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.