By Helen Briggs
Science reporter, BBC News
The fossil of an ape that lived 10 million years ago could hold clues to the dawn of human evolution.
Several details of its dentition place the ape in a class of its own
The ancient ape appears to be a close relative of the last common ancestor of gorillas, chimps and humans, according to a Kenyan-Japanese team.
The lower jaw bone and 11 teeth, found in volcanic mud deposits in northern Kenya, are unveiled in the Proceedings of the National Academy of Sciences.
Fossils from this critical time period in primate evolution are very rare.
Genetic studies suggest that the ancestors of humans and chimpanzees went along their separate pathways of evolution about five million to seven million years ago.
But until now there has been very little fossil evidence unearthed in Africa from the middle-to-late Miocene Epoch (12 to 7 million years ago), when gorillas, chimps and humans shared a common ancestor.
This has led some experts to propose that apes migrated out of Africa to Europe and Asia, only returning much later.
Study leader Yutaka Kunimatsu of Kyoto University's Primate Research Institute said the latest findings contradicted the so-called "out of Europe" theory.
"Now, we have a good candidate in Africa," he said. "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."
The team plans to return to the eastern edge of the Rift valley in Kenya next year to search for more fossils.
From the evidence available - a partial lower jaw and 11 teeth - they have pieced together clues to the ape's dentition and diet.
Two of the fossil teeth
"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," said Dr Kunimatsu.
"We only have some jaw fragments and some teeth... but we hope to find other body parts in our future research," he added.
Professor Fred Spoor, an anatomist at University College, London, UK, said that knowledge of great ape evolution had been hampered by a lack of fossil evidence.
"We have an extra piece in the puzzle of what was going in ape evolution," he told BBC News.
"Perhaps we might start to understand a little more about the common ancestor of African great apes and humans and whether it lived in Africa or Eurasia."
The new species has been christened Nakalipithecus nakayamai. It is the second rare fossil discovery from Africa announced in recent months.
In August, an Ethiopian-Japanese team announced the discovery in Ethiopia of 10-million-year-old teeth from a previously unknown species of great ape, Chororapithecus abyssinicus.