6 The Earliest Putative
Hominids
Brigitte Senut
Abstract
Following the molecular studies, the hominid family has long been considered as
emerging during the Pliocene. But today we have good evidence of hominids in
the African Upper Miocene strata. However, reconstructing our earliest history is
a difficult task as the Miocene data remain poorly known and fragmentary.
Moreover, the tendency in anthropology to consider the modern chimpanzee as
a good model for the last common ancestor of African apes and hominids has
obscured our understanding of evolution because the apelike features are exclusively defined on the basis of a modern animal and not on those of Miocene
hominoids. On the basis of detailed studies of Miocene apes and modern
hominoids, it appears that bipedalism is probably the most reliable feature for
defining hominids. Of the new hominoid taxa discovered in the Upper Miocene,
only Orrorin tugenensis exhibits clear evidence of adaptation to bipedalism. At
this stage, bipedalism in Sahelanthropus tchadensis and Ardipithecus kadabba still
needs to be demonstrated. A common idea in hominoid evolution is that
hominids emerged in dry, savannah‐like environments; but the data gained
from the Upper Miocene levels in Baringo (Kenya) clearly demonstrate that the
environment of the earliest hominids was more forested and humid than
expected. Finally, first discoveries of modern‐looking apes have been made in
12.5Ma‐old strata at Ngorora (Kenya) and 6‐Ma old deposits at Kapsomin and
Cheboit, indicating that the dichotomy between African apes and humans could
be much older than generally thought.
6.1
Introduction
Identifying the earliest hominids remains a difficult task because the definition of
the family varies widely from author to author. For some authors, the term
hominid should be restricted to humans and their bipedal predecessors, whereas
for others it should include all extant and fossil apes and humans; at its most
extreme definition all African Great Apes should be included in the genus Homo
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(Czelusniak and Goodman 1998), or in a slightly less extreme view, only chimpanzees and bonobos (with the exclusion of Gorilla) should be grouped with
Ardipithecus, Australopithecus, and humans in this genus (Wildman et al. 2003).
In the latter scenarios, the search for the oldest hominid leads to a strange
situation where the quest becomes that of identifying the earliest ape rather
than the earliest humans. This is definitely not what the theme of the research
on the origins of humans is today. The only consensus today among scientists
(molecularists and anatomists) in ape evolution is that Pongo, Pan, and Gorilla do
not belong to the same taxonomic group, a view that was widely accepted in the
last century; the family Pongidae is now restricted to Pongo (Greenfield 1979).
Homo is closely related to African apes and may be closer to Pan than to Gorilla.
Whatever systematic scheme is considered, the focus is on understanding the
ancestors of humans after their split from African apes. This is why it is more
appropriate to restrict the term Hominidae to humans and their fossil forerunners. The widespread confusion in the use of the term ‘‘hominids’’ is due to the
fact that authors usually do not define their position.
To understand the earliest hominids, we need to apply a geohistorical
approach. The opposition between Asia and Africa in the history of our origins
cannot be dismissed out of hand. However, today it seems clear from available
field data that the development of the human lineage occurred in Africa.
Finally, we must consider the fact that, for the last 30 years, the history of
research into the dichotomy between apes and humans has been dominated by
the controversial results obtained by paleontologists on the one hand and molecularists on the other. The debate has focused on two major aspects: chronology
and the search for the closest relative. There was a heated debate in the 1970s
concerning the molecular clock and its implications for hominid evolution versus
paleontological evidence and geological time. But discrepancies in the time scales
between various neontological studies have never been thoroughly debated, as
was pointed out by Arnason et al. (2001). We have known for almost two
centuries that the African apes are our closest relatives, but the research published
in the last three decades has attempted to focus on the question in greater detail
and this has led to another major issue: is the common chimpanzee the closest
relative to man? Or is it the bonobo? Or is it the group of African apes as a whole?
At this point it has become widely accepted, almost without debate, that the
closest relative to man is the chimpanzee, frequently claimed to share 98% or
more of its genetic material with humans (or even 99.4% for some authors such
as Wildman et al. 2003). This acceptance has occurred despite the fact that the
problem is not yet definitively solved (Marks 2002). It is within this complex
framework that research on our oldest ancestors has taken place during the past
three decades. In addition, preconceived ideas about our earliest relatives make it
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6
even more difficult to have a dispassionate discussion. A statement such as ‘‘the
common ancestor of human and chimpanzees was probably chimpanzee‐like, a
knuckle‐walker with small thin‐enamelled teeth’’ (Pilbeam 1996) takes us back
200 years, being no different from the quest for the missing link. This widespread
preconception is probably one of the reasons why most anthropologists have
used modern chimpanzees as the basic comparative material when researching
hominid origins.
This brings us to another aspect of the problem related to the definition of
the earliest hominid. Most anthropologists consider that apelike (i.e., chimpanzee‐like) features are primitive, and humanlike ones are derived. However, chimpanzees are not primitive; they are in fact highly derived in their locomotory and
dietary adaptations and the use of their features as ancestral traits is a major error.
Humans are also derived in their locomotion, but in a different way from
chimpanzees. It is, therefore, not possible to define the polarity of these traits
on the sole basis of some modern relict species; the Miocene apes were highly
diverse and this diversity has to be considered when reconstructing phylogenies.
The neontological approach, which has been in favor in some scientific circles,
turns out to be a total failure when dealing with the definition of the earliest
hominids. This approach leads to a search for magic traits, such as flat face, small
canine, and thick‐enameled teeth, which are considered almost universally to be
hominid features: for when only modern chimpanzees and humans are compared, these features seem to be obvious and clear. However, it is necessary to
understand their meaning and emergence before using them as a reference. When
Miocene hominoids are included in the study, these same features are found
to occur in many of them, suggesting that a flat face, small canines, and thick‐
enameled cheek teeth are plesiomorphic and that the elongated face of chimps,
their large canines, and thin‐enameled cheek teeth are apomorphies of the chimpanzee clade rather than plesiomorphies of hominoids. Exclusion of the fossils in
the comparisons of modern apes and humans erases the diversity of the past which
is the raw material for understanding our evolution.
