September 01, 2005
Thursday

Anthropologists working in East Africa's Great Rift Valley, where Earth's crust split apart millions of years ago, have discovered the first fossil evidence that chimpanzees, our closest genetic relatives, once lived side by side with the direct ancestors of modern humans.

The evidence consists of only three teeth - a single tiny molar and two pointed incisors. But they show clearly that some 500,000 years ago - and probably much further back in time than that - both chimps and a race of human forebears inhabited a verdant region marked by streams filled with fish, turtles and crocodiles and by forests and marshes abounding with buffalo, elephants, hippopotami, monkeys, ancestral antelopes and other animals.

Until now, scientists believed that the creation of the Great Rift Valley about 15 million years ago - caused when Earth's vast crustal plates began splitting chunks of East Africa apart - would have confined the chimps to more verdant areas well west of the region. But it appears from the findings that the land to the east was both forested and well-watered, and that both chimps and our human ancestors thrived together there.

The discovery could prove beneficial as scientists try to shed new light on the evolution of both humans and their primate relatives.

Nina G. Jablonski, chair of anthropology at the California Academy of Sciences in San Francisco, and Sally McBrearty, of the University of Connecticut, reported their findings in the latest issue of the scientific journal Nature.

Scientists have long believed that the ape and human lines - both known as hominids - diverged from an unknown common ancestor at some time between 5 million and 8 million years ago.

"The chimps are part of our shared evolution," says Jablonski.

McBrearty and her team of fossil-hunters made their discovery in a region of Kenya where they have been working for more than a decade. The region, marked by signs of ancient volcanic activity and known as the Kapthurin formation, is by the shores of Lake Baringo, about 180 miles north of Nairobi, the Kenyan capital.

They turned the chimp teeth and other animal fossils, as well as the stone tools of the human ancestors known as Homo erectus, over to Alan Deino of the Berkeley Geochronology Center, who dated them at 510,000 to 512,000 years old.

"These fossils show that humans and chimps lived in broadly similar environments in the Middle Pleistocene," said McBrearty. "To me, that suggests they must have done so in the distant past, and we know from the geology that there was a lake there, as well as streams and a freshwater spring nearby. In fact, the habitat is not that different from the shore of Lake Baringo today, if there were no people living there."

McBrearty said she and her crew came upon a single, tiny tooth that looked like a molar, while hunting for bones and artifacts in June 2004 during a routine annual prospecting trip. Then last January they found the fossils of two chimp incisors - their front biting teeth.

When she first saw the fossils, McBrearty said, she thought they must have come from some kind of nonhuman primate, but they seemed much too large to have been the teeth of ancient monkeys, and its enamel was too thin. So, she figured they might have come from an ape of some kind and set them aside for Jablonski, an expert on extinct primates, who traveled to the National Museum of Kenya in Nairobi to examine them.

"I must say that chimp fossils were about the last thing I would have expected to find there," McBrearty said.

Jablonski said in her office at the Academy in San Francisco this week that when she first saw the three fossils she realized they were too small to have come from humans - not even from the pre-human tool-using primates known as Homo erectus whose fossilized bones and tools McBrearty has been studying for years.

"From my own work in Asia," Jablonski said, "I thought they might have come from gibbons, but I knew that no gibbons have ever existed in Africa, so I borrowed the skeletons of modern chimps from Kenya's national museum, compared them meticulously with the fossil teeth, and determined they were indeed teeth from Pan troglodytes - the chimpanzee."

That realization "was a real ah hah! moment," Jablonski recalled.

It led her and McBrearty to ponder the light it might shed on the separate evolution of both species - particularly on the way humans evolved to walk upright while chimpanzees and other nonhuman primates remained basically four-legged.

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