Hominid Fossils
There are in fact some thousands of fossils of organisms that appear to be intermediate between
apes and humans; they have been classified with us in the family Hominidae, and are known as
hominids. These organisms were originally divided into two genera, Australopithecus and Homo.
Since then, some of these have been reclassified into more genera, but for now let's just use the
older names. All the Australopithecus fossils (there are several species) have been found in
Africa. Fossil species of Homo include Homo habilis, Homo erectus, and Homo sapiens. All of
these are found in Africa, and the earliest example of each is African. Later Homo erectus and
Homo sapiens fossils are found in Asia and Europe as well. The chart below summarizes the
fossil hominid finds. Note that several species existed at the same time in Africa, often two or
three in a given location.
From Conroy, GC 1990. Primate Evolution. New York: WW Norton.
Most of the discussion here will center on the genus Australopithecus. Fossils of this genus have
been found in strata dating from about 4 MYA to about 1 MYA. Let's consider the foramen
magnum first. Shown in the center below is the skull of Australopithecus boisei. In general, the
species A. boisei and A. robustus, known as the "robust" australopithecines, seem to have been a
side branch evolutionarily. They had fairly small brains, and their molar teeth and jaw were
oversized, suggesting adaptation for a diet of tough plant material. At all times they coexisted
with more slender "gracile" australopithecines such as A. africanus and A. afarensis, or with the
relatively gracile Homo habilis.
Human and Chimp Skull Comparison
Australopithecines
Above: Proconsul was a Miocene ape, one of the earliest known ape ancestors. Note the progressive movement of
the foramen magnum (found in the nuchal area) from the back to the bottom of the skull. Australopithecus afarensis
and A. africanus are gracile australopithecines, with relatively small teeth and light bones. They are generally
regarded as ancestral to the genus Homo.
The robust australopithecines, Australopithecus robustus and A. boisei, are distinguished by their huge chewing
apparatus. The molars are broad, in some cases as large as a nickel. The sagittal crest on the top of the skull is a
point of attachment for jaw muscles, which then pass under the broadly flaring cheekbones. It is apparent from their
teeth and jaws that these species were specialized herbivores. They are generally regarded as a side branch which
became extinct, while other descendants of A. afarensis were A. africanus and Homo habilis and later Homo
erectus.
H. erectus
H. erectus and Australopithecus pics from Conroy. Human and chimp from Swindler and Wood, An Atlas of
Primate Gross Anatomy.
The position of the foramen magnum can be inferred from the drawings above, but here is a
better view. This is A. boisei here. The position of the foramen magnum in this organism is
typical of the genus Australopithecus, being quite far forward and underneath the skull. The
pictures are for reference. Look at the skulls in the classroom.
Chimpanzee skull - foramen
magnum is angled between
bottom and back, indicating
spine exits at an angle
Skull of Australopithecus
boisei. (Olduvai H5 specimen)
The foramen magnum is
directly under or toward the
front. (Green portions of this
specimen are based on other
A. boisei fossils; brown
portions are casts of the actual
fossil find.)
Human skull -- foramen
magnum is angled directly
under or to the front,
indicating spine is vertical.
Most of the hominid fossils, like most fossils of every kind, are fragmentary. Many consist of
teeth alone, or skulls; others include some other, or "postcranial" bones. The oldest fairly
complete hominid fossil was found at the Hadar site in Ethiopia in 1974 by Donald Johanson and
his co-workers. This fossil is dated to about 3.3 MYA, and is over 50% complete. Given that
the body should be more or less symmetrical, virtually the entire skeleton can be reconstructed
from this find, nicknamed "Lucy." Lucy's left hip bones were well preserved, and are mirrored
and diagrammed below; you can compare them to human and chimp hips.
Hips of Australopithecus afarensis, from the specimen known as "Lucy".
Chimp hips. The blade of the ilium is flat against
the back, that is, lies parallel to a line drawn
across the back.
Human hips. The blade of the ilium wraps
around the side of the body, almost
perpendicular to a line drawn across the back.
Illustrations from Lovejoy, C.O. Evolution of human walking. Scientific American Nov. 1998, 118-125.
Did organisms of the genus Australopithecus walk upright? Explain your reasoning.
Please write your answer/response on the separate sheet.
Cranial volume can be measured from fossil skulls. The table below gives average volumes for
crania of several species. In most mammals, cranial volume has a direct correlation with body
mass. In order to express how much "extra" brain a given organism has, a measure called the EQ
(encephalization quotient1) is used. An EQ of 1 is the mean value for living mammals. Humans
have an EQ of 7.6, indicating that the human brain is on average 7.6 times larger than would be
expected in a typical mammal of the same body weight. You could compare these values with
the figure on the first page showing dates for these organisms in the fossil record.
average
cranial
volume
(ml)
395.0
Species
Pan troglodytes
Estimated
body mass EQ
(kg)
EQ as percent
Fossil dates,
of H. sapiens
MYA
value
45.0
2.6
34
-?-
Australopithecus
afarensis
413.5
37.1
3.1
41
3.7 - 2.7
A. africanus
441.2
35.3
3.4
45
3.0 - 2.3
A. robustus
530.0
44.4
3.5
46
2.1 - 1.4
H. habilis
640.2
48.0
4.0
53
2.3 - 1.2
H. erectus (early,
African and Asian)
937.2
53.0
5.5
72
1.7 - 0.5
H. erectus (late,
"Peking man")
1043.0
53.0
6.1
80
0.5 - 0.3
H. sapiens
1350.0
57.0
7.6
100
0.5 - now
From Conroy, GC 1990. Primate Evolution. New York: WW Norton.
1
EQ=E/0.12 P2/3 , where E = brain mass, and P = body mass.
Do the cranial volume and EQ data, along with the dates of the fossils, support the hypothesis
that humans and apes have a common ancestor? Explain.
Please write your answer/response on the separate sheet.
Teeth and skulls fossilize particularly well. Many fossils of Australopithecus teeth are
known. The illustration below compares the teeth of A. boisei to those of chimp and human.
Upper jaw of A. afarensis (one canine pulled
partly out from jaw)
Upper jaw of A. boisei
Chimpanzee upper jaw
Human upper jaw
From Conroy, GC 1990. Primate Evolution. New York: WW Norton.
See also this view of the teeth of A. africanus at http://www.talkorigins.org/faqs/homs/jaws.html.
Do the teeth and jaw of members of the genus Australopithecus support the hypothesis that
Australopithecus was intermediate between apes and humans? Explain
Please write your answer/response on the separate sheet.