By Pallab Ghosh
Fossils from Northern Kenya show that a new species of human lived two million years ago, researchers say.
The discoveries suggests that at least three distinct species of humans co-existed in Africa.
The research adds to a growing body of evidence that runs counter to the popular perception that there was a linear evolution from early primates to modern humans.
The research has been published in the journal Nature.
Anthropologists have discovered three human fossils that are between 1.78 and 1.95 million years old. The specimens are of a face and two jawbones with teeth.
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Nature was developing different human prototypes only one of which, our species, was ultimately successful”
Professor Chris Stringer
Natural History Museum, London
The finds back the view that a skull found in 1972 is of a separate species of human, known as Homo rudolfensis. The skull was markedly different to any others from that time. It had a relatively large brain and long flat face.
But for 40 years the skull was the only example of the creature and so it was impossible to say for sure whether the individual was an unusual specimen or a member of a new species.
With the discovery of the three new fossils researchers can say with more certainty that H.rudolfensis really was a separate type of human that existed around two million years ago alongside other species of humans.
For a long time the oldest known human ancestor was thought to be a primitive species, dating back 1.8 million years ago called Homo erectus. They had small heads, prominent brows and stood upright.
But 50 years ago, researchers discovered an even older and more primitive species of human called Homo habilis that may have coexisted with H. erectus. Now it seems H. rudolfensis was around too and raises the distinct possibility that many other species of human also existed at the time
This find is the latest in a growing body of evidence that challenges the view that our species evolved in a smooth linear progression from our primate ancestors. Instead, according to Dr Meave Leakey of the Turkana Basin Institute in Nairobi, who led the research the find shows that there was a diversity early on in the evolution of our species.
"Our past was a diverse past," she told BBC News, "our species was evolving in the same way that other species of animals evolved. There was nothing unique about us until we began to make sophisticated stone tools."
In other groups of animals many different species evolve, each with new traits, such as plumage, or webbed feet. If the new trait is better suited to the environment then the new species thrives, if not it becomes extinct. According to Professor Chris Stringer of the Natural History Museum in London, fossil evidence is increasingly suggesting that human evolution followed the same pattern.
The march of progress had many dead ends
"Humans seem to have been evolving in different ways in different regions. It was almost as if nature was developing different human prototypes with different attributes, only one of which, an ancestor of our species, was ultimately successful in evolutionary terms," he said.
According to Dr Leakey, the growing body of evidence to suggest that humans evolved in the same way as other animals shows that "evolution really does work".
"It leads to amazing adaptions and amazing species and we are one of them," she said.