Exercise 13
Hominid fossils (10 pts)
(adapted from Petersen and Rigby 1999, pp. 221–225)
INTRODUCTION
The first significant hominid fossils were found north of Düsseldorf,
Germany, in the Neander Valley in 1856. From then until now additional finds
have expanded the fossil collections of the Family Hominidae. These fossils
are among the most valuable objects of antiquity in the possession of
humankind.
Unlike most other fossil types, however, there are so few good hominid
fossils that each new discovery can have profound importance for
reconstructing the course of human evolution. Examples of single finds that
revolutionized evolutionary thinking include the discovery of the “Tuang
Baby” in Botswana in 1924, Homo habilis in Tanzania in 1962, “Lucy” in
Ethiopia in 1974, Australopithecus footprints in Tanzania in 1976, “Turkana
Boy” in Kenya in 1984, and the “Black Skull” in Kenya in 1985. Each find
represents either an extraordinary bit of good luck by the finder, or, more
typically, the result of deliberate, expensive, and painstaking excavations
whose purpose was to seek hominid fossils at known productive sites.
The most recent example of a spectacular new find is the 2001 discovery of
a hominid cranium and associated fragments in Chad by the French
paleoanthropologist Michel Brunet and his team. These fossils are dated at 6
to 7 million years old — the oldest hominid yet found. This find undoubtedly
will cause a wholesale revision of evolutionary scenarios, as previously the
oldest known hominid fossils were ~4.4 million years old.
Although there is no disagreement among scientists over the fact of human
evolution, there is uncertainty with respect to detailed evolutionary
relationships among the various known taxa. Modern hominids did not evolve
from modern pongids (gorillas, chimps, orangutans), because both groups are
contemporaneous. Clearly, however, modern hominids and modern pongids
must share a common ancestor from which they diverged to follow their
respective, independent evolutionary paths. The precise time of divergence
is unknown, but it almost certainly occurred late in the Miocene Epoch,
possibly around 7 million years ago.
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The single greatest obstacle to understanding evolutionary relationships
between hominids and pongids is inadequacy of the fossil record. There are
only a few hominoid (i.e., hominid plus pongid; Fig. 1) fossils from the critical
latest Miocene interval, 8 million to 5.3 million years ago. Morover, there are
very few pongid fossils of any age, let alone from the interval immediately
postdating their divergence from hominids. Evolutionary relationships
between pongids and hominids will remain equivocal until new discoveries
produce material from beds bracketing the time of divergence.
Figure 1—Classification of primates. Hominids and pongids are related through common ancestry, and therefore are
grouped together within the superfamily Hominoidea. (from Stanley 1998)
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Figure 2 contrasts the skulls and upper jaws of modern man and modern
chimpanzee. Note differences with respect to the following six features:
cranial capacity, brow ridge, jaw, diastema, palate and dentition. Obviously,
the most recent common ancestor to humans and chimps did not possess
anatomy identical to either modern human or modern chimps; rather, both
humans and chimps have diverged from the common ancestral state. For
purposes of this exercise, however, we will examine “undated” fossil hominid
skulls and jaws and attempt to arrange them in proper stratigraphic order on
the basis of their similarity to modern humans and chimps. The assumption
here is that a chimp-like hominid fossil is probably older than a modernlooking one, because its chimp-like features would place it closer to the most
recent common ancestor to both modern chimps and modern humans.
Figure 2—Comparison of skull and jaw features between modern man and modern chimpanzee.
(from Petersen and Rigby 1999)
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Part 1.
Examine the drawings of six fossil skulls in Figure 3, noting similarities and
differences between each fossil skull and modern human and chimp skulls.
The age of these fossil skulls is known, but not given here. Reconstruct what
you think to be the most likely stratigraphic sequence of the fossil skulls on
the basis of their various features. Label the individual skulls from 1 to 6,
with 1 being most chimp-like and 6 being most like modern man.
Figure 3—Selected fossil hominid skulls and their dentition (the jaw is to the left of its associated skull).
(from Petersen and Rigby 1999)
Part 2.
You probably discovered that ordering these skulls from most chimp-like to
most human-like is a highly subjective process that requires some arbitrary
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decisions. The procedure of ordering makes up a subdiscipline within the
science of taxonomy.
