Photograph by Joe McNally

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Photograph by Joe McNally

Sex With Humans Made Neanderthals Extinct?

The two species mated till the Neanderthal lineage melted into ours, study hints.

Neanderthals may have been victims of love, or at least of interspecies breeding with modern humans, according to a new study.

As the heavy-browed species ventured farther and farther to cope with climate change, they increasingly mated with our own species, giving rise to mixed-species humans, researchers suggest.

Over generations of genetic mixing, the Neanderthal genome would have dissolved, absorbed into the Homo sapiens population, which was much larger. (Get the basics on genetics.)

"If you increase the mobility of the groups in the places where they live, you end up increasing the gene flow between the two different populations, until eventually one population disappears as a clearly defined group," said study co-author C. Michael Barton, an archaeologst at Arizona State University's School of Human Evolution and Social Change.

Doing What Comes Naturally

Some theories suggest Neanderthals disappeared about 30,000 years ago because the species wasn't able to adapt to a cooling world as well as Homo sapiens. (See a prehistoric time line.)

Barton tells a different tale, suggesting that Neanderthals reacted to the onset of the Ice Age the same ways modern humans did, by ranging farther for food and other resources.

"As glaciation increased, there was likely less diversity in land use, so Neanderthals and modern humans alike focused on a particular survival strategy that we still see today at high latitudes," Barton said.

"They establish a home base and send out foraging parties to bring back resources. People move farther and have more opportunity to come into contact with other groups at greater distances. The archaeological record suggests that this became more and more common in Eurasia as we move toward full glaciation."

More frequent contact led to more frequent mating, the theory goes, as the two groups were forced to share the same dwindling resources.

"Other things might have happened," Barton said. "But in science we try to find the simplest explanation for things. This theory doesn't include massive migrations or invasions—just people doing what they normally do."

To estimate the effects of the assumed uptick in interspecies mating, Barton's team conducted a computational modeling study that spanned 1,500 Neanderthal generations.

In the end, the model results supported the not entirely new idea that Neanderthals were "genetically swamped" by modern humans.

"Extinction by Hybridization"

Though it's a relative underdog among Neanderthal-demise theories, genetic swamping is a well-known extinction cause among plant and animal species.

A smallish group of native, localized trout, for example, may lose their genetic identity after a large influx of a different species with which the native fish are able to breed.

"When endemic populations are specialized, and for some reason there is a change in their interaction with adjacent populations, and that interaction level goes up, they tend to go extinct—especially if one population is much smaller than the other," Barton explained.

"In conservation biology this is called extinction by hybridization."

Men on the Hunt

Paleoanthropologist Bence Viola said other models have produced different results, and some studies have concluded that relatively little interbreeding occurred.

But Viola, of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, is intrigued by Barton's research.

"From an archaeological and anthropological perspective, this sounds interesting and closer to what I believe—that you can have a lot of interbreeding," Viola said.

"Normally the first groups who [encounter] a new population are men, hunting parties perhaps. And men, being they way they are—if they meet women from another population, there is bound to be interbreeding."

Barton believes interbreeding caused other distinct human and human-ancestor groups to fade away.

"But their genes didn't disappear," he added. "And their culture probably didn't disappear either but was blended into a larger population of hunter-gatherers."

The Max Planck Institute's Viola believes interbreeding was a cause—but not the cause.

"Neanderthals disappeared around 30,000 years ago, and that was a period when the climate turned colder, and that likely made it physically harder for them to survive," Viola said.

"They also may have been exposed to some type of disease that modern humans brought from Africa and for which they had no immunity.

"Of course these are all things that are very hard to study archaeologically," Viola added. "So these models are a great tool for investigating ideas."

The Neanderthal-interbreeding study, co-authored by researchers at Arizona State University and the University of Colorado Denver, will be published in the December issue of the journal Human Ecology.

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