustus. Although Olson (1978) posited the presence of a broad, shallow fossa in TM 1517,
the circum–mastoid region of this specimen is very poorly preserved, and its condition cannot
be ascertained. A. boisei has a broad, shallow fossa, as represented by OH 5, KNM-ER 406,
KNM-ER 407 and KNM-ER 23000 (Brown et al., 1993).
44. Mandibular corpus area
This was calculated as the geometric mean of the area of the corpus at M1. Area was
calculated as an ellipse {area=[µ(height/2)(breadth/2)]}. The square root of this value was
used so as to counter-act the exponential growth of mandibular corpus area in relation to the
linear dimensions of the mandible. Our measurements supplemented those of Wood (1991).
Although there is slight overlap between taxa, two states are discernible: (0) small corpus area
(area <25·5), and (1) large corpus area (area >25·5). H. rudolfensis specimens are evenly
distributed across the range of this species, but that range straddles the boundary between
large and small area. Thus, this species is coded as being variable (small: KNM-ER 1482,
KNM-ER 1801, UR 501; large: KNM-ER 819, KNM-ER 1483, KNM-ER 1802). In light of
the demonstrable similarities between the UR 501 and KNM-ER 1802 specimens (Bromage
et al., 1995), it is worth noting that they are similar in size, falling just on either side of the
boundary. Gorilla is variable as well.
45. Orientation of the mandibular symphysis
S&M 49 characterize the orientation of the mandibular symphysis as ranging through six
states from ‘‘very receding’’ to ‘‘vertical’’. Our assignments are more conservative in that only
three states are recognized, (0) receding, (1) intermediate and (2) vertical. Contra S&M 49, the
symphysis in A. aethiopicus is considered to be vertical.
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46. Direction of the mental foramen opening
S&M 51 characterize the direction of the mental foramen opening as being either anterior or
lateral. In addition, the foramen in H. sapiens generally faces posteriorly, although a lateral
foramen may be present in 10% of individuals (DeVilliers, 1968). This character is considered
unordered. The direction of the foramen opening is variable in A. afarensis (lateral in AL
333w-60, LH 4, AL 400-1a; anterior in AL 288-1, AL 333w-12), A. africanus (lateral in MLD
40, Sts 52b; anterior in Sts 36, MLD 2) and G. gorilla.
47. Hollowing of the lateral mandibular corpus
Early hominid taxa differ with respect to the contour of the lateral face of the mandibular
corpus above and behind the mental foramen (White et al., 1981). Hollowing can be expanded
inferiorly to encompass the mental foramen, it can be reduced so that it is restricted to the
superior aspect of the corpus, or it may be absent. Three states are recognized: present
inferiorly (state 0), variably reduced or absent (state1), and absent (state 2). H. rudolfensis exhibits
state 1 because superior hollowing is present in UR 501 and KNM-ER 1802 but absent in
KNM-ER 1483. A. africanus is also variable, with superior hollowing present in Sts 52b, but
absent from Sts 36 and MLD 40. S&M 52 recognized a distinction between A. afarensis and the
great apes.
48. Width of the mandibular extramolar sulcus
S&M 53 recognized two states, viz. narrow and wide. Measurements of the width of the
extramolar sulcus suggest that a boundary between narrow and wide may be discerned at
about 6·5 mm. This boundary is approximate because the measurement is based on landmarks
that are difficult to define precisely. A. africanus is variable (MLD 40 is wide; Sts 36, Sts 52b and
MLD 22 are narrow), as is H. habilis (SK 15 is wide; SK 45, OH 7 and OH 13 are narrow).
49. Mandibular deciduous canine shape
Shape refers to the mesiodistal disposition of the apex, the height of mesial crown convexity,
and the height of the mesial end of the lingual cingulum (Grine, 1985). Two states are
recognized: (0) a centrally disposed apex with the convexity near the cervix (i.e., a low
convexity), and (1) a mesially positioned apex with the convexity closer to the tip (a high
convexity). Data are missing for A. aethiopicus, H. habilis and H. rudolfensis.
This character replaces S&M 18 (deciduous canine shape). They described this feature as
being either tall and narrow, or low, wide and blunt. Because these definitions reflect size as
well as shape, the character is redundant with our character 51 (canines reduced).
