Neanderthals, Humans Interbred, DNA Proves

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- A newly mapped Neanderthal genome provides strong evidence that humans and Neanderthals interbred.

- Between 1-4 percent of the DNA of many humans living today likely came from Neanderthals.

- People of European and Asian heritage are most likely to carry the Neanderthal genes.

It's official: Most of us are part Neanderthal. The first draft sequence of the Neanderthal genome has provided the strongest evidence yet that modern humans and Neanderthals interbred and that all non-Africans today have Neanderthal gene fragments in their genetic codes.

Although the Neanderthal contribution to the DNA of these individuals is estimated at being just one to four percent of the total, the finding, published in the latest issue of the journal Science, helps to resolve the long-standing controversy over whether or not humans mated with Neanderthals when the two groups encountered each other outside of Africa.

It also gives new life to Neanderthals that, as a species, went extinct 30,000 years ago.

"Neanderthals live on in non-Africans," co-author David Reich told Discovery News. "At least some Neanderthals were absorbed into the modern human population."

Reich is an associate professor of genetics at Harvard University who also serves as a population geneticist at the Broad Institute of MIT and Harvard.

He and his colleagues analyzed over one billion DNA fragments taken from Neanderthal bones -- dating to approximately 38,000 years ago -- found in Croatia, Germany, Russia and Spain.

Although 95 percent of the fragments consisted of bacteria and microorganisms that colonized the Neanderthal remains, special DNA isolation and anti-contamination measures enabled the scientists to piece together over 60 percent of the entire Neanderthal genome.

The researchers next compared the Neanderthal DNA to samples taken from present-day humans in southern Africa, western Africa, China, France and Papua New Guinea.

One of the first determinations concerned the point at which humans diverged from their common ancestor with Neanderthals, who lived in much of Europe and western Asia before they went extinct.

"According to our results, the ancestors of Neanderthals and modern humans went their separate ways about 400,000 years ago," said co-author Jim Mullikin, a computational geneticist at the National Human Genome Research Institute.

The new data, however, also supports that Neanderthals and humans encountered each other again around 45,000 to 80,000 years ago in the Middle East, when humans migrating out of Africa likely met Neanderthals who were already living there.

The meeting must not have been too unpleasant, since the researchers believe the interbreeding happened at this place and estimated time.

Supporting that conclusion is the fact that the Neanderthal DNA more closely matched that of the China, France and Papua New Guinea individuals in their study. All had the one to four percent Neanderthal contribution to their DNA, so the interbreeding must have first occurred before the humans migrating out of Africa colonized other, more distant regions in Europe, Asia and elsewhere.

Reich said humans and Neanderthals might have continued to mate with each other later, "because their populations overlapped in Europe and remains have been found for human individuals with Neanderthal-type features," but the genetic data cannot pinpoint when and where these later couplings might have taken place.

In addition to the interbreeding revelations, the study illuminates what makes modern humans unique.

The researchers found Neanderthal DNA is 99.7 percent identical to present-day human DNA, but Homo sapiens evolved distinct genes related to cognitive functions, metabolism and the development of cranial features, the collarbone and the rib cage.

Reich said three of the human-specific genes suggest that "skin and hair must have been particularly relevant to the evolution of modern humans," but additional analysis is needed.

In a second related Science study, Gregory Hannon of Cold Spring Harbor Laboratory and his colleagues explain a new process, called "array capture re-sequencing," that allows scientists to better study Neanderthal and other ancient DNA. Hannon said that the new process "enables us to extract from genomes important information, on a very selective basis, rapidly, very accurately, and at low cost."

Hannon described the sequencing of the Neanderthal genome as "a watershed event, a major historical achievement."

A reconstructed head based on a Neanderthal skull found in France is shown.