Scientists are postulating that a 10 million year old jawbone unearthed in Kenya's northern Nakali region may belong to the so-called evolutionary "missing link" - the common ancestor of African great apes and humans.

The fragment, along with 11 teeth, was discovered in 2005 in volcanic mud flow deposits by Japanese and Kenyan researchers. They have dubbed the new species - described by Reuters as "somewhere between the size of a female gorilla and a female orang-utan" - as Nakalipithecus nakayamai.

Frederick Manthi, senior research scientist at the National Museums of Kenya, declared at a press conference: "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."

Yutaka Kunimatsu at Kyoto University's Primate Research Institute told the news agency: "It could be positioned before the split between gorillas, chimps and humans."

If Nakalipithecus nakayamai is indeed the common ancestor of great apes and humans, the discovery supports the theory that it, and subsequently the great apes and humans, evolved entirely in Africa. An alternative theory - based on the fact that "so little fossil evidence in Africa dating between seven to 13 million years ago" - suggests accordingly that "the last common ancestor left Africa for Europe and Asia, and then returned later".

Kunimatsu elaborated: "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."

However, the team warned further evidence was required before a definitive explanation could be reached. Manthi admitted: "We have to find more fossils from a cross-section of sites to sustain that particular theory."

Kunimatsu added: "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."

What the researchers have deduced, though, is Nakalipithecus nakayamai's diet. Kunimatsu concluded: "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."

The team's findings are published in the latest issue of Proceedings of the National Academy of Sciences. ®

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