Fossils put new face on Lucy's species.

Investigators have recovered and pieced together the first nearly complete skull of the earliest known species in the human evolutionary family, Australopithecus afarensis. Fragments of the 3-million-year-old skull, as well as of a number of limbs and jawbones, turned up at the Hadar site in Ethiopia.

Fieldwork at Hadar in the 1970s yielded the first A. afarensisremains, including the partial female skeleton known as Lucy..

William H. Kimbel and Donald C. Johanson, both anthropologists at the Institute of Human Origins in Berkeley, Calif., and Yoel Rak, an anatomist at Tel Aviv University describe the new finds in the March 31 NATURE.

"There is no obvious sign of evolution in this prehuman species for about 1 million years," Kimbel says. "Yet later, in only a fraction of that time, (A. afarensis) gave rise to a great branching of the family tree."

The age estimate for the new skull, assembled from more than 200 fragments found in a sandy gully in March 1992, makes it the youngest known example of Lucy's kind. Analysis of two forms of argon in crystals of volcanic rock just above and below where the skull lay established its age. The fossil's anatomy confirms that a 3.9-million-year-old cranial fragment previously found at another Ethiopian site also belonged to A. afarensis, the scientists argue.

Lucy herself lived about 3.2 million years ago.

Since 1990, annual fieldwork at Hadar conducted by Kimbel and his coworkers has yielded 53 A. afarensis specimens.

Kimbel attributes the Hadar skull to a male much larger than the diminutive Lucy, who stood about 31/2 feet tall. The skull and other new Hadar material indicate that A. afarensis males were considerably larger than females, although average size differences between the sexes remain unclear, Kimbel notes.

A minority of researchers places smaller and larger A. afarensis in separate species. However, later australopithecines - including a lineage that died out 1 million years ago-also featured large size differences between sexes of the same species, writes Leslie C. Aiello, an anthropologist at University College in London in a comment accompanying the new report.

Reconstruction of the three-quarters complete Hadar cranium will yield an estimate of brain size, Kimbel notes.

A virtually complete ulna, or forearm bone, and a partial upper-arm bone found at Hadar also came from A. afarensis, the scientists say.

The ulna curves and is long relative to the upper arm, a pattern observed in chimpanzees. But the A. afarensis ulna lacks elbow features that allow chimps to support their body weight with the forelimbs while walking, Aiello says.

The thick upper-arm bone contains deep grooves where muscles attached, much like a corresponding A. afarensis fossil found at a nearby site (SN: 11/20/93, p.324), Kimbel points out.

Aiello calls the arm bones "ideally suited to a creature which climbed in the trees but also walked on two legs when on the ground."

Some researchers argue that female A. afarensis favored tree climbing, as indicated by their curving toe bones.

Kimbel suspects debate about how Lucy's kind moved about will continue. A. afarensis clearly could walk efficiently on two legs, "but I don't know if we can say whether they spent time in the trees," he adds.