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PostPosted: Thu Mar 25, 2010 12:08 am 
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Fingerbone points to a new type of human who fell off the family tree 30,000 years ago

http://www.timesonline.co.uk/tol/news/s ... 074975.ece

A new species of ancient human that lived side by side with Homo sapiens and Neanderthals as recently as 30,000 years ago has been discovered, rewriting the history of humanity’s spread around the world.

The creature, nicknamed “X-woman” by researchers, is the first human cousin to be identified purely from a DNA sample — extracted from a bone fragment of a little finger found two years ago in a Siberian cave.

The discovery, which has amazed and delighted scientists, shows that the human family was more diverse in prehistoric times than had been appreciated. It suggests that many different kinds of humans left Africa separately and then thrived for thousands of years, before Homo sapiens emerged as the sole survivor.

Only six years ago, only two hominin (ancient human) species alive 30,000 years ago were known to science: the Neanderthals and us. That number has since doubled, with the discovery of X-woman and the “hobbit” — the diminutive Homo floresiensis that lived on the Indonesian island of Flores until about 18,000 years ago.

“It looks like Nature was experimenting in how to be human, and we’re the last survivors,” said Professor Chris Stringer, head of human origins at the Natural History Museum in London.

The discovery of X-woman, published today in the journal Nature, also highlights the growing power of DNA sequencing to explain the human family’s past. The fossil fragment was recognised as a separate species only from its genetic code.

The section of finger bone was found in 2008 in Denisova cave, in the Altai mountains of southern Siberia, and dated to between 48,000 and 30,000 years old. Modern humans and Neanderthals are known to have lived in the same region at the same time, and it was initially assumed to belong to one of these species.

However, when a small sample of DNA was extracted and analysed by Johannes Krause, of the Max Planck Institute in Leipzig, Germany, it contained striking anomalies.

“It was a sequence that seemed in some ways human, but very distinct from humans and Neanderthals,” he said. “All the tests suggested it was a new hominin lineage.”

Dr Krause then rang his boss, Professor Svante Pääbo, who was away at a conference in the US, to discuss his findings. “I told him to sit down,” he said.Professor Pääbo said: “It was absolutely amazing, and I thought he was pulling my leg. Whatever carried this DNA out of Africa is some new creature that has not been on our radar screens. It suggests there were perhaps three different families of humans in this area about 40,000 years ago, and also the hobbits in Indonesia.”

The DNA shows that the Denisova creature last shared a common ancestor with modern humans and Neanderthals about one million years ago. This date indicates that the fossil does not belong to Homo erectus, a more archaic species that left Africa about 1.9 million years ago, or to Homo sapiens or Homo neanderthalis, which share a common ancestor from 500,000 years ago.

The initial analysis has looked at the hominin’s mitochondrial DNA, a small part of the genome. The “X-woman” nickname was chosen because mitochondrial DNA is always inherited in the maternal line, though scientists do not yet know whether the individual was male or female. Further analysis of nuclear DNA, which makes up most of the genome, is being conducted. This should provide a more definitive picture of X-woman’s relationship to humans. A formal scientific name will be chosen when this is complete.

Professor Stringer, who was not involved in the research, said it is possible that X-woman is related to other enigmatic fossils found in Asia.

He said: “This new DNA work provides an entirely new way of looking at the still poorly understood evolution of humans in central and eastern Asia.”

Another possibility that cannot yet be ruled out is that X-woman was the result of interbreeding between a male human or Neanderthal and a female from a more archaic human species, which passed its mitochondrial DNA to its descendents. The nuclear DNA analysis should shed light on this.

Little is known of what X-woman might have looked like but the proportions of the bone are similar to those of humans and Neanderthals, indicating a similar body shape. Its size suggests it belonged to a child aged 5 to 7.

The findings point to a previously unknown hominin migration out of Africa. At present, only three such events are definitively known, involving Homo erectus, the ancestors of Neanderthals, and modern humans. The new creature probably left Africa soon after the date of its last shared ancestor with humans and Neanderthals, about a million years ago.

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