I

f you want someone to thank for your immunity to a whole host of pathogens — or, someone to blame for your allergies — look no further than Neanderthals.

Humans carry three important genes that hail from two ancient, human-like species: Neanderthals and Denisovans, both of which have been extinct for tens of thousands of years. The new finding, published recently in the American Journal of Human Genetics, points to a growing body of evidence of interbreeding between humans and other archaic species.

Modern humans seem to have made out well from those genetic exchanges.

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“When modern humans bred with them, they took away the best things these archaic species had,” said Janet Kelso, an evolutionary geneticist at Germany’s Max Planck Institute for Evolutionary Anthropology and lead author of the study.

The three genes tied back to the ancient species — known as TLR1, TLR6, and TLR10 — are part of the area of the genome that regulates the body’s initial immune response to foreign bacteria.

Since Neanderthals and Denisovans had lived in the Eurasian environment for hundreds of thousands of years before Homo sapiens and humanoids came along, their immune systems had time to adapt to the environment, including any threatening pathogens. By retaining those genes, modern humans boosted their own immune responses.

“You’re able to take from a population that’s well-adapted in their immunity,” Kelso said.

There is, however, a downside: The same genes are also responsible for the immune system’s overactive response to some benign environmental triggers, like pollen and grass, so modern humans picked up more allergies.

“We see it as a trade-off,” Kelso said.

Comparing human genes to those of Neanderthals and other ancient species can provide telling clues about our evolution.

“Some of these changes may be responsible for why modern humans [were able to] expand in numbers, spread over all of the world, and fly to the moon, whereas Neanderthals and other now-extinct groups did not,” said Svante Pääbo, an evolutionary geneticist who has also studied Neanderthal genetics at the Max Planck Institute but was not affiliated with the study.

Humans almost always held onto their own genes in interbreeding with Neanderthals. So the fact that we retained several implies they were quite useful for the modern human, Kelso said. It also means that Neanderthals haven’t completely disappeared from the face of the Earth.

“They live on a little bit in many of us today,” Pääbo said.

This article was originally published on Jan. 7, 2016.

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