Evolution And History

160 CHAPTER 7 | The First Bipeds

Although one fossil specimen often referred to as the “Black

Skull” (see A. aethiopicus in Table 7.1) is known to be as much

as 2.5 million years old, some date to as recently as 1.1 mya.

Like robust australopithecines from South Africa, East

African robust forms possessed enormous molars and pre-

molars. Despite a large mandible and palate, the anterior

teeth (canines and incisors) were often crowded, owing to

the room needed for the massive molars.

The heavy skull, more massive even than seen in the ro-

bust forms from South Africa, has a sagittal crest and promi-

nent brow ridges. Cranial capacity ranges from about 500 to

530 cubic centimeters. Body size, too, is somewhat larger;

whereas the South African robust forms are estimated to

have weighed between 32 and 40 kilograms, the East African

robusts probably weighed from 34 to 49 kilograms.

Because the earliest robust skull from East Africa

(2.5 million years old), the so-called Black Skull from Kenya,

retains a number of ancestral features shared with earlier

East African australopithecines, it is possible that it evolved

from A. afarensis, giving rise to the later robust East African

forms. Whether the South African robust australopithecines

represent a southern offshoot of the East African line or

The first robust australopithecine to be found in East
Africa was discovered by Mary Leakey in the summer of
1959, the centennial year of the publication of Darwin’s On
the Origin of Species. She found it in Olduvai Gorge, a fossil-
rich area near Ngorongoro Crater, on the Serengeti Plain of
Tanzania. Olduvai is a huge gash in the earth, about 40 kilo-
meters (25 miles) long and 91 meters (300 feet) deep, which
cuts through Plio-Pleistocene and recent geologic strata
revealing close to 2 million years of the earth’s history.
Mary Leakey’s discovery was reconstructed by her hus-
band Louis, who gave it the name Zinjanthropus boisei (Zinj,
an old Arabic name for “East Africa,” boisei after the bene-
factor who funded their expedition). At first, he thought this
ancient fossil seemed more humanlike than Australopith-
ecus and extremely close to modern humans in evolutionary
development, in part due to the stone tools found in associa-
tion with this specimen. Further study, however, revealed that
Zinjanthropus, the remains of which consisted of a skull and
a few limb bones, was an East African species of robust aus-
tralopithecine. Although similar in many ways to A. robustus,
Zinj is now most commonly referred to as Australopithecus
boisei (see Table 7.1). Potassium-argon dating places this
early species at about 1.75 million years old.
Since the time of Mary Leakey’s original A. boisei find, nu-
merous other fossils of this robust species have been found at
Olduvai, as well as north and east of Lake Turkana in Kenya.

Figure 7.11 The differences between gracile and robust
australopithecines are related primarily to their chewing
apparatus. Robust species have extremely large cheek teeth,
large chewing muscles, and a bony ridge on the top of their skulls
for the attachment of large temporal muscles for chewing. The
front and back teeth of gracile species are balanced in size, and
their chewing muscles (reflected in a less massive skull) are more
like those seen in the later genus Homo. If you place your own
hands on the sides of your skull above your ears while opening
and closing your jaw, you can feel where your temporal muscles
attach to your skull. By moving your hands toward the top of your
skull you can feel where these muscles end in humans.

Gracile

No crest

Smaller

zygomatic

arch (cheekbone)

Face

lower

on skull

Front and back teeth

of similar sizes

Robust Sagittal crest

in males (the

bony point at

the top of skull)

Wide flaring

zygomatic

arch

Face higher

on skull

Unbalanced dentition with

very large molar teeth

The robust australopithecines and the earliest members of genus Homo

inhabited the earth at the same time. These skulls and leg bones

were all found along the eastern shores of Lake Turkana in Kenya and

are dated to between 1.7 and 1.9 mya. The two specimens with the

rounded skulls are classified by many paleoanthropologists as members

of the species Homo habilis. The robust australopithecine at the top of

the photograph has the bony ridge along the top of its skull.

© John Reader/Photo Researchers Inc.© John Reader/Photo Researchers Inc

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