American anthropologists have discovered the first chimpanzee fossils, filling one of the biggest gaps in the ancient primate record.

While hominid fossils abound, no one had previously found a fossil belonging to our closest living relative, Sally McBrearty of the University of Connecticut and Nina G. Jablonski of the California Academy of Sciences report in the current issue of Nature.

Unearthed in the Rift Valley, East Africa, in a sediment of the Kapthurin Formation, a region by the shores of Kenya's Lake Baringo, the chimp fossils consist of three teeth — two incisors and a molar — probably belonging to the same individual, a chimp that lived a half a million years ago.

As well as providing new insight into the evolution of chimps, the landmark discovery shatters the widespread belief that humans and chimps did not coexist since they diverged from a common ancestor five to eight million years ago.

The researchers found startling evidence of the cohabitation as they unearthed fossils attributed to Homo erectus or Homo rhodesiensis in the same geologic layer less than a mile away.

Since modern chimp populations are now confined to wooded west and central Africa, whereas most hominid fossils have been found in the semi-arid East African Rift Valley, it has been long speculated that ancient chimps and humans diverged from their common ancestor when hominids left the jungles and moved east to the less wooded grasslands.

"People used to believe that the origin of humans had to do with their leaving the forest and beginning to walk on two legs. Our discovery shows that the forest-savanna dichotomy used to explain the split between the chimpanzee and human lines does not add up," McBrearty, who discovered the fossils, told Discovery News.

McBrearty and Jablonski also found fossilized remains of fish, hippopotami, crocodiles, turtles, gastropods and other moisture-loving animals.

The remains would suggest that 500,000 years ago that chimps and ancient Homo inhabited a wet, wooded area surrounding a lake.

"The environment of the Rift Valley was more diverse than people tend to think ... . It was a bit more wooded than at present, and there were a number of animal species there that are not there now. Humans have had a big impact on the vegetation and the other animals in the last 100 to 1,000 years," McBrearty said.

The researchers estimated that the chimp was seven or eight years old when it died. Grooves on the teeth point to nutritional stress during the chimp's youth.

According to McBrearty, the teeth are quite similar to those of modern chimps. Except for size, they resemble the common chimpanzee ( Pan troglodytes ) rather than the 'pygmy' bonobo chimp (Pan paniscus).

"It's difficult to diagnose species from teeth alone. Because we have absolutely no other fossils of chimpanzees, people have assumed that chimps did not change much over time. In fact, we don't know exactly what chimpanzee ancestors may have looked like, and if there may have been a number of different species in the past that are extinct today," McBrearty said

More chimp fossils may lie in the Rift Valley, McBrearty said. She hopes to resume her search in Kenya in December.

According to anthropologist John Hawks of the University of Wisconsin, Madison, significant developments may follow the discovery.

"I think the interesting thing is the hint from genetics that East African chimpanzees diverged from central African chimpanzees only relatively recently — within the past two or three hundred thousand years. If that is accurate, then these fossils must represent some extinct form of chimpanzee that no longer exists — almost a chimpanzee version of a Neanderthal," Hawks told Discovery News.

http://dsc.discovery.com/news/briefs/20050829/chimpfossil.html