Team unearths oldest known human ancestor.

Fieldwork at an Ethiopian site in 1992 and 1993 has yielded the remains of the oldest members of the human evolutionary family yet discovered. These chimpanzeelike creatures, who lived 4.4 million years ago, apparently served as a common ancestral stock for all later hominid species. Their existence supports the theory that a common ancestor of apes and humans lived in Africa no more than 6 million years ago.

The scientific team that found the new fossils, led by Tim D. White, an anthropologist at the University of California, Berkeley, assigns them to a new species, Australopithecus ramidus. For nearly 20 years, the earliest hominid remains were those of "Lucy" and other members of A. afarensis, a species that lived from 4 million to 3 million years ago (SN: 4/2/94, p.212).

A description of A. ramidus, a dating analysis of associated volcanic ash, and evidence that this hominid inhabited forests rather than savannas appear in the Sept. 22 NATURE. The fossils may display enough anatomical differences to qualify as a new genus distinct from Australopithecus, argues Bernard Wood, an anthropologist at the University of Liverpool in England, in an accompanying comment.

"Whatever you call this creature, it was Lucy's ancestor, and it was considerably more primitive [anatomically] than Lucy," White contends.

Investigators discovered the fossils near the village of Aramis, about 45 miles south of Hadar, where Lucy and her kin were found. White and his coworkers uncovered the remains of 17 individual hominids. These specimens consist of skull fragments, teeth, arm bones, and part of a child's lower jaw. Carnivores apparently broke apart most of the bones before fossilization.

A. ramidus displays several differences from all other hominids: smaller cheek teeth, relatively larger canine teeth, thinner tooth enamel, and a cranial base and deciduous first molar tooth more like those of chimpanzees.

Yet three nearly complete bones from an A. ramidus individual's left arm look much like those of A. afarensis (SN: 11/20/93, p.324). Both hominids possessed heavily muscled arms with elbows that could not support knuckle walking. This arrangement suggests that they split their time between tree climbing and two-legged walking, White asserts.

An analysis of the ratio of two forms of argon in grains of volcanic ash found just below the fossils, as well as data on magnetic reversals in the ancient soil, produced the age estimate of 4.4 million years. Giday WoldeGabriel, a geologist at Los Alamos (N.M.) National Laboratory, directed this effort.

The researchers note that A. ramidus probably lived in a wooded environment. Later hominids resided on drier, more open savannas, where bones have a much greater chance of fossilizing. Extensive volcanic ash at Aramis helped preserve the bones, White says.

In addition to hominids, the scientific team recovered fossils of rhinos, elephants, birds, bats, rodents, the first large bear known to inhabit eastern Africa, and numerous bones of ancestors of monkeys and antelope that now live in forests and wooded areas. Fossilized plant and seed remains also come from varieties still common in African woodlands and forests.

A. ramidus probably ate whatever plants and other foods it could find on the forest and woodland floor, White holds. "Like all hominids, it was an omnivore," he says.

An estimate of the earliest known hominid's body size and the degree to which it relied on two-legged walking must await the discovery of foot, leg, and hip fossils. The scientists plan to return to Aramis in November to look for more hominid remains.

Further study of pollen, carbon isotopes, soil, and fossilized wood and small mammals at Aramis should clarify the nature of the habitat favored by A. ramidus.

White suspects that the common ancestor of humans and apes, as well as the early members of human and ape evolutionary families, all lived in wooded areas. Therefore, the chances of finding fossils for these animals are slim.

"We're in uncharted territory at Aramis for hominids and other mammals, as well as for Earth history," White maintains.