6.2
The rise and fall of the Ramapithecus–
Kenyapithecus group
During the 1960s and 1970s, it was widely claimed that Kenyapithecus (sometimes
considered to be an African relative of the Asian Ramapithecus) was a hominid,
aged ca. 15 Ma. This idea (Leakey 1961, 1962; Simons 1961; Simons and Pilbeam
1965; Andrews 1971) was questioned by some morphologists (Genet‐Varcin 1969;
Greenfield 1978). The divergence between apes and humans was thus considered
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The earliest putative hominids
by these authors to be very ancient (about 16 Ma or even close to 20 Ma). But in
the 1970s, the development of molecular biology and the application of the
molecular clock led to the notion that the dichotomy was considerably more
recent, and ages of divergences for the African great apes and hominids of about
2–4 Ma were proposed (Wilson and Sarich 1969). It was in this context, 30 years
ago, that ‘‘Lucy’’ and the Afar australopithecines were discovered in 3 million‐
plus‐year‐old deposits (Johanson and Taieb 1976). Considered at the time to
be the earliest hominid, these fossils were later formalized as Australopithecus
afarensis (Johanson et al. 1978). This major discovery was widely acclaimed: for
the first time, we could examine a quasi‐complete fossil skeleton of an early time;
the earliest stages of bipedalism (a key feature in the definition of hominids)
could be seen, and we could get information on body proportions of the earliest
members of our family Hominidae. Not least, Lucy was considered by many to
occur in the range of dates estimated by the famous molecular clock, which at the
time suggested that the dichotomy between apes and humans took place about 4
Ma (Wilson and Sarich 1969). Subsequently, it was demonstrated that the
molecular clock was not reliable as it did not run at a constant rate in mammals
and in particular in primates (Britten 1986; Pickford 1987; Stanyon 1989). In
numerous papers, a large variety of dates was advanced for the dichotomy
between apes and humans, depending on the calibration ages accepted in
the studies and the type of protein used for the demonstration: these dates ranged
from 2.5 up to 4 Ma. But the fossils seemed always to give an earlier date. The
question of an early or a late divergence was addressed by Greenfield (1980):
he had already proposed that Sivapithecus (¼Ramapithecus) brevirostris and
Sivapithecus (¼Kenyapithecus) africanus were size variants of Sivapithecus, and
he suggested a late divergence for the group of Homo, Pan, and Gorilla from the
pongid stock. However, the age of this dichotomy that he published (10–5 Myr)
was greater than the one proposed by geneticists. Following general acceptance of
the molecular evidence that supported a divergence between chimpanzees and
humans around 4 Ma, the matter of chronology was thus thought to be solved
and neither Ramapithecus nor Kenyapithecus was subsequently considered by
many authors to be hominid. By the end of the 1970s, the Kenyapithecus material
had been restudied, especially in the light of sexual dimorphism in modern and
fossil apes (Greenfield 1978, 1979; Pickford 1986; Pickford and Chiarelli 1986). It
transpired that the group of Ramapithecines–Kenyapithecines did not belong to
Hominidae and as a result the Middle Miocene estimates for the dichotomy
between apes and humans were abandoned.
Eventually a meeting organized at the Vatican in 1982 led to a consensus
between paleontologists and molecularists: 7 Ma was the date of the divergence
adopted at this meeting.
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However, despite the fact that several isolated fossils of putative early hominid ancestors were already known at that time, such as the Lothagam mandible
thought to be close to 7‐Myr old (Patterson et al. 1970), the Lukeino lower molar
(Pickford 1975), and the Kanapoi humerus (Patterson 1966; Patterson and
Howells 1967), these materials were too fragmentary to be taken seriously by
most paleoanthropologists and there was a tendency to avoid them in the
phylogenies. Then Coppens (1983) reconsidered all the hominoids, some of
which had been attributed to hominids. Taking into account environmental
changes, chronology and geography, he formalized his ‘‘East Side Story.’’
Until the end of the twentieth century, australopithecines were considered
to be direct ancestors of humans, even though some scholars pointed out
that more modern forms existed during the same period, which implied that
early Plio‐Pleistocene hominids were more diverse than expected and that
australopithecines might have been a side‐branch of human evolution (see
Senut 1992 for a review). But by the end of the twentieth century, it was
widely accepted that the earliest hominid ancestor was to be found in the Early
Pliocene.
6.3
Bipedalism and its impact on the origins
of Hominidae
Among living primates, facultative bipedalism is frequent; but even if primates
can sit with the back upright, stand on two feet, and walk bipedally for short
distances, humans are the only ones that can move on two legs for long distances
and extended periods of time. This difference is reflected in the skeletal characters
of extant humans, often defined in comparison with chimpanzees. While a suite
of features linked to femoral, pelvic, or sacral morphology appears to be soundly
based, others are questionable. This is the case with the position of the foramen
magnum. For the past 80 years, following Dart (1925), most scientists have
considered that an anterior position of the foramen magnum indicates bipedality
in hominids. Clark Le Gros (1950) used the anterior position of the occipital
condyles to confirm the hominid nature of the australopithecines and proposed
a ‘‘condylar position index,’’ but he noticed that in modern humans this position
varied between dolichocephalous and brachycephalous individuals. Later (in
1972), the same author warned: ‘‘It has been assumed that the condylar‐position
index, by itself, is always correlated with the degree of postural erectness. The
fallacy of this assumption is exposed by the fact that the index varies quite considerably even in modern H. sapiens.’’ However, generic, specific, and/or populational studies remained limited before 1960, despite the fact that Schultz (1955)
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The earliest putative hominids
highlighted the variability of this feature. Since then, it has been shown that an
anterior position of the foramen is not linked exclusively with bipedalism but
could be related to the development of the brain (Biegert 1963). Several authors
demonstrated that its position relative to the cranial foramina was variable
(Dean and Wood 1981, 1982; Schaeffer 1999; Granjean 2005). It is difficult to
discriminate individuals on these isolated features as there is a great overlap
between apes and humans.
Different forms of bipedalism have existed in the past. Of these, the most
debated concerns Oreopithecus bambolii, discovered in Late Miocene lignites of
Tuscany (Hürzeler 1958; Schultz 1960; Straus 1963; Sarmiento 1983; Tardieu
1983; Senut 1989) and which was recently demonstrated to be bipedal (Köhler
and Moyà‐Solà 1997; Rook et al. 1999). For the pedal features, these authors
showed that this Miocene ape could move bipedally when on the ground, but
with a stabilization morphology that differed from those of humans and australopithecines. Oreopithecus lived in an island environment where the absence of
large predators and limited trophic resources played an important role in the
evolution of mammals (Köhler and Moyà‐Solà 1997).
