Robust hominids: tooth and consequences.

Robust hominids: Tooth and consequences

It is a face that only a mother and apaleoanthropologist could love. The teeth are immense and hammer-like, particularly at the back of the mouth. Massive jaws and a broad face slope toward the back of the head, where a small brain is encased.

The face belongs to a member of therobust australopithecines, a group of hominids, or human-like creatures, that evolved at the same time as the lineage that led to modern humans, but became extinct around 1 million years ago. The size and shape of their fossil skulls, found in eastern and southern Africa, led to the view that they were large, heavily built creatures. That view appears to be wrong, according to Yoel Rak of Tel Aviv University in Israel.

Rak studied several skulls belonging tothe east African species Australopithecus boisei. "Robust' facial features reached their peak in A. boisei, which has been dated at 1.2 million to 2.2 million years old.

"I was astonished at how delicate muchof the boisei skull is,' reported Rak at the annual meeting of the American Association of Physical Anthropologists in New York City last week. "It appears that a massive [chewing] system was imposed on a relatively small creature.'

For example, says Rak, the walls of A.boisei's brain case were "amazingly thin.' Cranial thickness reaches no more than 4 millimeters in the largest specimens, and no more than 2 millimeters in a smaller skull. Fragile bone also surrounds the eye openings.

Anumber of features typical of A. boiseiskulls appear, he notes, to have been evolutionary modifications to cope with massive teeth and jaws. Among them are a flared, bony crest running over the top of the head, a visor-like crest over the eyes and the triangular shape of the brain case, all of which helped to anchor enormous facial muscles.

Rak's analysis feeds into the emergingview that east African australopithecines were not more "robust' in stature than their south African counterparts, who have been described as smaller or "gracile' (SN: 1/24/87, p.58). But the east African variety does appear to be characterized by larger teeth and thicker tooth enamel, said Frederick Grine of the State University of New York at Stony Brook, at a press conference held the day before the physical anthropology meeting began. His remark was generated by a five-day workshop in Stony Brook on the robust australopithecines attended by an international group of researchers.

It is difficult to make inferences aboutaustralopithecine biology, cautions Pat Shipman of Johns Hopkins University in Baltimore, because "it's hard to tell which heads go with which bodies.' Nevertheless, in independent studies presented at the workshop, Henry M. McHenry of the University of California at Davis and William L. Jungers of the State University of New York at Stony Brook conclude that fossil remains provide no evidence of marked differences in body size between geographically separated australopithecines. Furthermore, they suggest that later australopithecines were about the same size as earlier australopithecines and early members of the human lineage.

Both scientists report that while southAfrican robust hominids had thick tooth enamel, that of east African australopithecines was even thicker.

Most workshop participants agreedwith McHenry and Jungers. "The terms "robust' and "gracile' should refer to australopithecine teeth only,' comments Milford H. Wolpoff of the University of Michigan in Ann Arbor. Adds William H. Kimbel of the Institute of Human Origins in Berkeley, Calif., "The concepts of "gracile' and "robust' australopithecines should probably be dropped and we should just refer to species names.'

One australopithecus species, however,may require revision, according to Ronald J. Clarke of the University of the Witwatersrand in Johannesburg, South Africa. At the workshop, he discussed a recently excavated skull of a creature known as A. africanus and proposed that it and other specimens previously found at the same site may in fact represent two species. A. africanus has been found only in southern Africa and is estimated to have arisen between 2.5 million and 3 million years ago.

Clarke says some of the specimenshave larger teeth and flatter faces and brows, indicating that they were an ancestral stock for both southern and eastern robust australopithecines. Other skulls have smaller teeth and more prominent brows and nasal bones. This species may have been ancestral to the human lineage, he notes.

Kimbel and his colleagues also suggestthat A. africanus specimens may represent more than one species, based on a study of fossil teeth. "If this proves to be the case,' he says, "there may be interesting changes in how we reconstruct early hominid evolution.'

Photo: Skull and artist's conception of robustaustralopithecine known as A. robustus.