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."
Reich says it's hard to pin down exactly how much DNA Neanderthals contributed to modern humans. "We estimate about 1 to 4 percent of the genetic ancestry of non-Africans is from Neanderthals," he says.