6.4
The case of Australopithecus afarensis (¼antiquus)
At one time every single fossil older than A. afarensis from Afar in Ethiopia was
considered to be the earliest human ancestor; and this ancestor was almost always
interpreted as being in the direct line leading to the genus Homo and then to us.
However, this approach ignored or underestimated the probable diversity of
Pliocene hominids. In fact, in the late 1970s, several authors had already pointed
out that there might be a taxonomic problem with the species Australopithecus
afarensis: was it, as claimed, a single bipedal species? Did this species include
two different taxa, one of which was a combination of a climber and a terrestrial
biped and the other a more advanced species which was primarily a ground
dwelling biped? (see a review of Australopithecus afarensis locomotor adaptations
in Stern 2000 and Coppens and Senut 1991). The difficulty derived mainly from
the fact that before the 1970s, scientists had built their phylogenetic trees almost
exclusively on the basis of dental anatomy, whereas the incorporation of locomotor traits led to a modification of these phylogenies. The picture became more
complex in subsequent years, with a crop of new species of australopithecines being created; several of these had specimens also included in other hypodigms. This was especially clear with Praeanthropus africanus, Australopithecus
afarensis, and Australopithecus anamensis. The use of cladistic methods did not
clear up the problem (Strait et al. 1997; Strait and Grine 1999, among others), and
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6
the phylogenetic approach became more and more confused, various scholars
using the same species in different ways without defining them.
As discoveries became more and more numerous, several genera were resurrected or created: Praeanthropus, Ardipithecus, Orrorin, Sahelanthropus, and
Kenyanthropus. New specimens of a Pliocene hominid were found at Sterkfontein
in South Africa (Clarke 1995). A major question remains today: is Australopithecus afarensis a direct ancestor or a side‐branch of our family? This question is still
unanswered.
6.5
The new challenge
The majority of scenarios concerning the dichotomy of apes and humans, with
the exception of the East Side Story of Coppens (1983), failed to take into account
the environment. Coppens’ hypothesis was eco‐geographic in nature, the African
Rift Valley constituting an ecological barrier between the apes in the west and
early hominids in the east from about 7 to 8 Ma. But the most important
elements of his hypothesis were chronological (the divergence took place between
8 and 7 Ma) and ecological (climatic change engendering modifications in
regional vegetation patterns, etc.), whereas, despite its name, the geographic
element was subsidiary in terms of the evolutionary scenario. As soon as we
began to look for early and/or putative hominids in strata older than the Pliocene,
we found them. In 2000, the discovery of early hominid remains (O. tugenensis)
in the Upper Miocene strata of Kenya, and subsequent finds in Middle to Upper
Miocene sediments of the same country, shed new light on the question of our
divergence from the African apes. The Orrorin discovery was subsequently followed by finds in Ethiopia (Ardipithecus kadabba) and then Chad (Sahelanthropus
tchadensis). The debate mainly focused on the C/P3 complex and on adaptations
to bipedalism, and the status of these two species is still a matter of debate. But
the main disagreement lies in the fact that in several studies comparisons were
made basically with modern apes and later hominids, and very little with Miocene
apes. As pointed out above, structures or features supposed to be hominid
apomorphies might well be retained from older Miocene apes and some of the
modern African ape features, usually considered to be primitive, might not be so.
6.5.1 Ardipithecus ramidus
In 1994, Australopithecus ramidus was published and in 1995 it was attributed to a
new genus Ardipithecus. This hominoid from Aramis localities 1–7 in the Middle
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Awash (Afar Depression in Ethiopia) (White et al. 1994, 1995) was announced as
the earliest known hominid. All the specimens, except the humerus (which was
found above the Daam‐Aatu Basaltic Tuff) come from a level located between the
Daam‐Aatu Basaltic Tuff and the Gaala Vitric Tuff complex. The tuff complex,
situated at the base of the sections, has been dated at 4.39 0.013 Myr and an age
between 4.2 and 4.5 Ma can be estimated for the fossil hominid (WoldeGabriel
et al. 1994). At the time of the discovery, these fossils were among the few
supposed hominids older than 4 Ma. Recently, a few more specimens have been
described from the Early Pliocene at As Duma in the Gona Western Margin
(Ethiopia), the ages of which have been estimated at 4.51–4.32 Ma (Semaw et al.
2005). According to its discoverers, the new genus was based on differences from
Australopithecus: the reduced megadontia of the postcanine teeth, the greater
width of the upper and lower incisors compared with postcanine teeth, a narrow
and obliquely elongated lower dm1 with a large protoconid and a small, distally
place metaconid without an anterior fovea, a small, low talonid with reduced
cuspule development, absolutely and relatively thinner canine and molar enamel,
lower P3 and upper P3 more strongly asymmetrical, with more dominant buccal
cusps. With a canine that is not mesiodistally elongated, it is distinguishable from
modern African apes. However, some of the cited features—including the thin
enamel in the molars, asymmetrical upper and lower third molars, and the size
relationships between the canines and jugal teeth—places Ardipithecus closer to
the chimpanzee than to any of the oldest hominids known. The first deciduous
molar shows resemblances to those of bonobos. But the morphology of the canine
distances Ardipithecus from apes: it is more incisiform than in the latter group.
Metric comparisons of the adult teeth were made with A. afarensis and underline
the diminutive size of Ardipithecus. The upper canine/lower anterior premolar
complex is typical of apes and was described as being ‘‘morphologically and
functionally only slightly removed from the presumed ancestral ape condition’’
(White et al. 1994 p 308). However, certain features taken as support for its
hominid status occur in female apes which have a reduced canine/premolar
complex compared to those of males. Its postcranial bones reveal several apelike
features, but the proximal humerus is more humanlike in the shallowness of the
bicipital groove. However, this character occurs not only in hominids but also in
other primates, such as Pongo, which is a cautious climber (Senut 1981). The
fragment of occipital preserved would suggest that the foramen magnum is
placed anteriorly relative to the carotid foramen, but for the reasons given
above we must be cautious with the interpretation of this feature. At the end of
1994, a skeleton was found in the Aramis strata, but it has not yet been published.