An approach that eliminates some, but not all, of the taxonomic subjectivity
involves quantifying the morphological characters on which the evolutionary
order of appearance is inferred. For example, a numerical value can be
assigned to each important feature of each skull in order to quantify the
degree to which each skull resembles modern man or modern chimpanzee:
Example Table 1 (skull of a modern chimp).
Can’t tell
(0 pts)
Cranial capacity
Brow ridge
Jaw
Diastema
Palate
Dentition
Chimplike Intermediate Humanlike
(1 pt)
(2 pts)
(3 pts)
1
1
1
1
1
1
According to this kind of scoring, a modern human would be assigned 18
points (3 points for each of six features) and a modern chimp 6 points (1
point for each of six features). Evolutionary stages in human evolution could
be inferred by this approach, known as “numerical taxonomy.”
a. Complete a table like Example Table 1 for each of the six skulls and then
determine the order of evolutionary appearance on the basis of resulting
scores.
Skull A
Can’t tell
(0 pts)
Chimplike Intermediate Humanlike
(1 pt)
(2 pts)
(3 pts)
Cranial capacity
Brow ridge
Jaw
Diastema
Palate
Dentition
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Skull B
Can’t tell
(0 pts)
Chimplike Intermediate Humanlike
(1 pt)
(2 pts)
(3 pts)
Can’t tell
(0 pts)
Chimplike Intermediate Humanlike
(1 pt)
(2 pts)
(3 pts)
Can’t tell
(0 pts)
Chimplike Intermediate Humanlike
(1 pt)
(2 pts)
(3 pts)
Can’t tell
(0 pts)
Chimplike Intermediate Humanlike
(1 pt)
(2 pts)
(3 pts)
Cranial capacity
Brow ridge
Jaw
Diastema
Palate
Dentition
Skull C
Cranial capacity
Brow ridge
Jaw
Diastema
Palate
Dentition
Skull D
Cranial capacity
Brow ridge
Jaw
Diastema
Palate
Dentition
Skull E
Cranial capacity
Brow ridge
Jaw
Diastema
Palate
Dentition
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Skull F
Can’t tell
(0 pts)
Chimplike Intermediate Humanlike
(1 pt)
(2 pts)
(3 pts)
Cranial capacity
Brow ridge
Jaw
Diastema
Palate
Dentition
b. Give the order from most chimp-like to most human-like:
c. What is the effect of poor fossil preservation? In other words, if a fossil
isn’t sufficiently complete to allow judging its similarity to a chimp or human,
how does this affect the numerical score?
Part 3.
The “art” of taxonomy is in determining which taxonomic features are more
useful than others in evolutionary studies. For example, an experienced
evolutionary biologist or paleontologist might have reason to believe that
differences in cranial capacity are taxonomically twice as important as
differences in dentition.
a. Select one or two morphologic features that you feel are taxonomically
important and repeat the scoring. This time, however, double the point
values assigned to the important features while keeping all the other point
values the same as before.
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Skull A
Can’t tell
(0 pts)
Chimplike Intermediate Humanlike
(1 pt)
(2 pts)
(3 pts)
Can’t tell
(0 pts)
Chimplike Intermediate Humanlike
(1 pt)
(2 pts)
(3 pts)
Can’t tell
(0 pts)
Chimplike Intermediate Humanlike
(1 pt)
(2 pts)
(3 pts)
Can’t tell
(0 pts)
Chimplike Intermediate Humanlike
(1 pt)
(2 pts)
(3 pts)
Cranial capacity
Brow ridge
Jaw
Diastema
Palate
Dentition
Skull B
Cranial capacity
Brow ridge
Jaw
Diastema
Palate
Dentition
Skull C
Cranial capacity
Brow ridge
Jaw
Diastema
Palate
Dentition
Skull D
Cranial capacity
Brow ridge
Jaw
Diastema
Palate
Dentition
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Skull E
Can’t tell
(0 pts)
Chimplike Intermediate Humanlike
(1 pt)
(2 pts)
(3 pts)
Can’t tell
(0 pts)
Chimplike Intermediate Humanlike
(1 pt)
(2 pts)
(3 pts)
Cranial capacity
Brow ridge
Jaw
Diastema
Palate
Dentition
Skull F
Cranial capacity
Brow ridge
Jaw
Diastema
Palate
Dentition
b. Give the new order from most chimp-like to most human-like:
c. Which feature or features did you weight? Did you arrive at the same
result as in Part 2? If not, how are the new results different?
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