Evolution And History

The Pliocene Environment and Hominin Diversity 157

by absolute brain size but also by the ratio of brain to body

size. Unfortunately, with such a wide range of adult weights,

it is not clear whether australopithecine brain size was larger

than a modern ape’s, relative to body size.

Much has been written about australopithecine teeth

because they are one of the primary means for distinguish-

ing among closely related groups. In A. afarensis, unlike hu-

mans, the teeth are all quite large, particularly the molars.

The premolar is no longer fully sectorial as in A. anamen-

sis, but most other features of the teeth represent a more

ancestral rather than derived condition. For example, the

rows of the teeth are more parallel (the ancestral ape con-

dition) compared to the arch seen in the human tooth

rows. The canines project slightly, and a slight space or gap

known as a diastema remains between the upper incisors

and canines as found in the apes (Figure 7.8).

The skull bones are particularly important for recon-
structing evolutionary relationships as well as for learning
about the cognitive capacities of ancestral species. The skull
of A. afarensis is relatively low, the forehead slopes back-
ward, and the brow ridge that helps give apes such massive-
looking foreheads is also present. The lower half of the face
is chinless and accented by jaws that are quite large, rela-
tive to the size of the skull. The brain is small and apelike,
and the general conformation of the skull seems nonhu-
man. Even the semicircular canal, a part of the ear crucial to
maintenance of balance, is apelike. Cranial capacity, com-
monly used as an index of brain size for A. afarensis, aver-
ages about 420 cubic centimeters (cc), roughly equivalent
to the size of a chimpanzee and about one-third the size of
living humans.^4 Intelligence, however, is indicated not only

But still, it’s hard to believe that aus-

tralopithecines would retain a chimpan-

zee-like big toe, if they couldn’t use that

big toe by inverting or dorsiflexing their

foot in any significant way. By all other

accounts, an abducted hallux (big toe)

would only impede effective bipedality.

It is of no use at all for a human-like pat-

tern of climbing. The only remaining util-

ity would be for small-branch grasping,

but small branches would seem unlikely

as a support for hominin arboreality.

One possibility is that Clarke and

Tobias were simply mistaken. That

appears to be the explanation favored by

Harcourt-Smith and Aielloc and McHenry

and Jonesd who concluded that all known

hominin feet appear to lack any “ape-like

ability to oppose the big toe.” They also

point to the Laetoli footprint trails, most

observers of which agree that the big toe

was adducted, not abducted.

I tend to favor that explanation—

australopithecines simply didn’t have

a grasping foot. But they may not have

shared the medial longitudinal arch, at

least not in the human configuration,

and without it one might doubt that their

gait featured as strong a toe-off as that

of later humans. Who knows?

a DeSilva, J. M. (2009). Functional mor-

phology of the ankle and the likelihood

of climbing in early hominins. Proceed-

ing of the National Academy of Sci-

ences, USA 106, 6567–6572.

b Clarke, R. J., & Tobias, P. V. (1995).

Sterkfontein Member 2 foot bones of the

oldest South African hominid. Science

269 , 521–524.

c Harcourt-Smith, W. E. H., & Aiello, L. C.

(2004). Fossils, feet and the evolution

of human bipedal locomotion. Journal of

Anatomy 204 , 403–416, at 412.

d McHenry, H. M., & Jones, A. L.

(2006). Hallucial convergence in early

hominids. Journal of Human Evolution

50 , 534–539.

Ape Laetoli-Hadar

Dental arcade and diastema

Later Australopithecus and Homo

Chimpanzee upper jaw Early Australopithecus Human upper jaw

Figure 7.8 The

upper jaws of an ape,

Australopithecus, and

modern human show

important differences in

the shape of the dental

arch and the spacing

between the canines

and adjoining teeth.

Only in the earliest

australopithecines can

a diastema (a large gap

between the upper canine

and incisor) be seen.

diastema A space between the canines and other teeth allow-

ing the large projecting canines to fit within the jaw.

(^4) Grine, F. E. (1993). Australopithecine taxonomy and phylogeny: Histori-
cal background and recent interpretation. In R. L. Ciochon & J. G. Fleagle
(Eds.), The human evolution source book (pp. 201–202). Englewood Cliffs,
NJ: Prentice-Hall.