Hey, Neanderthal!

To find out whether the two human species interbred, scientists from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, launched an audacious effort to sequence all 3 billion letters of Neanderthal DNA. As hard as it may be to believe, fragments of DNA can survive in bones that are more than 30,000 years old. New techniques can sequence those fragments, and new software can line the fragments up.

The main strategy for finding whether modern humans and Neanderthals exchanged genes was to compare the Neanderthal DNA with DNA from modern humans " some whose ancestors had remained in Africa, and others who had migrated to Europe, Asia and the New World. If the Neanderthal DNA was more similar to the non-African DNA, it would indicate that Neanderthals contributed their genes after the first modern humans migrated out of Africa.

Reich says that's what they found.

"The non-African is more closely related to the Neanderthal," Reich says. "Quite definitely. Highly statistically significantly overwhelming evidence that they're more closely related on average to Neanderthals."

In other words, Reich and his Leipzig co-authors are totally sure that Neanderthal genes found their way into modern humans when the two species intersected. They report their findings in the journal Science.

"The simplest possible explanation is that it occurred once," Reich says. "But it very easily could have occurred on multiple occasions. Perhaps it's likely that it occurred on multiple occasions."