The fossil remains of an ape, described in tomorrow's issue of Science, shine new light on an extremely murky phase of ape evolution and force researchers to reexamine the family tree of our distant ancestors.
The fossil--which includes a jaw with teeth and bones from the spine, rib cage, arms, wrists, and hands--was found in 1993 by fossil hunter Boniface Kimeu on a slope in the Tugen Hills of north central Kenya. It is the most complete ape fossil known from about 11 million to 16 million years ago--a crucial transition time when primitive apes looking something like howler monkeys were evolving into the ancestors of the living great apes.
Initially the research team, led by paleoanthropologist Steve Ward of the Northeastern Ohio Universities College of Medicine in Rootstown, Ohio, thought the skeleton belonged to a 15-million-year-old primate called Kenyapithecus, first found by paleontologist Louis Leakey in 1961. Leakey had identified two species, K. wickeri and K. africanus; impressed by its modern-looking teeth, he declared Kenyapithecus to be "a very early ancestor of man himself." Now most paleontologists consider Kenyapithecus to be more primitive. They think the common ancestor of modern great apes and humans may have been a later European ape, which might have evolved from Kenyapithecus.
The new fossil suggests that both sides might be right. It belongs to K. africanus, but with the added detail provided by this fossil, the team realized that K. africanus and K. wickeri were profoundly different apes, belonging to two separate genera. In their report, the researchers therefore rechristen Kenyapithecus africanus as Equatorius africanus. They think Equatorius was the earliest known ape to occasionally leave the treetops for the ground, about 15 million years ago, when the dense African rainforests began to turn into a more open woodland. In Ward's words, it was "an animal about the size of a big adult male baboon, an animal whose arms and legs were about equivalent length, with a long, flexible vertebral column and powerful grasping hands and feet." Meanwhile, K. wickeri shows tantalizing similarities with modern apes. "We're looking at Equatorius on one side of the divide and wickeri on the other," says Ward.
This new view of Kenyapithecus has "very important implications for the whole picture of [ape] evolution," says Carol Ward (no relation to Steve) of the University of Missouri, Columbia. Comparing the two genera shows how ancient apes followed an evolutionary path from primitive to more specialized traits, she says.