50. Maxillary incisor size
This character represents the summed MD means of the I1 and I2. Data for Koobi Fora
specimens are from Wood (1991), for Olduvai Gorge specimens are from Tobias (1991), for
H. sapiens are from Jacobson (1982), for A. afarensis are from White et al. (1981), and for
southern African fossils are from Grine (pers. obs.). Three states are recognized. State 0
(largest) includes Pan (20·9) and Gorilla (22·4). State 1 includes H. ergaster (19·7), A. afarensis
(18·3), A. africanus (18·1) and H. habilis (18·0). State 2 (smallest) includes A. boisei (16·0), H. sapiens
(16·0) and A. robustus (15·6). With reference to H. rudolfensis, only a single I1 is known (KNM-ER
1590; MD=12·3). This is the largest early hominid central incisor known in the species
considered here. In addition, the chord length of the incisor alveolus (prosthion–distal margin
of I2) of KNM-ER 1470 (ca. 18·0) is greater than that of specimens of A. afarensis, A. africanus
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and H. habilis and is less than that of H. ergaster, all of which are assigned state 1. Therefore,
H. rudolfensis is reasonably assigned state 1. Similarly, although no maxillary incisor has been
attributed to A. aethiopicus, the alveolar chord of KNM-WT 17000 (ca. 20·0) falls within the
range of state 1 taxa, and is 33% longer than that of OH 5, which possesses the largest incisors
of any A. boisei specimen. Thus, A. aethiopicus is assigned state 1.
This trait replaces S&M 8, 19 and 20. S&M 19 (‘‘I1 incisal edge length’’) is descriptively
similar, but we prefer to expand the character to included all incisors as a whole, since this
allows the inclusion of more specimens, particularly through the measurement of the incisor
alveoli. Incisor size is related to S&M 20 (‘‘position of I2 roots relative to the lateral margins
of the piriform aperture’’), because the position of the root is dependent on the size of the I1
and I2 crowns, and upon pyriform aperture breadth, which itself is known to vary among early
hominid taxa (cf., A. afarensis and A. boisei). This feature is, itself, related to S&M 8 (‘‘upper
canine jugae independent of margin of nasal aperture’’), because if the I2 is positioned beneath
the margin of the pyriform aperture, then the canine jugum cannot be.
51. Canines reduced
This trait relates to both maxillary and mandibular canines. Reduction is calibrated relative to
Pan and Gorilla, which do not have reduced canines. Walker et al. (1986) reported that
KNM-WT 17000 has small canines, an assessment that was presumably based on the size of
the canine alveoli. Although canine crowns are missing in other A. aethiopicus specimens, canine
roots in some specimens (e.g., Omo 18-1967-18) likewise appear small.
Skelton & McHenry (1992) include seven characters that redundantly describe canine size.
S&M 7 (‘‘upper canine juga and prominence’’) reflects the size of the upper canine root and
S&M 21 (‘‘projection of upper canine’’) is redundant with size because large canines project
whereas small canines do not. Likewise, S&M 22 (‘‘upper canine labial crown profile’’) reflects
size because large canines are asymmetrical while small ones are symmetrical. Similarly, mesial
and distal contact facets (S&M 23), diastemata (S&M 27), a long and steep mesial occlusal edge
(S&M 24) and a long distal edge are associated only with large canines. These seven traits
were, therefore, subsumed under our canine size character (51).
52. Prominence of median lingual ridge of mandibular canine
Like S&M 25, this feature is characterized as being either prominent or weak. S&M 25 stated
that the ridge is prominent in SK 23 and KNM-ER 3230, and thus coded A. robustus and
A. boisei as being variable. We disagree, and consider all specimens of these species to have a
weak ridge. However, A. africanus is variable, because Sts 50 and Stw 21 have a strong ridge,
whereas that of Sts 51 is weak.
53. Premolar crown area
This character and the following (54, molar crown area) replace S&M 28 (‘‘postcanine tooth
area’’). Their single character has been divided because premolars and molars do not
necessarily change (either increase or decrease) in size at the same rate in all species, as
revealed not only by values for individual specimens that possesss both premolars and all three
molars, but also by species values determined from crown area means.
Premolar crown area was determined for specimens with associated P3 and P4 or P3 and P4.
Crown areas of either the upper or lower teeth were added to provide a single value for each
specimen, and the square root of the summed area value was used in order to obviate the effect
of exponential enlargement of area with a linear increase in MD and BL dimensions.
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This character is a composite of maxillary and mandibular crowns area distributions. While
maxillary and mandibular ranges may differ, there is consistancy in the trends that they
established. The character states are determined by the degree of overlap and disjunction
among the observed ranges, especially those for maxillary teeth.
