Fossil finds expand early hominid anatomy.

Fossil finds expand early hominid anatomy

Renewed field research at the Ethiopian site of Hadar, where "Lucy" and other members of the earliest known hominid species turned up in the mid-1970s, has yielded a new batch of fossils that significantly expand the anatomical diversity of the more than 3-million-year-old species.

The finds, announced last week by project co-leaders Donald C. Johanson, William H. Kimbel and Robert C. Walter of the Institute of Human Origins in Berkeley, Calif., promise to reignite debates over whether Lucy and her cohorts represent one species or two, and whether at least some members of the species spent more time in trees than walking upright.

During two months of field exploration that began last October, a 10-member scientific team from the United States and Ethiopia found 18 fragmentary fossils that represent 15 individuals belonging to Lucy's species, Australopithecus afarensis. The specimens lay on the surface of heavily eroded layers of fossil-rich earth.

"We found fossil remains of hominids [the evolutionary family that includes modern humans] throughout the geologic horizon at Hadar," says Johanson, who led the team that discovered Lucy. "There should be many more to come in future field work."

A total of 15 tooth and jaw fragments possess characteristics similar to previous A. afarensis finds, but three fossils show anatomical features new to the Hadar hominid collection.

A specimen consisting of an upper jaw and partial face has bony pillars on each side of the nasal opening that resemble those of A. africanus, a South African hominid dating to at least 2.5 million years ago. But the shape and depth of the jaw clearly place it within Lucy's species, Johanson asserts. Moreover, the specimen contains much shallower roots for the front teeth than previously observed in A. afarensis.

A lower jaw with some teeth still in place shows primitive features much like those of 8- to 12-million-year-old extinct apes recently linked to A. afarensis (SN: 6/23/90, p.390), Kimbel says. The row of cheek teeth curves inward, and the premolar tooth behind the small canine has one cusp -- features found on some previous A. afarensis specimens but not on later hominids.

Finally, a robust upper right arm bone with its ends chewed off, perhaps by a hyena prior to fossilization, preserves two large grooves for muscle attachments near its top end. Such shoulder muscles offer powerful assistance in motions such as hoisting the body with the arms.

While the fossils await further study, scientists who argue that smaller Hadar hominids spent much of their time in trees (SN: 7/2/83, p.8) will likely greet the new limb bone with enthusiasm.

A. afarensis certainly spent some time in the trees, but analyses of its feet, legs and knees indicate it was well on the way to a full-time upright stance, Johanson argues in anticipation of any new tree-dwelling hypotheses.

Although the recent finds reveal that A. afarensis was more anatomically diverse than previously realized, "nothing in the sample screams out: 'More than one species,'" Kimbel maintains.

Researchers led by Walter also collected volcanic ash and rock samples, which they expect to yield more reliable age estimates of Hadar's sediments.