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Deep in Human DNA, a Gift From the Neanderthals

Long ago, Neanderthals probably infected modern humans with viruses, perhaps even an ancient form of H.I.V. But our extinct relatives also gave us genetic defenses.

People of Asian and European descent — almost anyone with origins outside of Africa — have inherited a sliver of DNA from some unusual ancestors: the Neanderthals.

These genes are the result of repeated interbreeding long ago between Neanderthals and modern humans. But why are those genes still there 40,000 years after Neanderthals became extinct?

As it turns out, some of them may protect humans against infections. In a study published on Thursday, scientists reported new evidence that modern humans encountered new viruses — including some related to influenza, herpes and H.I.V. — as they expanded out of Africa roughly 70,000 years ago.

Some of those infections may have been picked up directly from Neanderthals. Without immunity to pathogens they had never encountered, modern humans were particularly vulnerable.

“We were actually able to not only say, ‘Yes, modern humans and Neanderthals exchanged viruses,’” said David Enard, an evolutionary biologist at the University of Arizona and co-author of the new paper, published in the journal Cell. “We are able to start saying something about which types of viruses were involved.”

But if Neanderthals made us sick, they also helped keep us well. Some of the genes inherited from them through interbreeding also protected our ancestors from these infections, just as they protected the Neanderthals.