Some hominids show fidelity to the tooth.

Among the ancient members of the human evolutionary family, called fossil hominids by anthropologists, Paranthropus boisei cuts a striking profile. Its skull revolves around huge jaws that encase small front teeth and immense, pegshaped back teeth. A flattened face and flared cheekbones slope back to a visorlike crest over the eyes. A bony ridge runs over the top of the head, where it meets a small, triangular braincase.

A new study now indicates that P. boisei also exhibited a remarkably stubborn devotion to its distinctive look for more than 1 million years, until the Paranthropus lineage hit an evolutionary dead end. The basic features of P. boisei jaws and teeth remained unchanged during a time of marked brain growth and tooth-size reduction in direct human ancestors, contends anthropologist Bernard Wood of the University of Liverpool in England.

"I suspect P. boisei underwent little evolutionary change of any kind," Wood asserts.

The finding coincides with Wood's view that hominid species directly ancestral to modern humans also experienced few anatomical changes before their relatively abrupt evolution to succeeding species (SN: 6/20/92, p.408).

P. boisei belonged to a group of African hominids, referred to as robust australopithecines by some investigators, which first appeared about 2.6 million years ago. P. boisei lived in east Africa from around 2.2 million to 1 million years ago, in Wood's view. Some anthropologists argue that the discovery of the so-called black skull extends the antiquity of P. boisei to 2.5 million years ago, a claim that continues to spark controversy (SN: 1/24/87, p.58).

Wood studied 144 fossil jaws and teeth that belonged to P. boisei at various points in its evolutionary history He could not conduct a similar anatomical survey of lower-body bones, because fossil hunters have found a scant collection of such specimens for P. boisei.

Only a few, marginally important changes took place over time in the extinct hominid's jaws and teeth, Wood reported last week in San Francisco at the annual meeting of the American Anthropological Association. Nine out of 47 anatomical features measured by Wood displayed significant change in a comparison of early and late P. boisei specimens.

Both the thickness and the height of the hominid's powerful lower jaw stayed constant over time, Wood notes. The overall size of the lower jaw increased slightly in later specimens, but the British scientist calls this trend "weak."

In contrast, a few tooth features underwent significant change without altering the crucial aspects of P. boisei dentition, Wood asserts. For example, canines enlarged from early to later specimens, but these teeth played a minor role in chewing and grinding, which was handled largely by the heavily enameled molars at the back of the mouth, he maintains.

Although premolar teeth just behind the canines also became larger, no general trends in dental evolution accompany this change, Wood holds.

Anatomical disparities among the teeth and jaws of some P. boisei fossils probably reflect differences between the sexes, he says.