Oldest fossil ape may be human ancestor.

Newly discovered fossils in Uganda from a more than 20-million-year-old, apelike creature, combined with previously unearthed remains, establish the find as the oldest known ancestor of modern humans, apes, and gibbons, a team of researchers now contends.

Two characteristics of the African animal, which weighed between 45 and 90 pounds, solidify its pivotal evolutionary standing, according to the scientists-skeletal signs of a relatively upright posture and a shoulder that allowed for hoisting the body and swinging through the trees.

The ancient ape rates a new genus and species, Morotopithecus bishopi, assert anthropologist Daniel L. Gebo of Northern Illinois University in DeKalb and his colleagues.

"This is the earliest evidence for a significantly apelike body plan in the primate fossil record," says anatomist Laura MacLatchy of the State University of New York at Stony Brook, a member of Gebo's team.

After finding parts of its face, teeth, and spinal column at Uganda's Moroto site in the early 1960s, researchers classified the prehistoric animal as any of several established fossil apes that lived in Africa between 25 million and 15 million years ago. These include Proconsul and Afropithecus.

Excavations in 1994 and 1995 by Gebo's group yielded additional remains from the same animal's upper leg and shoulder. The leg bones are extremely thick and would have allowed for cautious climbing, as seen in living orangutans and lorises, the team conte nds. The shoulder shows evidence of enhanced mobility, suggesting that Morotopithecus used its arms to hang or swing from branches.

Prior studies of its vertebrae had indicated that it had a relatively short, stiff back, consistent with knuckle-walking while on the ground.

Together, the old and new Ugandan material suggests a creature whose anatomy differs markedly from that of other fossil apes, MacLatchy says.

Analysis of the decay rate of a radioactive form of the element argon in volcanic ash at Moroto dates fossils at the site to at least 20.6 million years ago, she and her coworkers report in the April 18 Science.

Knowledge of the anatomy and evolutionary relationships of fossil apes that inhabited Africa and Asia from 25 million to 10 million years ago remains fragmentary, and Morotopithecus has both backers and doubters.

Monte L. McCrossin of Southern Illinois University in Carbondale suspects that the Ugandan specimens belong to Afropithecus, which has been found in western Kenya and dates to about 17 million years ago.

Additional shoulder fossils will be needed to certify the Moroto ape as an arm-swinger, McCrossin argues.

He and his colleagues have found fossils of a 15-million-year-old African ape, Kenyapithecus (see p. 240). That creature was more closely related to living African apes and humans, in their view.

William J. Sanders of the University of Michigan in Ann Arbor tentatively accepts the new classification of the Ugandan finds. Sanders had previously studied vertebrae from Morotopithecus.

Either the creature is a tree-climbing ancestor of living apes, humans, and gibbons, or it died out after evolving several features in common with later apes, Sanders remarks.

"The early age assigned to Morotopithecus requires us to interpret these fossils cautiously," the Michigan anthropologist adds.