More recently, Semaw et al. (2005) briefly described a proximal third of a pedal
proximal phalanx from the deposits of As Duma. They write: ‘‘The transversely
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6
broad oval proximal facet is oriented dorsally, a character diagnostic of bipedality,
and a trait also seen in Ardipithecus kadabba.’’ But, Rose (1986) had already
described the same feature in Sivapithecus from the Miocene of Pakistan. Given
such weak evidence, it is difficult to accept bipedalism in Ardipithecus ramidus.
On the basis of the fauna, the botanical and sedimentological indications, the
environment of Ardipithecus ramidus at Aramis is a forested one (WoldeGabriel
et al. 1994). In the Gona sites, the faunal association, carbon isotopes, and
sedimentology suggest a moderate rainfall woodland and woodland/grassland
(Semaw et al. 2005).
6.5.2 Orrorin tugenensis
The discovery of Orrorin led to the elucidation of several aspects of early
hominids (Senut et al. 2001). The specimens come from four sites: Cheboit,
Kapsomin, Kapcheberek, and Aragai in the Lukeino Formation aged ca. 6 Ma
(6–5.8 Ma) (Bishop and Chapman 1970; Bishop and Pickford 1975; Chapman
and Brook 1978; Kingston et al. 1994; Pickford and Senut 2001) (> Figure 6.1).
The Lukeino Formation overlies the Kabarnet Trachyte dated by K/Ar, paleomagnetism, and biochronology at 6.1 Ma and is overlain by the Kaparaina Basalt, the
age of which is estimated to be 5.7 Ma (Sawada et al. 2002). In the section,
Cheboit and Aragai are the oldest sites, followed by Kapsomin and then Kapcheberek which lies in the upper level of the formation. Up to now, 20 specimens of
Orrorin have been found consisting of the posterior part of a mandible in two
pieces, a symphysis and several isolated teeth, as well as three femoral fragments, a
partial humerus, a first phalanx, and a distal thumb phalanx. The genus is defined
by its jugal teeth being smaller than those of australopithecines, an upper canine
short with a shallow and narrow vertical mesial groove and a low apical height, a
small triangular upper M3, a lower P4 with offset roots and oblique crown, small
Homo‐like rectangular lower M2 and M3, thick enamel on the lower cheek teeth, a
buccal notch well developed on the cheek teeth, no cingulum on the molars, a
femur with a spherical head rotated anteriorly, the femoral neck elongated and
oval in section, a medially salient lesser trochanter, a deep digital fossa, a humerus
with a vertical brachioradialis crest, a curved proximal manual phalanx, and a
dentition that is small relative to body size. Orrorin differs from Australopithecus
in the morphology of the cheek teeth, which are smaller and less elongated
mesiodistally. It differs from Ardipithecus by the greater thickness of enamel. It
differs from both by the presence of a mesial groove on the upper canine. The
upper and lower canines exhibit an apelike morphology, seen in female chimpanzees and Miocene apes; they are reduced in comparison with Pan. The apex of
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The earliest putative hominids
. Figure 6.1
Remains attributed to Orrorin tugenensis in 2000
the upper canine is pointed and almost sectorial, and a poorly developed lingual
wear facet is visible.
The femora reveal that Orrorin was bipedal (Senut et al. 2001; Pickford et al.
2002; Galik et al. 2004). However, the other postcranial bones suggest that it could
climb trees. The distal phalanx of the thumb exhibits features which are classically (but
probably erroneously) associated with the manufacture of tools; these traits could
be related to grasping abilities when climbing trees (Gommery and Senut, 2006).
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6
At the time of its discovery, Orrorin was the first known bipedal hominid older
than 5 Ma and indicated that the dichotomy between the African apes and the
hominids had to be older than 6 Ma and that the classic recent dates of divergences estimated by molecular biologists did not fit with the paleontological
evidence. On the other hand, the locomotor and dental features suggest that
Orrorin was different from Australopithecus afarensis. It was a microdont animal
with small postcanine teeth and a rather large body size, whereas Australopithecus
was a megadont hominid with large postcanine teeth and small body size.
Modern humans are microdont.
Orrorin did not live in an open environment but in a more forested one as
suggested by the faunal remains, such as impalas, colobines, water chevrotain,
arboreal civets, and floral remains, which contain large leaves with drip points
(Pickford and Senut 2001; Senut and Pickford 2004; Senut 2006) from which it is
concluded that in its early stages, bipedalism was not related to dry environments
(Senut 2006). The humid conditions persisted in the Lower Pliocene (Pickford
et al. 2004).
6.5.3 Ardipithecus kadabba
Material discovered in Ethiopia (Haile‐Selassie 2001) from sediments aged between 5.2 and 5.7 Ma was identified as belonging to a subspecies of Ardipithecus
ramidus, more recently raised to the specific rank Ardipithecus kadabba (Haile‐
Selassie et al. 2004). The material was collected at five different sites: Digiba Dora,
Asa Koma, Alayla, Saitune Dora, and Amba East, from the Asa Koma Member of
the Adu Asa Formation. The first four are in the Asa Koma Member of the Adu
Asa Formation, and the deposits which have yielded the hominids are securely
dated at 5.54–5.77 Myr by radiometric methods applied to underlying and
overlying basalts. The Amba East material is slightly younger, being from the
Kuseralee Member of the Sagantole Formation dated at 5.2–5.6 Ma (Renne et al.
1999). The morphology of the upper canine crown with a more rounded outline
differs from Orrorin; but it also differs from Ardipithecus ramidus in the crest
pattern of the same tooth, as well as in the morphology of the lower premolar
which is more asymmetric in outline, and by the presence of a small anterior
fovea. Moreover, the lingual cusps are more salient and sharp in the lower M3 and
the upper M3 bears four cusps.
The species Ardipithecus kadabba differs from extant apes by its canines, which
have a tendency to be incisiform as in A. ramidus, and by the presence of a clearly
defined fovea on the lower P3, which is isolated from the mesial marginal ridge by
a fold‐like buccal segment. The postcranial morphology (Haile‐Selassie 2001)
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indicates several similarities to African apes and selected specimens from the
Hadar, but the shape of the ulnar olecranon differs from that of hominids. A
proximal pedal phalanx resembles the ones from Hadar, and on the basis of the
dorsal orientation of the proximal facet of the bone, it supposedly belongs to a
bipedal animal. However, the curvature seen in the Ardipithecus kadabba phalanx
might be linked with arboreal adaptations as discussed by several authors (Stern
and Susman 1983; Susman et al. 1984), and we must remain careful when
assessing a locomotor complex on the basis of restricted material. Ardipithecus
kadabba is associated with relatively wet and wooded environments as indicated
by the fauna. As for the Amba East site, it seems slightly drier (WoldeGabriel et al.
