African fossil pushes back human ancestry.

A 2.3-million-year-old fossil unearthed in Ethiopia comes from the oldest known member of the human evolutionary lineage, a scientific team announced last week.

The new Homo specimen, which consists of an upper jaw holding nine relatively complete teeth and part of the lower nasal area, is about 400,000 years older than the next-oldest securely dated Homo fossils. Those discoveries were made more than 30 years ago at Tanzania's Olduvai Gorge. Many researchers now divide the Olduvai bones into two species, H. habilis and H. rudolfensis.

Preliminary comparisons indicate that the new fossil's remaining teeth bear close similarities to those of H. habilis, but the Ethiopian find cannot confidently be assigned to a species, assert anthropologist William H. Kimbel of the Institute of Human Origins in Berkeley, Calif., and his coworkers. Kimbel's group will present its findings in the December Journal of Human Evolution.

"The new specimen is complete enough to assign to Homo and has a pretty solid date," remarks anthropologist G. Philip Rightmire of the State University of New York at Binghamton. "It helps to clear away some of the mist surrounding early Homo evolution."

Investigators found the partial jaw at the Hadar site, where prior work yielded the remains of "Lucy" and other members of Australopithecus afarensis. At Hadar, these upright-walking, small-brained creatures date to between 3.4 million and 3 million years ago.

A number of stone tools lay in the sediment that held the Hadar jaw. The researchers have not established that these implements were made by early Homo, but this is the oldest association between a human ancestor's fossils and stone tools. The oldest known stone implements come from a nearby Ethiopian site and date to around 2.6 million years ago (SN: 4/15/95, p. 237).

Fossils of antelopes, hippopotamuses, and other animals recovered near the Hadar jaw suggest that early Homo lived in an open, grassy locale with stands of trees and water nearby, the scientists contend. In contrast, when A. afarensis lived at Hadar, the area was wetter and more densely wooded.

Analyses of two isotopes of the element argon contained in volcanic ash situated just above the Homo fossil allowed the researchers to calculate the jaw's approximate age.

"I wouldn't be surprised if further Homo fossils are discovered that date to around 2.5 million years ago, when the lineage probably originated," Rightmire says.