Mensurational data are from Mahler (1973) for Pan and Gorilla, Jacobson (1982) for
H. sapiens, Tobias (1991) for Olduvai Gorge specimens of H. habilis, Johanson et al. (1982),
White (1977, 1980), Kimbel et al. (1994) and White et al. (1993) for the Hadar, Laetoli and
Maka specimens of A. afarensis, and Grine (pers. obs.) for all other specimens. With regard to
P. aethiopicus, only a single upper premolar of disputed allocation is known (Suwa, 1989; Grine
et al., 1991), but the mandibular premolars of KNM-WT 16005 and L. 860-2 are intermediate
in size between those of P. robustus and P. boisei.
54. Molar crown area
This value was calculated for specimens with associated M1–M3, and/or M1–M3, where the
crown areas of the three teeth could be summed to a single value. The square root of the
summed area value was employed in order to preclude the effect of exponential area
enlargement with the linear increase of MD and BL dimensions.
The character is a composite of maxillary and mandibular crown area distributions, which
are similar, but not identical. Although the ranges may differ, there is overall consistancy in
their trends. Character states are determined by the degree of overlap and disjunction among
the observed ranges, especially for maxillary teeth. Mensurational data are taken from the
same sources as for character 53.
Although there is no associated M1–M3 or M1–M3 for any H. rudolfensis specimen, the M1
and M2 of KNM-ER 1590, and the M1s and M2s of KNM-ER 1820 and UR 501 are
comparatively large (especially compared to H. habilis homologues). In addition, Wood (1992)
has noted that H. rudolfensis specimens do not exhibit M3 reduction, in comparison to H. habilis
specimens. Thus, we are reasonably confident in our assignment of a character state to
H. rudolfensis as being equivalent to A. africanus and P. robustus, and larger than H. habilis.
A. aethiopicus is accepted as having large molar teeth. Although there is no complete crown
associated with KNM-WT 17000, it has very large tooth roots (cf. OH 5), and while there is
no M3 that has been definitely attributed to A. aethiopicus, the roots and alveoli of Omo
57.4–1968–41 indicate the presence of three large molars. This, together with the KNM-WT
16005 crowns provide for a reasonably confident assignment of overall molar crown size.
55. Mesial crown profile of dm1
This character replaces S&M 29 (‘‘distal crown profile of dm1’’) which was taken from White
et al. (1981). Grine (1984) has demostrated that because of its intraspecific polymorphism, the
distal crown profile of the dm1 fails to distinguish adequately between taxa. The mesial crown
profile, on the other hand, is a feature that has been shown to differ between species, and to
exhibit a low degree of variability within OTUs (Robinson, 1954; Grine, 1984, 1985).
This character comprises several inter-related traits, which were atomized by Grine (1985).
These are: (a) the position of the protoconid relative to the metaconid, (b) the disposition of the
mesial marginal ridge, and (c) the structure of the fovea anterior. Three states are recognized:
(0) there is virtually no mesial marginal ridge present, the protoconid is situated well mesial,
and the anterior fovea exhibits a wide lingual opening, (1) the mesial marginal ridge is slight,
the protoconid is situated mesial of metaconid, the fovea anterior opens lingually via a fissure
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ET AL.
between the mesial marginal ridge and metaconid, (2) the mesial marginal ridge is welldeveloped and horizontally disposed, the protoconid is nearly level with metaconid, and the
anterior fovea is enclosed lingually.
Information for H. habilis and H. rudolfensis is lacking because no dm1 is definitively
attributable to either, because we accept that the KNM-ER 820 and KNM-ER 1507 likely
represent H. ergaster. Omo 222-2744 is a very early specimen of Homo (Grine, 1984), but its
specific attribution is unresolved. The datum for A. aethiopicus is based on L.704-2 from
Member D.
56. Distal marginal ridge of dm2
This character is taken from Grine (1985). Two states are recognized: (0) the distal marginal
ridge is high at its buccal end, being confluent with the metacone near its apex; and (1) the
ridge is buccally low and narrow, attaining confluence with the metacone near its base. Data
are not available for H. ergaster or A. aethiopicus because no dm2 is definitively attributable to
either species.
57. Separation of premolar and molar cusp apices
This character replaces S&M 31, which alludes simply to the separation of the cusps of the P4.
