WASHINGTON -- A baboon-sized ape that lived in East Africa about 15 million years ago may have been among the first primates to leave the treetops and live on the ground, a key step in the evolutionary path that theoretically led eventually to humans.
The fossilized partial skeleton of the animal is distinctly different from other ancient apes, prompting researcher Steve Ward and his colleagues to identify it as the only member of a new ape genus they call Equatorius.
Ward, a primate anatomy expert at Northeastern Ohio Universities College of Medicine in Rootsville, Ohio, is senior author of a study to be published Friday in the journal Science.
Hand, finger, arm and shoulder bones show strong evidence that Equatorius "spent a lot of time on the ground," said Ward in a telephone interview. "This is the first time that we see evidence of that in the fossil record of apes."
He called this "a very important finding" in understanding the evolutionary steps leading eventually to modern primates that spend almost no time in trees and are able to walk upright.
Equatorius is not thought to be a direct ancestor of humans or of modern apes, said Ward. The animal probably was an evolutionary dead end, a species that disappeared after about 1« million years, but it provides important evidence of a poorly understood era that Ward calls "the golden age of ape evolution."
"There was a large group of primitive apes that appeared in East Africa sometime before 23 million years ago," said Ward. "They exploded in an evolutionary sense into many, many genera and species."
Equatorius, said Ward, was a late representative of that era and was probably among the first to abandon the tree top home of earlier apes species.
Ape species around 10 to 14 million years ago moved from Africa into Europe and Asia where they initially thrived, but eventually most became extinct.
The remains of Equatorius were found in 1993 by a fossil hunter who discovered a single tooth projecting from a solid rock wall in the Tugen Hills of north central Kenya.
Careful excavation of the site uncovered a jaw with more teeth, along with bones from the spine, arms, shoulders, wrists, fingers and hand. Ward said the specimen is the most complete ape skeleton yet found from a critical era of ape evolution.
Equatorius was about the size of an adult male baboon. Its arms and legs were of about equal length. Powerful gripping hands and feet suggest that although it spent much of its life on the ground, it was also adept at tree-climbing, said Ward.
At first it was thought the Equatorius specimen belonged in the genus of another ancient ape called Kenyapithecus. There are only small collections of fossils for this genus and experts thought it contained two species, K. wickeri and K. Africanus.
The sparse fossils from the two species were "a mixed bag", said Ward, and "garbled the signals we were getting from the bones" about that era of ape evolution.
But he said a comparison of the more complete skeleton of Equatorius makes it clear that K. Africanus really belongs in the new genus of Equatorius.
"By recognizing Equatorius for what it is, it helps us to understand a lot about the early ape evolution," said Ward.
It is now clear, Ward said, that Equatorius is more primitive and preceded K. Wickeri in evolutionary history by 1 to 1« million years.
Other paleontologists praised the new finding, but some are not ready to accept that Equatorius is in a different genus than Kenyapithecus.
"This discovery tells us much more about the diversity of animals from that period of ape evolution," said Eric Delson, a paleontology professor at Lehman College in New York and a researcher at the American Museum of Natural History.
Delson said Equatorius is the most complete fossil collection to indicate that apes of that era lived on the ground. There have been suggestions from other fossils, but that evidence was only "fragmentary," he said.
However, Delson said he is not persuaded that Kenyapithecus and Equatorius belong in separate genera. And, in any case, it is not clear if either of these ancient apes are part of the evolutionary lineage that led eventually to humans and modern apes, he said.
Many theorists believe the primate lineage that eventually evolved into humans and modern apes arose from an unknown common ape ancestor sometime around 6 to 7 million years ago in Africa. However, very few African primate fossils have been found for the period from 14 million to 6 million years ago, a time Ward calls the "Miocene gap."
"Those are the key fossils that we need to find," Ward said.