The next time you're tempted to call some oaf a Neanderthal, you might want to take a look in the mirror.

According to a new DNA study, most humans have a little Neanderthal in them—at least 1 to 4 percent of a person's genetic makeup.

The study uncovered the first solid genetic evidence that "modern" humans—or Homo sapiens—interbred with their Neanderthal neighbors, who mysteriously died out about 30,000 years ago.

What's more, the Neanderthal-modern human mating apparently took place in the Middle East, shortly after modern humans had left Africa, not in Europe—as has long been suspected.

"We can now say that, in all probability, there was gene flow from Neanderthals to modern humans," lead study author Ed Green of the University of California, Santa Cruz, said in a prepared statement.

"They've finally seen the light ... because it's been obvious to many us that this happened," said Trinkaus, of Washington University in St. Louis, Missouri, who wasn't part of the new study.

Trinkhaus adds that most living humans probably have much more Neanderthal DNA than the new study suggests.

"One to 4 percent is truly a minimum," Trinkaus added. "But is it 10 percent? Twenty percent? I have no idea."

Surprising Spot for Neanderthal-Human Mating

The results showed that Neanderthal DNA is 99.7 percent identical to modern human DNA, versus, for example, 98.8 percent for modern humans and chimps, according to the study. (Related: "Neanderthals Had Same 'Language Gene' as Modern Humans.")

In addition, all modern ethnic groups, other than Africans, carry traces of Neanderthal DNA in their genomes, the study says—which at first puzzled the scientists. Though no fossil evidence has been found for Neanderthals and modern humans coexisting in Africa, Neanderthals, like modern humans, are thought to have arisen on the continent.

"If you told an archaeologist that you'd found evidence of gene exchange between Neanderthals and modern humans and asked them to guess which [living] population it was found in, most would say Europeans, because there's well documented archaeological evidence that they lived side by side for several thousand years," said study team member David Reich.

For another thing, Neanderthals never lived in China or Papua New Guinea, in the Pacific region of Melanesia, according to the archaeological record. (See "Neanderthals Ranged Much Farther East Than Thought.")

"But the fact is that Chinese and Melanesians are as closely related to Neanderthals" as Europeans, said Reich, a population geneticist at the Broad Institute of MIT and Harvard University.

Neanderthal-Human One-Night Stand?

So how did modern humans with Neanderthal DNA end up in Asia and Melanesia?

Neanderthals, the study team says, probably mixed with early Homo sapiens just after they'd left Africa but before Homo sapiens split into different ethnic groups and scattered around the globe.

The first opportunity for interbreeding probably occurred about 60,000 years ago in Middle Eastern regions adjacent to Africa, where archaeological evidence shows the two species overlapped for a time, the team says.

And it wouldn't have taken much mating to make an impact, according to study co-author Reich. The results could stem from a Neanderthal-modern human one-night stand or from thousands of interspecies assignations, he said.

More DNA Evidence for Neanderthal-Human Mating

The new study isn't alone in finding genetic hints of Homo sapiens-Homo neanderthalensis interbreeding.

Genetic anthropologist Jeffrey Long, who calls the Science study "very exciting," co-authored a new, not yet published study that found DNA evidence of interbreeding between early modern humans and an "archaic human" species, though it's not clear which. He presented his team's findings at a meeting of the American Association of Physical Anthropologists in Albuquerque, New Mexico, last month.

Long's team reached its conclusions after searching the genomes of hundreds of modern humans for "signatures of different evolutionary processes in DNA variation."

Like the new Science paper, Long's study speculates that interbreeding occurred just after our species had left Africa, but Long's study didn't include analysis of the Neanderthal genome.

"At the time we started the project, I never imagined I'd ever see an empirical confirmation of it," said Long, referring to the Science team's Neanderthal-DNA evidence, "so I'm pretty happy to see it."