2001).
6.5.4 Sahelanthropus tchadensis
The discovery of Sahelanthropus in Chad was published in 2002 (Brunet et al.
2002; Vignaud et al. 2002). Announced as the earliest known hominid, this status
has been the subject of debate (Wood 2002; Wolpoff et al. 2002). It was found
at Toros‐Menalla (Chad) in deposits dated between 6 and 7 Ma, maybe closer
to 7 Ma, by comparison with the Lukeino Formation and Nawata Formation
(Vignaud et al. 2002). The following diagnostic features of the species have been
published: orthognathic face and weak subnasal prognathism, small ape‐sized
braincase, long and narrow basicranium, small canines, robust supraorbital torus,
absence of supratoral sulcus, marked postorbital constriction, small, posteriorly
located sagittal crest and large nuchal crest, wide interorbital pillar, low‐crowned
jugal teeth and enamel thickness between that of Pan and Australopithecus, and
anterior position of the foramen magnum. It is considered different from all the
living great apes because of the relatively small canines, the apical wear of the
canines, and a probable non-honing C/P3 complex. The claimed hominid status
is based on the small, apically worn canine and on the structure of the face.
However, when these complexes are considered among all fossil and extant
hominoids, it appears that they are more frequent than believed. The maxillofacial complex in extant apes varies according to sex just as it does in Miocene
hominoids (Proconsul, Kenyapithecus, Ramapithecus). It was this combination of
features that originally led to Ramapithecus being proposed as a hominid, whereas
it is today thought to be the female of Sivapithecus. Bipedalism in Sahelanthropus
has been inferred from the position of the foramen magnum, but again, for the
reasons expressed above, this feature can be misleading. The cranial base and
nuchal area of Sahelanthropus (with its strongly developed nuchal crest and the
flatness of the occipital) seem more apelike to some authors (Wolpoff et al. 2002),
The earliest putative hominids
6
suggesting a quadrupedal posture and locomotion despite the reconstruction
proposed by Zollikofer et al. (2005), which fails to bring more evidence to the
debate. The orientation of the plan of the foramen magnum falls within the range
of variation of modern apes (Pickford 2005). Whatever Sahelanthropus is, its
status as a hominid is still being debated (Wood 2002).
Sahelanthropus was found in perilacustrine sandstones and the sedimentological context suggests a mosaic of environments between lake and desert, which
have been compared with the Okavango delta.
6.6
An earlier dichotomy?
During the past decade, several apelike fossils have been discovered in the Baringo
District (> Figure 6.2): a lower molar in the 12.5 Ma Ngorora Formation and
three fragmentary teeth from the Lukeino Formation found in the same strata as
O. tugenensis.
6.6.1 Ngorora
In 1999, a lower molar was collected at Kabarsero, Ngorora Formation, Tugen
Hills (Pickford and Senut 2005) 12.5 Ma (Bishop and Pickford 1975). This tooth,
probably a lower M2 (Pickford and Senut 2005), is close in morphology to
Dryopithecus (Begun 2002) and chimpanzees and distant from similarly aged
Kenyapithecus and Otavipithecus (Conroy et al. 1992; Ward and Duren 2002). An
upper molar had already been described from Ngorora (Bishop and Chapman
1970; Leakey 1970; Bishop and Pickford 1975), but it appears to be closer in
morphology to Kenyapithecus (Ishida and Pickford 1998; Pickford and Ishida
1998; Senut 1998), and the thickness of the enamel and the more centralized
. Figure 6.2
(a) Bar 910 99, right lower molar, stereo occlusal view; (b) Pan paniscus right m/2, occlusal
view; (c) Pan troglodytes, right m/2, occlusal view; (d–f) Dryopithecus brancoi lower molars
from Europe, d¼Trochtelfingen, e¼Salmendingen, f¼Ebingen; (g) Gorilla gorilla lower m/
2, occlusal view; (h–j) Bar 17570 02, Kapsomin large ape, occlusal, lingual, and oblique views;
(k) Gorilla gorilla, M2/, oblique view; (l) Australopithecus afarensis, upper M2/, occlusal view;
(m) Gorilla gorilla upper M2/, occlusal view; (n) Pan troglodytes, upper M1/‐M3/, occlusal
view; (o) O. tugenensis upper molar row, occlusal view; (p) Bar 17570 02, detail of dentine‐
enamel junction at hypoconulid; (q) KNM LU 335, Orrorin tugenensis, left m/3, occlusal view;
(r) Praeanthropus africanus cast of upper incisor from Laetoli, lingual view; (s) BAR 10010 01,
Kapsomin large ape, lingual view; (t) Gorilla gorilla upper central incisor, lingual view; (u)
Praeanthropus africanus cast of upper incisor from Laetoli, distal view; (v) BAR 10010 01,
Kapsomin large ape, distal view; (w) Gorilla gorilla upper central incisor, distal view
1531
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The earliest putative hominids
Figure 6.2 (continued)
The earliest putative hominids
6
cusps suggest that it belongs to a different hominoid from that of the lower molar.
Two different kinds of hominoids would have coexisted at Kabarsero, a possibility
which is also suggested in the Lower (Napak) and Middle (Moroto) Miocene sites
of Uganda (Gommery et al. 1998, 2002).
If the derived characters of the Ngorora tooth are homologous to those of
chimpanzees, then it would indicate that chimps were already a separate lineage
by the end of the Middle Miocene, a suggestion that accords with some interpretations of the molecular data (Arnason et al. 2001). The resemblances between
the Ngorora tooth and Dryopithecus indicate that the latter genus may have
originated in Africa and migrated toward Europe about 12.5 Ma.
6.6.2 Kapsomin
In 2002, half an upper molar of a large hominoid was found at Kapsomin,
Lukeino Formation, aged 5.9 Ma (Pickford and Senut 2005). This tooth is larger
than those of O. tugenensis and the crown morphology is different. The trigon is
wide, the distal fovea broad, the main cusps high and less inflated, and there is a
deep buccal slit. The dentine penetrance is also high. Most of these features occur
in Gorilla and are different from Pan.
