Experts find jawbone of pre-human great ape in Kenya

NAIROBI (Reuters) - Researchers unveiled a 10-million-year-old jaw bone on Tuesday they believe belonged to a new species of great ape that could be the last common ancestor of gorillas, chimpanzees and humans.

The Kenyan and Japanese team found the fragment, dating back to between 9.8 and 9.88 million years, in 2005 along with 11 teeth. The fossils were unearthed in volcanic mud flow deposits in the northern Nakali region of Kenya.

The species -- somewhere between the size of a female gorilla and a female orangutan -- may prove to be the "missing link", the key step that split the evolutionary chains of humans and other primates, Kenyan scientists said.

"Based on this particular discovery, we can comfortably say we are approaching the point at which we can pin down the so-called missing link," Frederick Manthi, senior research scientist at the National Museums of Kenya, told reporters.

"We have to find more fossils from a cross-section of sites to sustain that particular theory," he added, speaking at a desk where the approximately four-inch sliver of bone was displayed alongside human and gorilla skulls.

It was the latest important finding in east Africa's Rift Valley -- a region long regarded as the "cradle of humankind".

"The teeth were covered in thick enamel and the caps were low and voluminous, suggesting that the diet of this ape consisted of a considerable amount of hard objects, like nuts or seeds, and fruit," Yutaka Kunimatsu at Kyoto University's Primate Research Institute said in a telephone interview.

"It could be positioned before the split between gorillas, chimps and humans," he added.

However, it was hard to determine what the new species, named Nakalipithecus nakayamai, looked like.

"We only have some jaw fragments and some teeth ... but we hope to find other body parts in our future research. We plan to go back next year. We will try to find bones below the neck to tell us how the animal moved," Kunimatsu said.

Published in the latest issue of the Proceedings of the National Academy of Sciences, the finding is significant as it gives credence to the theory that the evolution from ape to man may have taken place entirely in Africa.

Prior to this finding, there had been so little fossil evidence in Africa dating between 7 to 13 million years ago that some experts began to surmise that the last common ancestor left Africa for Europe and Asia, and then returned later.

"Now, we have a good candidate in Africa. We do not need to think the common ancestor came back from Eurasia to Africa. I think it is more likely the common ancestor evolved from the apes in the Miocene in Africa," Kunimatsu said.

The Miocene is a period extending from 23.03 million to 5.33 million years ago.

"Some apes (then) left Africa and migrated to Eurasia. They then became orangutans in Southeast Asia. Today's orangutan evolved from the apes that left Africa," he said.

(Additional reporting by Tan Ee Lyn in Hong Kong; Editing by Caroline Drees)