A 15 million-year-old skeleton unearthed in Kenya belongs to a previously unknown animal type that may be a sort of great-uncle of our oldest direct relative, scientists say.

The recent find predates by several million years the long-sought "holy grail" of paleoanthropology: the last common ancestor that great apes and humans had before each of those groups diverged and developed separately.

But the new and unexpectedly copious remains are an important step in the search, apparently ruling out one leading candidate for that illustrious evolutionary position, and narrowing the range of characteristics for which fossil-hunters will look.

The partial skeleton, extracted from a stone formation after a protruding tooth first signaled its presence in 1993, came from an animal approximately "the equivalent of a large modern male baboon," said co-discoverer Steve Ward of Northeastern Ohio Universities College of Medicine and Kent State University. It weighed about 60 pounds and stood 4 to 5 feet tall with a long, flexible spine and strong grasping hands.

That creature arose during a key epoch in pre-human evolution called the Middle Miocene (about 16 million to 11 million years ago) in which the lush rain forests of Africa gradually gave way to more open plains and savannas. During this period, the animals whose lineage would eventually produce gibbons, great apes and humans first ventured down from treetops and spent a little time on the ground.

Very few fossils have been recovered from this crucial interval. The best known are some teeth and jaws found in western Kenya nearly 30 years ago by renowned paleontologist Louis Leakey. He named them Kenyapithecus ("Kenya ape") and separated them into two distinct species: K. wickeri and K. africanus. Both have long been considered as possible candidates for humankind's oldest ancestor.

(Small subgroups of animals are classified by genus and species. In our group, Homo sapiens, Homo is the genus and sapiens is the species. Other species of Homo, such as H. erectus and H. neanderthalensis, are now extinct.)

The new find, reported by Ward and colleagues in today's issue of the journal Science, contains many more associated parts than any of the Kenyapithecus specimens, including portions of hand, collarbone, shoulder blade, arm bones and spine. That allowed the researchers to determine that their fossils represent a new genus they call Equatorius.

The researchers argue that Equatorius is more primitive than K. wickeri but extremely similar to K. africanus, which they reclassify as a form of Equatorius.

That leaves only one major candidate at present for the type of animal that evolved into our first ancestor, although more will doubtless appear as research continues. "It is possible," said Ward, that the younger "Kenyapithecus is a potential ancestor of later great apes and humans. Equatorius I would divorce from that ancestry," he said. Moreover, "we have no idea whether Equatorius had a role in the appearance of Kenyapithecus."

Also unknown is whether the creatures that produced our first forebears arose exclusively in Africa, or whether they might have traveled elsewhere first. Kenyapithecus-like fossils aged 16 million to 14 million years have been found as far north as Austria and as far east as Turkey.

That, said Ward, is "one of the really exciting questions: Did [some of these ape-like creatures, called hominoids] leave East Africa and go through Asia Minor and East Asia, while something remained in Africa that evolved into apes and humans? Or was there this great radiation after 12 million years ago, and then something migrated back into Africa and then evolved into humans?"

It may be quite a while before the issue gets clearer, said William H. Kimbel, science director of the Institute of Human Origins at Arizona State University. "How exactly K. wickeri is related to later forms is still unspecifiable and will remain debated" because, owing to the paucity of fossil evidence, "K. wickeri, too, is still poorly known."

In the meantime, said paleoanthropologist Bernard Wood of George Washington University and the Smithsonian Institution's Human Origins Program, the new specimen will "tidy up the Miocene hominoid cupboard" and help experts "make much more sense of the limited material they were struggling to come to terms with."

CAPTION: PIECES OF THE PUZZLE (This graphic was not available)