THERE'S NO SUCH THING AS SPECIES:

According to a new analysis of gene ancestry data published Monday in Nature Ecology & Evolution, populations of humans and Neanderthals mated and procreated multiple times over during the 30,000 years when the two species overlapped. Our snippets of Neanderthal DNA are not the result of one-off hookups.

"The history of human-Neanderthal interactions is complex," says Joshua Schraiber, a genomics researcher at Temple University and a co-author of the new study. "It wasn't just one time that humans and Neanderthals ran into each other and interbred. They overlapped for tens of thousands of years, and multiple interactions occurred."

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