The condition exhibited by the P4 applies equally to the P3, the lower premolars, and the
upper and lower molars. This character reflects the postion of the cusp tips being nearer the
crown margin in occlusal view, or closer to the crown center (Clarke, 1977; Tobias, 1991;
Grine & Strait, 1994). Three states are recognized: (0) apices widely separated and
approximate the outer crown margins, (2) apices are close together, and (1) a condition
intermediate between the first two.
S&M 41 (‘‘degree of lower molar cusp swelling’’) is related to the disposition of the cusp tips,
because when apcies are close together, the cusps appear swollen.
58. P3 metaconid development
This feature is a slight alteration of S&M 32. Their characterization of this character, and
therefore the character state distributions that they recognized are altered. We refer to the
frequency of a well-developed P3 metaconid within each species sample. Three states are
recognized, being (0) metaconid absent, (1) metaconid infrequent, and (2) metaconid frequent.
In addition to S&M 32, Skelton & McHenry (1992) employed four other characters that
reflect P3 metaconid development. S&M 33 (‘‘P3 talonid height’’) is clearly related to the size
of the metaconid, and indeed, the codes that they provide for these ‘‘two’’ features (S&M 32
and 33) have exactly the same distributions. S&M 34 (‘‘robust’’ features of P3) is not a single
character, but a compilation of several components, which are related to the overall size of the
crown, the size of the metaconid, and talonid development. This morphology has been
accommodated in the present study by characters 53 and 58. S&M 35 (‘‘crown shape index of
P3’’) is related to metaconid development, because a large metaconid will result in lingual
expansion of the crown and, therefore, an increased BL diameter. Also, because the P3s in Pan,
Gorilla, and A. afarensis are set at an angle to the tooth row, their MD and BL diameters are not
strictly comparable with those recorded for the premolars of other species (White et al., 1981).
Finally, S&M 35 is little more than a metrical statement of S&M 36 (‘‘P3 occlusal outline
shape’’).
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59. Enamel thickness
Metrical data derive from Beynon & Wood (1987), and Grine (unpubl.) for A. boisei, Beynon &
Wood (1986, 1987) and Bromage et al. (1994) for H. rudolfensis, Beynon & Wood (1986) and
Grine (unpubl.) for H. habilis, Grine & Martin (1988) for A. africanus, Grine & Martin (1988) for
A. aethiopicus, Grine & Martin (1988) for A. robustus, and Grine & Martin (1988) and Grine
(unpubl.) for Pan, Gorilla and H. sapiens.
Three categories of relative enamel thickness are recognized. These are (0) thin, (1) thick,
and (2) hyperthick. There is no metrical data available for A. afarensis. Although White et al.
(1994) distinguish Ardipithecus ramidus from A. afarensis on tooth enamel thickness, Wood (1991)
makes reference to KNM-ER 5431 as having ‘‘absolutely and relatively thin’’ enamel on the
fractured P4. The attribution of KNM-ER 5431 is open to dispute, but it may be referrable to
A. afarensis. Thus, the state assignment of A. afarensis (thick) is based on the observation by
Leakey et al. (1995) that enamel in A. anamensis is of comparable thickness to that in A. afarensis,
and that it is relatively thicker than that of A. ramidus.
60. Dental development rate
Dean (1985, 1987, 1988b) and Smith (1986, 1994) have described differences in the
calcification and eruption patterns of the permanent teeth in extant hominoids and fossil
hominids. The results of their independent investigations are in broad agreement with regard
to general patterns of development in that A. afarensis and A. africanus are observed to display
a pattern consistent with that exhibited by Pan and Gorilla. This pattern (state 0) entails delayed
maturation of the permanent incisors, premolars and especially the canines relative to the
development of the M1. Humans display a condition (state 1) in which there is greater
concordance of developmental timing among these teeth. Although Smith (1986) expressed
some doubt as to the pattern that best described KNM-ER 820 and KNM-ER 1507
(attributed here to H. ergaster), Dean (1987, 1988b) and later Smith (1994) considered their
incisor and molar patterns to be comparable to those of modern humans. Dean (1988b; Dean
et al., 1993) and Smith (1986, 1994) are in agreement that A. robustus and A. boisei display a
pattern of dental development in which their incisors, canines and premolars are developmentally advanced relative to the M1 by human standards (state 2). According to Smith (1988,
1994), KNM-ER 1590, attributed here to H. rudolfensis, exhibits the primitive ape-like pattern
(state 0). Data are unavailable for A. aethiopicus and for H. habilis. The specific attribution of the
early Homo juvenile mandible Omo 222-2744 is uncertain, as is that (SKX 21204) from
Swartkrans Member 1 ‘‘Lower Bank’’ (Grine, 1993); in neither case has the specimen been
examined in relation to this question.
