But what, exactly, do these fragments of Neanderthal and Denisovan DNA do? What survival advantage did they confer on our ancestors? Scientists are starting to pick up hints. Some of these genes are tied to our immune system, to our skin and hair, and perhaps to our metabolism and tolerance for cold weather, all of which might have helped emigrating humans survive in new lands.

“What allowed us to survive came from other species,” said Rasmus Nielsen, an evolutionary biologist at the University of California, Berkeley. “It’s not just noise, it’s a very important substantial part of who we are.”

Illustration by Lucy Reading-Ikkanda for Quanta Magazine, based on a map by Sriram Sankararaman.

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The Tibetan plateau is a vast stretch of high-altitude real estate isolated by massive mountain ranges. The scant oxygen at 14,000 feet—roughly 40 percent lower than the concentrations at sea level—makes it a harsh environment. People who move there suffer higher rates of miscarriage, blood clots, and stroke on account of the extra red blood cells their bodies produce to feed oxygen-starved tissue. Native Tibetans, however, manage just fine. Despite the meager air, they don’t make as many red blood cells as the rest of us would at those altitudes, which helps to protect their health.

In 2010, scientists discovered that Tibetans owe their tolerance of low oxygen levels in part to an unusual variant in a gene known as EPAS1. About 90 percent of the Tibetan population and a smattering of Han Chinese (who share a recent ancestor with Tibetans) carry the high-altitude variant. But it’s completely absent from a database of 1,000 human genomes from other populations.

In 2014, Nielsen and colleagues found that Tibetans or their ancestors likely acquired the unusual DNA sequence from Denisovans, a group of early humans first described in 2010 that are more closely related to Neanderthals than to us. The unique gene then flourished in those who lived at high altitudes and faded away in descendants who colonized less harsh environments. “That’s one of the most clear-cut examples of how [interbreeding] can lead to adaptation,” said Sriram Sankararaman, a geneticist and computer scientist at the University of California, Los Angeles.

The idea that closely related species can benefit from interbreeding, known in evolutionary terms as adaptive introgression, is not a new one. As a species expands into a new territory, it grapples with a whole new set of challenges—different climate, food, predators, and pathogens. Species can adapt through traditional natural selection, in which spontaneous mutations that happen to be helpful gradually spread through the population. But such mutations strike rarely, making it a very slow process. A more expedient option is to mate with species that have already adapted to the region and co-opt some of their helpful DNA. (Species are traditionally defined by their inability to mate with one another, but closely related species often interbreed.)