Previous studies have confirmed interbreeding among humans and Neanderthals. Now, a new genetic survey has revealed gene flow between humans and Neanderthals was mediated by viral transmissions.

"It's not a stretch to imagine that when modern humans met up with Neanderthals, they infected each other with pathogens that came from their respective environments," David Enard, an assistant professor in ecology and evolutionary biology at the University of Arizona, said in a news release. "By interbreeding with each other, they also passed along genetic adaptations to cope with some of those pathogens."

Scientists think humans first interacted with Neanderthals in Eurasia, after migrating out of Africa 70,000 years ago. Humans brought viruses that Neanderthals had no natural immunity to. Humans also became exposed to new Eurasian viruses for the first time -- viruses that Neanderthals had built up defenses against.

Through interbreeding, scientists think some humans and Neanderthals traded genetic immunities.

"Some of the Neanderthals had adaptive mutations that gave them advantages against these pathogens, and they were able to pass some of these mutations on to modern humans," said Enard. "That's called positive natural selection -- it favors certain individuals that carry these advantageous mutations."

In 2016, while working at Stanford University with researcher Dmitri Petrov, Enard found roughly a third of all protein adaptations in the human genome were triggered by viral infections. For the latest study, Enard and Petrov identified 4,000 genes in the human genome that interact with viruses.

When researchers compared the 4,000 genes with the Neanderthal genome, they found significant evidence of gene flow between the two species.

"Many Neanderthal sequences have been lost in modern humans, but some stayed and appear to have quickly increased to high frequencies at the time of contact, suggestive of their selective benefits at that time," Petrov said. "Our research aims to understand why that was the case. We believe that resistance to specific RNA viruses provided by these Neanderthal sequences was likely a big part of the reason for their selective benefits."

Researchers think their findings -- published this week in the journal Cell -- could inform investigations of ancient epidemics. Viral DNA itself is easily degraded, but the genetic mutations they inspired can be identified in the genomes of ancient populations.

"You can think of these genetic adaptations like footprints from long-extinct dinosaurs preserved in fossilized mud," Enard said. "Even without having access to the viruses themselves, scientists who study prehistoric epidemics will be able to learn about the pathogens that drove them."

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