The particularly small Homo erectus skull, shown from above with a large skull of the same species (from Olduvai, Tanzania), to demonstrate the gorilla-like size variation.

Credit: National Museums of Kenya/F. Spoor and J. Reader.

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PARIS: The discovery of two fossils has challenged the belief that our early human ancestor Homo erectus evolved from Homo habilis and suggests they co-existed.

The finds, on the eastern bank of Lake Turkana in Kenya – detailed today in the British journal Nature – are evidence that the two species may have intermingled for some 50,000 years in East Africa.

The team that found the remains was led by mother-daughter team Louise and Meave Leakey of the famed Kenyan anthropological family who have uncovered a host of critical human and hominid remains in east Africa.

Stirring controversy

One of the fossils is a 1.4-million-year-old upper jaw bone of H.habilis, which is the most recent fossil of the species known. The second is a remarkably well preserved skull of H.erectus, which paradoxically dates back even further, to some 1.55 million years ago.

"What is truly striking about this fossil is its size," said Fred Spoor of University College London in the U.K., and one of the paper's authors. "It's the smallest Homo erectus found anywhere in the world."

The recent discovery of the two fossils has created a stir among academics tracing humankind's roots, as it challenges the presumed evolutionary timeline of the species: H.habilis to H.erectus to Homo sapiens.

"Their co-existence makes it unlikely that Homo erectus evolved from Homo habilis," said Meave Leakey of the Koobi Fora Research Project at the National museums of Kenya in Nairobi. "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."

H.habilis is a species of the genus Homo, which is thought to have lived from approximately 2.5 million to 1.8 million years ago.

Great variation

The name, literally "clothed man" was given because crude stone tools were found near the sites of their remains. They were thought to have been succeeded by the H.erectus, or upright man, whose remains were first found in Asia. But later their fossilised remains ranging from between 1.8- and one-million-years-old were unearthed in Africa, and Europe as well as in Indonesia, Vietnam, and China.

H.erectus is thought to be an important human ancestor because it may have been the first to leave Africa.

The weaker teeth and jawbones of H.erectus suggest a food regime including more meat, animal fat and softer food unlike the H.habilis who largely fed on nuts and tubers

The variation in the skull size of the East African H.erectus fossils – from the petite new skull to a large specimen previously discovered in neighbouring Tanzania – points to sexual dimorphism, or a large difference in form between male and female individuals of the same species.

"In gorillas, males are much larger than females and this sexual dimorphism is related to their strategy of having multiple mates," said co-author and anthropologist Susan Anton of New York University In New York City, USA."The new Kenyan fossil suggests that, contrary to common belief, this may have been true of Homo erectus as well."

The authors point out that like gorillas and chimpanzees, who currently cohabit in Africa without entering into conflict, the two genus species had likely lived side by side on the continent for nearly half a million years.