Appendix 2: Omitted characters
A number of characters employed by Skelton & McHenry (1992) characters were rejected
either because the validity of the character was questioned, or because it failed to discriminate
among taxa. The character number employed by them is referred to as ‘‘S&M #’’. These
characters are summarized below.
Multiple infraorbital formaina present (S&M 5)
Skelton & McHenry (1992) characterized A. afarensis as having multiple foramina whereas all
other hominid maxillae except Sts 52a do not. However, the A. afarensis juvenile AL 333-105
has a single foramen. Furthermore, although multiple grooves are present on the malar of
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ET AL.
AL 58-22, the exterior surface of this specimen is not preserved. Thus, we are unsure whether
multiple foramina were present or if these grooves represent the path taken by the anterior
superior alveolar nerves. Depending on the morphology of AL 58-22, A. afarensis either has a
single foramen or is variable. If the former is true, then the trait fails to discriminate between
hominid taxa and should be discarded. If the trait is variable in A. afarensis, then it supports a
Homo+Paranthropus clade. Because there is doubt as to the morphology of A. afarensis, this
character was rejected.
Anterior projection of the subnasal region relative to the nasal aperture (S&M 11)
This feature was quantified in two ways, as a ratio of the distance from subnasali to prosthion
(data from Wood, 1991) compared to the distance between prosthion and a line tanget to the
distal margins of the M3s, and as the angle formed by the subnasili–prosthion chord and the
occulsal plane. The occlusal plane was used instead of Frankfurt Horizontal in order to include
a larger number of specimens. Neither measurement sorted hominid species into discernible
groups.
P3 occlusal outline (S&M 30)
Contra Skelton & McHenry (1992), this feature is variable within A. boisei, A. africanus, H. habilis,
and H. sapiens. Consequently, it was judged to be a poor discriminator of hominid species.
P3 crown wear (S&M 37)
Tooth wear patterns are poor cladistic characters because they are unlikely to be heritable in
a strict genetic sense. Rather they reflect the mechanical properties of ingested food items.
Furthermore, this character is likely to be influenced by canine size (character 51), and the
relative proportions of the metaconid and protoconid (character 58).
Predominant P3 root conformation (S&M 38)
An ordered morphocline of root configuration was arranged by Skeleton & McHenry (1992).
According to this arrangement, root morphology proceeds from the most primitive condition,
as in the great apes, to the derived condition exhibited by modern humans, and then to the
derived condition shown by A. boisei and A. robustus. However, the states exhibited by modern
humans, on the one hand, and by the ‘‘robust’’ australopithecines, on the other, represent
extremes at opposite poles (Wood et al., 1988).
In addition, the level of variability of each species makes assignment of a single character
state impossible (Wood et al., 1988; Wood, 1993). Although Wood’s (1988) attempt to provide
an evolutionary characterization of this morphocline is laudible, the hypodigms of most species
considered here comprise two (or more) states. Thus, according to the four categories of
mandibular premolar root form identified by Wood et al. (1988), their data reveal P. troglodytes
P3s to exhibit category 1 morphology in 33% of individuals and category 3 morphology in
67% of cases. Similarly, of 15 A. afarensis specimens, 25% display category 1 morphology, while
the remaining 75% evince either category 2 or 3. Of four A. africanus individuals thus scored,
one displays morphology 1, another displays morphology 2, while the remaining two possess
root form 3. With regard to P. robustus, 70% of specimens evince category 2 and 30% have
category 3 roots. In A. boisei, about 33% show root form 3 while 67% possess category 4 roots.
Of the six specimens attributed to H. habilis for which such data have been recorded, 67% have
root form 1, 17% have form 2, and 17% have form 3. The situation for the five specimens of
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H. rudolfensis is similar, in that 20% have form 1, 60% have form 2, and 20% have category 3
roots. Of the two specimens of H. ergaster for which P3 root morphology has been scored, one
exhibits category 2 while the other has type 3 morphology.
‘‘Robust’’ features of the P4 (S&M 39)
A summary score, as provided by Suwa (1988: pp. 206–207) does not represent a single,
discrete or independent character. The atomization of this complex into the three components
recognized by Suwa (1988)—i.e., buccal wall slope, crown wall convexity, and transverse crest
prominence—does not yield satisfactory separation of individual species. Moreover, we are
unable to score the three non-metric variables identified by Suwa (1988, 1990) with any degree
of confidence or repeatibility. While these features appear to display certain trends, they
also exhibit considerable overlap. Finally, the states that have been recognized for these
features differ between publications, such that Suwa (1988) posited the existence of three states
for the first feature (i.e., buccal wall slope), but subsequently (Suwa, 1990) recognized four
states for it.
