JOSHUA AKEY, a geneticist at Princeton university, grew dissatisfied with the methods used to look for Neanderthal DNA in living people. 

The standard method was built on the assumption that most Africans had no Neanderthal DNA at all.

Dr. Akey and his colleagues figured out a new method, which they call JBDMix, that takes advantage of the fact that relatives share stretches of matching DNA.

Siblings, for example, share many long, identical stretches of DNA. But their children will have fewer identical segments, which also be shorter. Distantly related cousins will have tinier matching segments that require sophisticated methods to uncover.

Dr. Akey and his colleagues figured how to search the DNA of living humans and remains of Neanderthals for those minuscule matching segments.
Then they pinpointed the segments that came from a relatively recent ancestor - and therefore were a sign of interbreeding. 

The scientist searched 2,504 genomes of living humans for segments that matched those in a Neanderthal genome. When the scientist tallied up the results, the results took Dr. Akey by surprise.

The human genome is detailed in units called base pairs, about 3 billion such pairs in total. The scientist found that Europeans on average had 51 million base pairs that matched Neanderthal DNA, and East Asians had 35 million.

Dr. Akey's previous research had indicated that Asians carried far more Neanderthal ancestry than did Europeans.
Africans on average had 17 million base pairs that matched Neanderthal DNA - far more than predicted by the original models describing how humans and Neanderthal interbred.

''That was just so completely opposite to my expectations,'' said Dr. Akey. ''It took a while to convince ourselves that we are finding with this new approach was actually true.''

Looking at the size these shared segments and how common they were around the world, Dr. Akey and his colleagues concluded that some were the result of interbreeding very early in human history.

They believe that a group of modern humans expanded out of Africa perhaps 200,000 years ago and interbred with Neanderthals.

Those modern humans then disappeared.

But Neanderthals who lived after that disappearance inherited some modern human DNA.

Other experts said the new study offered compelling support for earlier hints for this ancient expansion. Last year, for example, a team of scientists reported finding a modern human skull in Greece dating back over 210,000 years.

Despite his hesitation over the analysis of African DNA, Dr. Reich said the new findings do make a strong case that modern humans left Africa much earlier than thought.

''I was on the fence about that, but this paper makes me think it's right,'' he said.

The World Students Society thanks author, Carl Zimmer.


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