In 2000, an upper central incisor was found in the same strata as Orrorin.
Originally assigned to O. tugenensis, its morphology did not seem to fit with the
early hominid. After a restudy of the specimen, it appears that it differs strongly
from australopithecines and other hominids because of the lack of fossa on the
lingual side of the tooth. Moreover, the crown is relatively low compared to root
length, whereas in hominids the crown is higher with a scoop‐shaped profile. In
contrast, in Gorilla incisors, the lingual fossa is missing and the crown is wedge
shaped.
In 2003, a lower molar was found at Cheboit, near the site of discovery of the
first hominid tooth from the Lukeino Formation (Pickford 1975). The morphology of the tooth is compatible with the half upper molar from Kapsomin and the
specimens probably belong to the same taxon. As for Ngorora and the Ugandan
sites, two different hominoids would have coexisted at Kapsomin in the same
strata, 6 Ma.
6.7
Conclusions
The debate about our earliest origins is probably not closed and is fueled by the
poverty of fossils in the time period between 12 and 4 Ma. This is why it is
1533
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The earliest putative hominids
necessary to continue excavation and prospecting in different areas of Africa in
order to fill the gaps and extend our knowledge of variation. One of the most
troubling aspects of the research done to date on the origins of hominids
is related to the comparative samples. Most scientists still focus on modern
hominoids as good reference for primitive morphologies. However, these animals
are highly derived in their cranial and postcranial anatomy. As long as Miocene
apes are not properly considered in these studies, we will remain trapped in the
quest for a mythical missing link.
Of all the features used to define hominids, probably the least controversial is
bipedalism. We know that in the past there have been several types of bipedalism,
but there is definitely a basic one that is known in australopithecines, Orrorin and
Homo. In this group, adaptation to arboreality is variable: greater in some taxa,
less in some others, and very little in Homo. There was probably a variety of early
forms of hominids; the oldest widely accepted biped (supported by postcranial
evidence) is Orrorin and we await further data on Ardipithecus and Sahelanthropus to clarify their status (> Figure 6.3). What the evidence from the Upper
Miocene tells us is that we cannot continue to support an origin of the earliest
. Figure 6.3
Proposed relationships between early hominids and late apes
The earliest putative hominids
6
hominids in dry savannah‐like conditions: in contrast, they inhabited humid to
forested environments.
These early hominids cohabited with apes, and we are only just beginning to
uncover the history of modern apes: this is the challenge of the third millennium.
References
Andrews P (1971) Ramapithecus wickeri mandible from Fort Ternan, Kenya. Nature 230:
192–194
Arnason U, Gullberg A, Schweitzer‐Burguette A,
Janke A (2001) Molecular estimates of primate divergences and new hypotheses for
primate dispersals and the origin of modern
humans. Hereditas 133: 217–228
Begun D (2002) European hominoids. In:
Hartwig WC (ed) The primate fossil record.
Cambridge University Press, Cambridge, pp
339–368
Biegert J (1963) The evaluation of characters of
the skull, hands and feet for primate taxonomy. In: Washburn SL (ed) Classification and
human
evolution,
Aldine,
Chicago,
pp 116–145
Bishop WW, Chapman GR (1970) Early Pliocene sediments and fossils from the northern
Kenya Rift Valley. Nature 226: 914–918
Bishop WW, Pickford M (1975) Geology fauna
and palaeoenvironments of the Ngorora
Formation, Kenya Rift Valley. Nature 254:
185–192
Britten R (1986) Rates of DNA sequence evolution differ between taxonomic groups. Science 231: 1393–1398
Brunet M, Guy F, Pilbeam D, Mackaye H, Likius
A, Ahounta D, Beauvilain A, Blondel C,
Bocherens H, Boisserie J‐R, De Bonis L,
Coppens Y, Dejax J, Denys C, Duringer P,
Eisenman V, Fanone G, Fronty P, Geraads
D, Lehmann T, Lihoreau F, Louchar A,
Mahamat A, Merceron G, Mouchelin G,
Otero O, Campomanes P, Ponce de Leon M,
Rage J‐C, Sapanet M, Schuster M, Sudre J,
Tassy P, Valentin X, Vignaud P, Viriot L,
Zazzo A, Zollikofer C (2002) A new hominid
from the Upper Miocene of Chad, Central
Africa. Nature 418: 145–151
Chapman GR, Brook M (1978) Chronostratigraphy of the Baringo Basin, Kenya. In:
Bishop WW (ed) Geological background to
fossil man. Geological Society of London,
Scottish Academic Press, pp 207–223
Clark Le Gros WE (1950) New palaeontological
evidence bearing on the evolution of the
Hominoidea. Q J Geol Soc London CV:
225–263
Clark Le Gros WE (1972) The fossil evidence
for human evolution. University of Chicago
Press, Chicago
Clarke R (1995) First ever discovery of a well‐
preserved skull and associated skeleton of
Australopithecus. S Afr J Sci 94: 460–463
Conroy GC, Pickford M, Senut B, Van
Couvering J, Mein P (1992) Otavipithecus
namibiensis, nov. gen., nov. sp., first Miocene
hominoid from Southern Africa (Berg
Aukas, Namibia). Nature 356: 144–148
Coppens Y (1983) Les plus anciens fossiles
d’Hominidés. Recent advances in evolution
of primates. Pontificiae Academiae Scientiarum Scripta Varia 50: 1–9
Coppens Y, Senut B (eds) (1991) Origine(s) de
la bipédie chez les Hominidés. Cahiers de
Paléoanthropologie. CNRS, Paris
Czelusniak J, Goodman M (1998) Where DNA
sequence place Homo sapiens in a phylogenetic classification of primates. DUAL
Congress 1998, Johannesburg, Abstract Volume, Organising Committee of Dual Congress, 14
Dart R (1925) Australopithecus africanus: The
man‐ape of South Africa. Nature 115: 195–199
Dean MC, Wood BA (1981) Metrical analysis of
the basicranium of extant hominoids and Australopithecus. Am J Phys Anthropol 54: 53–71
Dean MC, Wood BA (1982) Basicranial anatomy of the Plio‐Pleistocene hominids from
1535
1536
6
The earliest putative hominids
East and South Africa. Am J Phys Anthropol
59: 157–174
Galik K, Senut B, Pickford M, Gommery D,
Treil J, Kuperavage AJ, Eckhardt RB (2004)
External and internal morphology of BAR
10020 00 Orrorin tugenensis femur. Science
305: 1450–1453
Genet‐Varcin E (1969) Les singes actuels et
fossiles. Boubée, Paris
Gommery D, Senut B (2006) La phalange du
pouce d’Orrorin tugenensis, Miocène supérieur des Tugen Hills, Kenya. Geobios 39:
372–384
Gommery D, Senut B, Pickford M (1998) Nouveaux restes postcrâniens d’Hominoidea du
Miocène inférieur de Napak, Ouganda.