Degree of mesial appression of M1 and M2 hypoconulids (S&M 40)
Skelton & McHenry (1992) noted that this trait is difficult to score because interproximal
attrition introduces a degree of uncertainty. It is even more difficult to score this character
because its description does not indicate whether the position of the hypoconulid vis-à-vis the
hypoconid and entoconid is under consideration, or whether the size of the hypoconulid is
relevant. If the character refers to the former, then the states posited by Skelton & McHenry
(1992) do not seem plausible. Because of this uncertainty, the character in question was not
considered in the present analysis.
Wear disparity between buccal and lingual molar cusps (S&M 42)
The disparity between the amount of wear on the buccal and lingual cusps of a molar crown
is related not only to a suite of morphological features, such as the relative cuspal height, the
degree of cuspal apex separation, and the angle at which the cusp tips diverge from one
another, but also to physiological factors, such as the direction and magnitude of the power
stroke, that are variably related to the morphological variables. Moreover, it is obvious that
individual dietary proclivities will also impact upon the degree of relative cuspal reduction. In
light of this complex web of interactive masticatory variables, it is difficult to justify this as an
independent character.
Mandibular robusticity (S&M 48)
This feature is an index of mandibular corpus breadth to height. Skelton & McHenry (1992)
do not indicate at which point along the tooth row robusticity was measured, but M1 is the
most common site. Our measurements supplemented those of Wood (1991). Although this
character has been used to distinguish ‘‘robust’’ australopithecines from other hominids, the
ranges of taxa overlap extensively and discernable groups are not evident (A. afarensis: 49–75;
A. africanus: 55–79; A. robustus: 56–77; A. boisei: 57–80; H. habilis: 44–70; H. rudolfensis: 59–71;
H. ergaster: 60–64). Skelton & McHenry assigned states according to the taxon mean rank
order, and thus recognized differences between mean index values of 61·7 and 62·5. Given the
considerable variability of this feature, such fine distinctions are not warranted.
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ET AL.
Position of mental foramen relative to mid corpus (S&M 50)
We measured this as an index of mental foramen height (from the base of the mandibular
corpus) to corpus height at P4. Our measurements supplemented those of Wood (1991).
Although there is a tendency for the foramina of A. afarensis to be below mid corpus, and those
of A. boisei and A. robustus to be above mid corpus, most species are variable [as was recognized
by Skelton & McHenry (1992)]. A. boisei, A. africanus, H. habilis, H. rudolfensis and H. ergaster each
have specimens that are above and below mid corpus. This variablitiy is too strong to identify
distinct groups of taxa.
Height of mandibular ramus origin on corpus (S&M 54)
Skelton & McHenry (1992) described the height of the mandibular corpus origin as being
either high or low. We measured this feature as a percentage of corpus height. This
measurement failed to discriminate groups of taxa.
Inflection of mastoids beneath cranial base (S&M 63)
Skelton & McHenry (1992) described the mastoid inflection as being either strong or reduced.
Kimbel et al. (1985), in response to Olson (1981), noted the considerable variability of this trait
even within a single species, such as P. troglodytes. However, Kimbel et al. (1988) later used this
feature to characterize KNM-WT 17000 and all ‘‘post-A. afarensis’’ hominids. We find it
difficult to categorize species using this character. An index of bi-mastoid breadth (measured
at the mastoid tips) to bi-supramastoid breadth failed to subdivide hominid species into
discerable groups. Further, the mastoid processes, and especially their tips, are damaged in
many hominid specimens, making this character difficult to assess.
Branch of middle meningeal artery from which middle branch is derived (S&M 67)
Saban (1985) stated that apes have a simple middle meningeal pattern in which there are only
two branches (anterior and posterior). A. boisei and A. robustus were said to have a third (middle)
branch that is derived from the posterior branch., while Homo has a middle branch that is
derived from the anterior branch. However, middle meningeal branching patterns are
notoriously variable among conspecifics (Schepers, 1946; Tobias, 1967, 1991; Aiello & Dean,
1990), and even within individuals (e.g., OH 5, Tobias, 1967). Furthermore, the terminology
used to describe this feature is inconsistently applied, making it difficult to independently assess
the morphology.