Kenyan fossils unveil new hominid species.

Excavations in northern Kenya have yielded the remains of a new species in the human evolutionary family that lived between 4.2 million and 3.9 million years ago, according to a new report. These hominids, named Australopithecus anamensis by their discoverers, display the earliest evidence of consistent two-legged walking and may have been the ancestors of A. afarensis, the species that includes the 3.2-million-year-old partial skeleton known as Lucy.

"We conclude that [a two-legged gait] had evolved at least half a million years before the previous earliest evidence, the footprints at Laetoli, suggests," contend Meave G. Leakey of the National Museums of Kenya in Nairobi and her colleagues. Footprints preserved in volcanic ash at Tanzania's Laetoli site date to about 3.6 million years ago.

Leakey and Alan Walker of Pennsylvania State University in University Park directed excavations at Kanapoli and Allia Bay, two sites near Kenya's Lake Turkana. Craig S. Feibel of Rutgers University in New Brunswick, N.J., and Ian McDougall of Australian National University in Canberra dated volcanic ash above and below fossil-bearing soil by measuring the decay of a radioactive form of the element argon in single crystals of the ash.

A total of 21 A. anamensis fossils has been recovered so far at the Lake Turkana sites, Leakey and her coworkers report in the Aug. 17 Nature.

A. anamensis combines humanlike limbs with relatively apelike jaws and teeth, the researchers assert. Both a partial shin bone and an upper-arm bone, the latter unearthed 30 years ago at Kanapoi by other investigators, closely resemble corresponding bones in living humans.

The mix of features in A. anamensis suggests it may have evolved into A. afarensis, according to Leakey's group. A. afarensis lived from about 4 million to 3 million years ago. Its fossils have been found at two East African sites separated by 900 miles. A. anamensis, the researchers contend, has more in common with A. afarensis finds from the older site, which lies closer to Lake Turkana.

The new jaws and teeth differ in important ways from those of the earliest known hominid, which lived in East Africa 4.4 million years ago (SN: 10/1/94, p.212). That creature, now considered part of the new genus Ardipithecus, may have been a "sister species" to A. anamensis and later hominids directly ancestral to modern humans, Leakey and her associates suggest.

However, anatomical variations on the adaptation for two-legged walking may have caused several hominid species to arise around 4 million years ago, they add. Any of those species, not just A. anamensis, could have served as the evolutionary root of Homo sapiens.

Many questions remain about the evolutionary identity of the Kenyan finds, notes Peter Andrews of the Natural History Museum in London in an accompanying editorial. Two species may have lived at Kanapoi, he maintains. Human-like limbs found there may be as many as 500,000 years younger than apelike jaws and teeth from deeper sediment, in the British researcher's opinion.

Or, Andrews proposes, the unusual combination of ape and human features in A. anamensis may render it distinct from australopithecines such as Lucy. Instead, the Kenyan hominid could have belonged to a lineage that directly linked ancient apes to Homo, which arose about 2 million years ago.

Whatever the case, other evidence indicates that early hominids apparently retained a capacity for tree climbing along with upright walking, Andrews holds (SN: 7/29/95, p.71). Evidence of considerable forest and smaller wooded regions at Kanapoi and other ancient hominid sites suggests that creatures such as A. anamensis lived much as modern apes do, he contends.