(Hommage à W.W. Bishop). Ann Paléontol
(Vert.‐Invert.) 84(3–4): 287–306
Gommery D, Senut B, Pickford M, Musiime E
(2002) Le squelette d’Ugandapithecus major.
Ann Paléontol 88: 167–186
Granjean E (2005) Etude de la position relative
du foramen magnum dans l’axe antéro‐
postérieur des primates actuels et des hominidés fossiles (Australopithecus et Homo
habilis). Mémoire de Maı̂trise du Quaternaire: Géologie, Paléontologie Humaine,
Préhistoire, MNHN, Paris, 37p
Greenfield LO (1978) On the dental arcade
reconstruction of Ramapithecus. J Hum
Evol 7: 345–359
Greenfield LO (1979) Ontheadaptativepatternof
‘‘Ramapithecus’’. Am J Phys Anthropol 50(4):
527–548
Greenfield LO (1980) A late divergence hypothesis. Am J Phys Anthropol 52: 351–365
Haile‐Selassie Y (2001) Late Miocene hominoids
from the Middle Awash. Nature 412: 178–181
Haile‐Selassie Y, Suwa G, White TD (2004) Late
Miocene teeth from Middle Awash, Ethiopia,
and Early hominid dental evolution. Science
303: 1503–1505
Hürzeler J (1958) Oreopithecus bambolii Gervais. Verhandlungen der Naturforschung
Gesellschaft. Basel 65(1): 88–95
Hürzeler J (1968) Questions et réflexions sur
l’histoire des Anthropomorphes. Ann
Paléont (Vertébrés) LIV: 195–233
Ishida H, Pickford M (1998) A new Late Miocene hominoid from Kenya: Samburupithecus
kiptalami gen. et sp. nov. CR Acad Sci Paris
IIa, 325: 823–829
Johanson D, Taieb M (1976) Plio‐Pleistocene
discoveries in Hadar, Ethiopia. Nature 260:
293–297
Johanson DC, White TD, Coppens Y (1978) A
new species of the genus Australopithecus
(Primates, Hominidae) from the Pliocene of
Eastern Africa. Kirtlandia 28: 1–14
Kingston JD, Marino BD, Hill AP (1994) Isotopic evidence for Neogene hominid palaeoenvironments in the Kenya Rift valley. Science
264: 95–959
Köhler M, Moyà‐Solà S (1997) Ape‐like or
hominid‐like? The positional behavior of
Oreopithecus bambolii reconsidered. Proc
Natl Acad Sci USA 94: 11747–11750
Leakey LSB (1961/1962) A new lower Pliocene
fossil primate from Kenya. Ann Mag Nat Hist
13(4): 689–696
Leakey LSB (1970) In: Bishop WW, Chapman
G (eds) Early Pliocene sediments and fossils
from Northern Kenya Rift Valley. Nature 226:
914–918
Marks J (2002) What does it mean to be 98%
chimpanzee? California Press, Berkeley, 312
Patterson B (1966) A new locality for Early
Pleistocene fossils in north‐western Kenya.
Nature 212: 577–581
Patterson B, Howells WW (1967) Hominid humeral fragment from Early Pleistocene of Northwestern Kenya. Science 156:
64–66
Patterson B, Behrensmeyer AK, Sill WD (1970)
Geology and fauna of a new Pliocene locality
in north‐western Kenya. Nature 226:
918–921
Pickford M (1975) Late Miocene sediments and
fossils from the Northern Kenya Rift Valley.
Nature 256: 279–284
Pickford M (1986) Sexual dimorphism in Proconsul. In: Pickford M, Chiarelli B (eds) Sexual dimorphism in primates. Il Sedicesimo,
Florence, pp 133–170
Pickford M (1987) The diversity, zoogeography
and geochronology of monkeys. Hum Evol
2(1): 71–89
Pickford M (2005) Orientation of the foramen
magnum in Late Miocene to extant African
apes and hominids. Anthropologie 43: 103–110
The earliest putative hominids
Pickford M, Chiarelli B (eds) (1986) Sexual
dimorphism in primates. Il Sedicesimo, Florence
Pickford M, Ishida H (1998) Interpretation of
Samburupithecus, an Upper Miocene hominoid from Kenya. CR Acad Sci Paris Ser. IIa
326: 299–306
Pickford M, Senut B (2001) The geological and
faunal context of Late Miocene hominid
remains from Lukeino, Kenya. CR Acad Sci
Paris 332: 145–152
Pickford M, Senut B (2005) Hominoid teeth
with chimpanzee‐ and gorilla‐like features
from the Miocene of Kenya: Implications
for the chronology of ape‐human divergence
and biogeography of Miocene hominoids.
Anthropol Sci 113: 95–102 (publié en ligne
le 13 juillet 2004)
Pickford M, Senut B, Gommery D, Treil J
(2002) Bipedalism in Orrorin tugenensis
revealed by its femora. CR Palevol 1: 191–203
Pickford M, Senut B, Mourer‐Chauviré C
(2004) Early Pliocene Tragulidae and peafowls in the Rift Valley, Kenya: Evidence for
rainforest in East Africa. CR Palevol 3:
179–189
Pilbeam D (1996) Genetic and morphological records of the Hominoidea and hominid origins: A synthesis. Mol Biol Evol 5:
155–168
Renne PR, WoldeGabrielG, Hart WK, Heiken G,
White TD (1999) Chronostratigraphy of the
Miocene‐Pliocene Sagantole Formation,
Middle Awash Valley, Afar Rift, Ethiopia.
