By Ann Gibbons, ScienceNOW

After 40 years of searching, researchers can finally put a new face on a mysterious human ancestor whose skull was discovered 40 years ago in Kenya. The find is giving scientists a better look at an enigmatic species that was alive soon after the dawn of our genus Homo about 2 million years ago. It also shows that there were several species of Homo present 1.78 million to 2.03 million years ago in the Rift Valley of Africa, and that they probably had to adapt in different ways to coexist.

Ever since paleoanthropologist Meave Leakey got her first look at the skull of a strange, new kind of human ancestor in 1972 at Koobi Fora, famed fossil beds on the east side of Africa’s Lake Turkana where several different species of human ancestors have been found since the 1960s, she and others have searched in vain for more members of this species. The 2-million-year-old skull had a big brain that made it a member of our own genus Homo. But its long, flat face and other features distinguish it from the other two members of early Homo known at the time, so many researchers thought of it as a new species, Homo rudolfensis. Some questioned whether it was a new species, however, or just an unusual member of Homo habilis, which lived 2.3 million to 1.4 million years ago in East Africa. "It was always an anomaly," says Leakey, of the Turkana Basin Institute in Kenya and Stony Brook University in New York. "We always knew we had to find more of it."

When fossil hunters found the well-preserved fossilized bones of the mid-face and teeth of a juvenile protruding out of rock in 2008, it "was really exciting," Leakey says. The face looked like a small "pocket version" of the original H. rudolfensis skull, known as KNM-ER 1470 – with an unusually flat visage, as opposed to the more jutting upper jaw found in H. habilis, says co-author Fred Spoor of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. Its small size also ruled out the older view that H. rudolfensis skulls were invariably larger than those of H. habilis or that the larger specimens were males and the smaller were females.

With the discovery of a remarkably complete lower jaw in 2009, the team got an even better look at this elusive species (1470 did not have a lower jaw). The jaw and the new face revealed that H. rudolfensis had an unusual, U-shaped palate, with canines facing the front of the jaw rather than aligned on the sides in a V-shaped palate, as in H. habilis. This suggests a significant developmental difference between two species, rather than variation within one species, Spoor says.

The new fossils, described online today in Nature, were all found on the Karari Ridge of Koobi Fora, within 10 kilometers of the fossil beds where the 1470 skull was found – and within the same region where fossils of H. habilis and H. erectus have been discovered. Some researchers still think that 1470 and the new fossils could be members of the same taxa (or biological group), H. habilis, because so few fossils of H. habilis have been found that "we still don't understand H. habilis," says paleoanthropologist Timothy White of the University of California, Berkeley.

But others say the new material "is really good evidence that there has to be *H. erectus *plus two or three other taxa," paleoanthropologist Bernard Wood of George Washington University in Washington, D.C., says, who was not involved in the new work. And if three species did coexist at roughly the same time and place, how did they compete with each other for food and sleeping sites? Did they eat different foods, inhabit different terrain, or use stone tools in various ways? Now it’s time to "think about hypotheses to explain how they might have divided up their world," says paleoanthropologist William Kimbel of Arizona State University, Tempe.

This story provided by ScienceNOW, the daily online news service of the journal Science.