"Between seven and ten million years ago, the environment in Africa changed," he said. "It became more open and barren."
Material from the dig probably came from both males and females, he added.
"We discovered most of the specimens within a very small area, probably about 10 to 20 square meters [110 to 220 square feet], so maybe they came from a group."
The team also found the remains of many other primates at the Nakali site, Kunimatsu said, showing that a richer diversity of monkeys and apes than was previously thought survived late into the Miocene epoch, which lasted from about 23 to 5 million years ago.
The Great Migration?
The new finds are a rare glimpse into Africa's recent fossil record, where a gap between around 12 million years ago and the present has clouded human ancestral origins.
"We have almost nothing with which to understand the divergence of the African great apes and humans," Kunimatsu said.
That gap has prompted some researchers to suggest that hominid apes left Africa for Europe and Asia about 20 million years ago, returning much later.
But the Nakali jawbone complements other recent African hominid discoveries in casting doubt on that theory, said Tim White, director of the Human Evolution Research Center at the University of California, Berkeley.
Though its relationship to Nakalipithecus has not yet been established, the "into Africa" proponents "have some explaining to do, now that the African record is starting to fill up," White said.
Study co-leader Kunimatsu said the discovery hinted that the migration may even have occurred in the opposite direction.
"It is highly probable that large-bodied hominids survived through the middle to late Miocene in Africa, giving rise to the last common ancestor of African great apes and humans," he said.
Now the search is on for additional fossils that might solve the puzzle definitively.
"The great news is we now have places and time periods to look in. We have [African] sediments with vertebrate fossils, and they're not easy to find—they are very rare in that time period," U.C. Berkeley's White said.
"The real hope, of course, is that they will find ... pieces of the rest of the body—leg bones, arm bones, that sort of thing."
The greatest prize, Kunimatsu said, would be to find the link between chimps and humans.
"That's the most interesting link. We would like to know, finally, how humans and chimpanzees diverged from each other."
"Evolution is an unbroken chain of links, and the more of those links we can recover, the more we're going to understand the chain," White added. "We still have pretty major gaps—not because the chain was broken, but because we haven't found the links yet.
"We are going to get closer and closer to that last common ancestor," he said. "And we are already pretty darn close. We're down there knocking on that door, five or six million years ago."
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