A Kenyan-based research team has found ancient evidence that undermines the idea that humans evolved in a more or less straight line from Homo habilis to Homo erectus to Homo sapiens.

Meave Leakey, a paleontologist with the Koobi Fora Research Project, and her colleagues have found fossil evidence suggesting that H. habilis and H. erectus lived at the same time.

Koobi Fora Research Project team member Frederick Manthi with the H. erectus skull he found.
(M.G. Leakey/Koobi Fora Research Project)

"Their co-existence makes it unlikely that Homo erectus evolved from Homo habilis, " Leakey said in a release Wednesday. The researchers believe that both species originated between two and three million years ago.

"The fact that they stayed separate as individual species for a long time suggests that they had their own ecological niche, thus avoiding direct competition," Leakey said.

Their research, to be published in the journal Nature on Thursday, started with the discovery of an upper jawbone of H. habilis dating from 1.44 million years ago, more recent than any other fossils of that species.

"This late-survivor shows that Homo habilis and Homo erectus lived side by side in eastern Africa for nearly half a million years," the release said.

H. habilis was probably tougher than H. erectus, as shown by their teeth and jaws. H. habilis probably ate more vegetation, while H. erectus consumed more animal meat and fats.

Erectus may be further from humans than thought

The researchers also found a second fossil, described as "an exquisitely preserved skull of Homo erectus," that dated to about 1.55 million years ago.

It was especially small, raising the possibility that H. erectus was like the gorilla, in that the females were much smaller than the males. The size difference is related to a strategy of having multiple mates.

"The new Kenyan fossil suggests that, contrary to common belief, this may have been true of Homo erectus as well," co-author Susan Antón said.

If that's the case, the researchers speculated that H. erectus "was not as human-like as once thought" because big size differences between the sexes is deemed to be a primitive feature of human evolution.

The scientists suggested that modern humans, H. sapiens, still evolved from H. erectus, although there may have been an intermediate species, which many researchers have recognized as a separate species.

That happened less than a million years ago in Africa.

Both fossils were found in 2000 east of Lake Turkana in Kenya.

The study's authors include Fred Spoor, Meave Leakey, Patrick Gathogo, Frank Brown, Susan Antón, Ian McDougall, Christopher Kiarie, Frederick Manthi and Louise Leakey.

The Koobi Fora Research Project is affiliated with the National Museums of Kenya.