The family tree of humanity as we know it - Homo sapiens - isn't as straightforward as "one species gave way to another." New evidence suggests that at least two different Homo species lived in Kenya about 2 million years ago.

Scientists report in the journal Nature that they have linked recently discovered fossils with a controversial cranium found in 1972 in Kenya. They believe these new remnants belonged to the same species as the skull, which has been dubbed Homo rudolfensis. The study is led by prominent paleontologist Meave Leakey.

The Homo rudolfensis skull, found near Lake Turkana, has a bigger brain case and a flatter face than specimens of Homo habilis, the other species of the Homo genus that appears to have lived around that time. Homo habilis is thought to have been a toolmaker because its hand bones were found next to stone tools.

It has been evident for a long time that there were several contemporaneous Homo species present in this area of Kenya called Koobi Fora, said Ian Tattersall, paleoanthropologist and curator at the American Museum of Natural History in New York, who wasn't involved in the study. This study helps confirm that, he said.

But some scientists not involved in the study said there is not enough evidence to claim that the new findings, together with the 1972 skull, represent a distinct Homo species.

Lee Berger, paleoanthropologist at the University of the Witwatersrand in Johannesburg, South Africa, called this argument based on associations of a few fossils "weak." He said in an e-mail that the fossils should also be compared to other potential human ancestors, such as Australopithecus africanus and Australopithecus sediba.

"East Africa is not an island," Berger said. "One must use the whole continent's evidence for human origins in such an important argument as the question of the origins of the genus Homo."

Ancient fossils question human family tree

The new fossils consist of a face, lower jaw and fragmentary lower jaw of different ages, none of which is quite as old as the first Homo rudolfensis specimen. These new fossils are believed to be 1.83 million and 1.95 million years old. They include a face that is "incredibly flat," as there is a straight line from eye sockets to where incisor teeth would have been, said study co-author Fred Spoor of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. The cheekbones on either side of the nose are far forward, contributing to that flatness. The incisor teeth are in a straight line, he said.

All of this information allowed the researchers to do a virtual reconstruction of what the head of a member of this mysterious species would have looked like.

Leakey, of the Turkana Basin Institute in Nairobi, Kenya, said the differences between this species and what we've known previously are "more extreme than you would expect" and are not merely regular variations of the same species. The skull's straight profile is distinctive, she said.

The environment in this area is very windy, sandy and extremely hot today, but it would have been less hostile 2 million years ago, Leakey said. Lake Turkana was a lot larger, and there was more vegetation, so there was more opportunity to eat different types of food. And even if Homo rudolfensis and Homo habilis lived around the same time, they may not have known one another.

"We can’t say they were standing next to each other and could shake hands," Spoor said.

Whether we descended from the species of these ancient creatures is unknown. There could be some other, yet undiscovered species from around this time that is a more probable ancestor.

Bernard Wood predicts, in an accompanying article in Nature, that by 2064, "researchers will view our current hypotheses about this phase of human evolution as remarkably simplistic."