Geol Soc Ann Bull 111: 869–885
Rook L, Bondioli L, Köhler M, Moyà‐Solà S,
Macchiarelli R (1999) Oreopithecus was a
bipedal ape after all: Evidence from the iliac
cancellous architecture. Proc Natl Acad Sci
USA 96: 8795–8799
Rose M (1986) Further hominoid postcranial
specimens from the Late Miocene Nagri
Formation of Pakistan. J Hum Evol 15:
333–367
Sarmiento E (1983) Some functional specializations in the skeleton of Oreopithecus bambolii
and their phylogenetic significance. Am J
Phys Anthropol 60: 248–249
SawadaY, PickfordM, SenutB, Itaya T, HyodoM,
Miura T, Kashine C, Chujo T, Fuji H (2002)
6
The age of Orrorin tugenensis, an early hominid from the Tugen Hills, Kenya. CR Palevol
1: 293–303
Schaeffer (1999) Foramen magnum–carotid
foramina relationship: Is it useful for species
designation? Am J Phys Anthropol 110:
467–471
Schultz AH (1955) The position of the occipital
condyles and of the face relative to the skull
base in primates. Am J Phys Anthropol 13(1):
97–120
Schultz AH (1960) Einige Beobachtungen
und Masse am Skelett von Oreopithecus. Z
Morphol Anthropol 2: 197–311
Semaw S, Simpson SW, Quade J, Renne PR,
Butler RF, McIntosh WC, Levin N, Dominguez‐Rodrigo M, Rogers MJ (2005) Early
Pliocene hominids from Gona Ethiopia.
Nature 433: 301–305
Senut B (1981) L’humérus et ses articulations
chez les Hominidés plio‐pléistocènes. Cahiers
de Paléontologie (Paléoanthropologie), publiés
sous la direction d’Y. Coppens, CNRS Paris
Senut B (1989) Le coude des primates hominoı̈des, Cahiers de Paléontologie (Paléoanthropologie). CNRS, Paris
Senut B (1992) French contribution to the
study of human origins: The case of Australopithecus afarensis. Hum Evol 7(4): 15–24
Senut B (1998) Grands singes fossiles et origine
des hominidés. Primatologie 1: 91–134
Senut B (2006) Bipédie et climat. CR Palevol 5:
89–98
Senut B, Pickford M (2004) La dichotomie
grands singes‐hommes revisitée. CR Palevol
3: 265–276
Senut B, Pickford M, Gommery D, Mein P,
Cheboi K, Coppens Y (2001) First hominid
from the Miocene (Lukeino Formation,
Kenya). CR Acad Sci Paris 332: 137–144
Simons EL (1961) The phyletic position of
Ramapithecus. Postilla YPM 57: 1–9
Simons EL, Pilbeam D (1965) A preliminary
revision of Dryopithecinae (Pongidae,
Anthropoidea). Folia Primatol 3: 81–152
Stanyon R (1989) Implications of biomolecular data for human origins with particular
reference to chromosomes. In: Giacobini
G (ed) Hominidae. Jaca Book, Milan,
pp 35–44
1537
1538
6
The earliest putative hominids
Stern JT (2000) Climbing to the top: A personal
Memoir of Australopithecus afarensis. Evol
Anthropol 9(3): 113–133
Stern JT, Susman R (1983) The locomotor anatomy of Australopithecus afarensis. Am J Phys
Anthropol 60: 279–317
Strait DS, Grine FE (1999). Cladistics and early
hominid phylogeny. Science 285: 1210
Strait DS, Grine FE, Moniz MA (1997). A reappraisal of early hominid phylogeny. J Hum
Evol 32: 17–82
Straus (1963) The classification of Oreopithecus. In: Washburn SL (ed) Classification and
human
evolution.
Aldine,
Chicago,
pp 146–177
Susman RL, Stern JT, Jungers WL (1984).
Arboreality and bipedality in the hadar
hominids. Folia Primatol 43: 113–156
Tardieu C (1983) Analyse morphofonctionnelle de l’articulation du genou chez les
primates. Application aux Hominidés fossiles. Cahiers de Paléoanthropologie. CNRS,
Paris
Vignaud P, Duringer Ph, Mackaye HT, Likius A,
Blondel C, Boisserie JR, de Bonis L,
Eisenmann V, Etienne ME, Geraads D, Guy
F, Lehmann T, Lihoreau F, Lopez‐Martinez
N, Mourer‐Chauvire C, Otero O, Rage JC,
Schuster M, Viriot L, Zazzo A, Brunet M
(2002) Geology and palaeontology of
the Upper Miocene Toros‐Menalla hominid locality, Djurab Desert. Nature 418:
152–155
Ward SC, Duren DL (2002) Middle and late
Miocene African hominoids. In: Hartwig WC
(ed) The primate fossil record. Cambridge
University Press, Cambridge, pp 385–397
White TD, Suwa G, Asfaw B (1994) Australopithecus ramidus, a new species of early
hominid from Aramis, Ethiopia. Nature
371: 306–312
White TD, Suwa G, Asfaw B (1995) Australopithecus ramidus, a new species of early hominid from Aramis Ethiopia: A corrigendum.
Nature 375: 88
Wildman DE, Uddin M, Liu G, Grossman LI,
Goodman M (2003) Implications of natural
selection in shaping 99.4% nonsynonymous
DNA identity between humans and chimpanzees: Enlarging genus Homo. Proc Natl
Acad Sci USA 100(12): 7181–7188
Wilson A, Sarich V (1969) A molecular time
scale for human evolution. Proc Natl Acad
Sci USA 63: 1088–1093
WoldeGabriel G, White TD, Suwa G, Renne P,
de Heinzelin J, Hart WK, Helken G (1994)
Ecological and temporal placement of early
Pliocene hominids at Aramis, Ethiopia. Nature 371: 330–333
WoldeGabriel G, Haile‐Selassie Y, Renne PR,
Hart WK, Ambrose SH, Asfaw B, Heiken G,
White T (2001) Geology and palaeontology of
the Late Miocene Middle Awash valley, Ethiopia, Afar Rift, Ethiopia. Nature 412: 175–181
Wolpoff MH, Senut B, Pickford M, Hawks J
(2002) Sahelanthropus or Sahelpithecus? Nature 419: 581–582
Wood BA (2002) Hominid revelations from
Chad. Nature 418: 133–135
Zollikofer C, Ponce de Léon M, Lieberman D,
Guy F, Pilbeam D, Likius A, Mackaye H,
Vignaud P, Brunet M (2005) Virtual reconstruction of Sahelanthropus tchadensis.
Nature 434: 755–759