Evolution And History

Early Representatives of the Genus Homo 167

australopithecines became extinct around 1 mya, the ro-

bust forms underwent relatively little change.^19

Evidently, the pattern in early human evolution has

been relatively short periods of marked change with

diversification, separated by prolonged periods of relative

stasis or stability in the surviving species. While robust

australopithecines continued this pattern, the new genus

Homo began a steady course of brain expansion that con-

tinued over the next 2.3 million years until brain size

reached its current state. With the appearance of this new

larger-brained hominin, the first stone tools appear in the

archaeological record.

Early Representatives

of the Genus Homo

Just as the Leakeys thought, Olduvai Gorge with its stone

tool assemblages was a good place to search for human

ancestors. Part of what is now Olduvai Gorge was once a

lake. Almost 2 mya, its shores were inhabited by numerous

wild animals including a variety of bipeds. In 1959—when

the Leakeys found the bones of the first specimen of robust

Australopithecus boisei in association with some of these

tools and the bones of birds, reptiles, antelopes, and pigs—

they thought they had found the remains of one of the tool-

makers. Fossils unearthed a few months later and a few feet

below this first discovery led them to change their mind.

These fossil remains consisted of more than one

individual, including a few cranial bones, a lower jaw, a

clavicle, some finger bones (Figure 7.16), and the nearly

complete left foot of an adult (Figure 7.17). Skull and jaw

fragments indicated that these specimens represented

a larger-brained biped without the specialized chewing

apparatus of the robust australopithecines.

The Leakeys and colleagues named that contempo-

rary Homo habilis (Latin for “handy man”) and suggested

that tool-wielding H. habilis may have eaten the animals

and possibly had the Australopithecus boisei for dessert.

Of course, we do not really know whether A. boisei from

Olduvai Gorge met its end in this way, but we do know that

cut marks from a stone tool are present on a 2.4-million-

year-old australopithecine jaw bone from South Africa.^20

This was done, presumably, to remove the mandible, but

for what purpose we do not know. In any event, it does

lend credibility to the idea of A. boisei on occasion being

dismembered by H. habilis.

Subsequent work at Olduvai has unearthed not only

more skull fragments but other parts of the skeleton of

Consistent with Wheeler’s hypothesis is the fact that the
system for drainage of the blood from the cranium of the
earlier australopithecines is significantly different from
that of the genus Homo (Figure 7.15).
Though paleoanthropologists cannot resolve every
detail of the course of human evolution from the available
data, over time the narrative they have constructed has
improved. Human evolution evidently took place in fits
and starts, rather than at a steady pace. Today we know
that bipedalism preceded brain expansion by several mil-
lion years. Bipedalism likely occurred as a sudden shift
in body plan, while the tempo for the evolution of brain
size differed considerably. For example, fragments of an
Australopithecus skull dated to 3.9 million years old are
virtually identical to the corresponding skull fragments
a million years later. Evidently, once a viable bipedal ad-
aptation was achieved, stabilizing selection took over, and
there was little change for at least a few million years.
Then, 2.5 mya, change was again in the works, result-
ing in the branching out of new forms, including sev-
eral robust species as well as the first appearance of the
genus Homo. But again, from about 2.3 mya until robust

Scalp vein

Emissary vein

Skull

Skull (Diploic) vein

Venous sinus Meningeal veins

Brain

External

jugular

vein

Internal

jugular

vein

Internal

carotid

artery

External

carotid

artery

Figure 7.15 In humans, blood from the face and scalp, instead
of returning directly to the heart, may be directed instead into the
braincase and then to the heart. Already cooled at the surface of
the skin, blood is able to carry heat away from the brain.

(^19) Wood, B., Wood, C., & Konigsberg, L. (1994). Paranthropus boisei:
An example of evolutionary stasis? American Journal of Physical
Anthropology 95, 134.
(^20) White, T. D., & Toth, N. (2000). Cutmarks on a Plio-Pleistocene hominid
from Sterkfontein, South Africa. American Journal of Physical Anthropology
111, 579–584.