Evolution And History

The Pliocene Environment and Hominin Diversity 161

Figure 7.12 The Pliocene fossil bipeds and the scientific names by which they have been known,

arranged according to when they lived. A. aethiopicus, A. boisei, and A. robustus are all robust

australopithecines. Whether the different species names are warranted is a matter of debate.

Millions of years ago

654321

MIOCENE PLIOCENE PLEISTOCENE

Sahelanthropus

tchadensis

Orrorin

tugenensis

Kenyanthropus

platyops

Ardipithecus Australopithecus

anamensis

Australopithecus

africanus

Australopithecus

robustus

Australopithecus boisei

Australopithecus

afarensis

Australopithecus garhi

Australopithecus aethiopicus

Epochs

© T. White 1998©^
T. White

1998

Photographer David Brill, a

specialist in images of fossils

and paleoanthropologists at

work, positions the upper jaw

and the other skull fragments

of Australopithecus garhi so

that the fragments are aligned

as they would be in a com-

plete skull.

shared by separate fossil groups as seen in the robust aus-

tralopithecines is referred to as a grade.

Many anthropologists believe that, by becoming a

specialized consumer of plant foods, the late australo-

pithecines avoided competing for the same niche with

early Homo, with which they were contemporaries. In

the course of evolution, the law of competitive exclu-

sion dictates that when two closely related species com-

pete for the same niche, one will out-compete the other,

bringing about the loser’s extinction. That early Homo

and late Australopithecus did not compete for the same

niche is suggested by their coexistence for something like

1.5 million years from about 1 to 2.5 mya (Figure 7.12).

convergent evolution from a South African ancestor has not

been settled; arguments can be presented for both interpre-

tations. In either case, what happened was that the later ro-

bust australopithecines developed molars and premolars that

are both absolutely and relatively larger than those of earlier

australopithecines who possessed front and back teeth more

in proportion to those seen in the genus Homo.

Larger teeth require more bone to support them, hence

the prominent jaws of the robust australopithecines. Larger

jaws and heavy chewing activity require more jaw muscu-

lature that attaches to the skull. The marked crests seen

on skulls of the late australopithecines provide for the at-

tachment of chewing muscles on a skull that has increased

very little in size. In effect, robust australopithecines had

evolved into highly efficient chewing machines. Clearly,

their immense cheek teeth and powerful chewing muscles

bespeak the heavy chewing required for a diet of uncooked

plant foods. This general level of biological organization

law of competitive exclusion When two closely related

species compete for the same niche, one will out-compete the

other, bringing about the latter’s extinction.