Emily Singer writes: Early human history was a promiscuous affair. As modern humans began to spread out of Africa roughly 50,000 years ago, they encountered other species that looked remarkably like them — the Neanderthals and Denisovans, two groups of archaic humans that shared an ancestor with us roughly 600,000 years earlier. This motley mix of humans coexisted in Europe for at least 2,500 years, and we now know that they interbred, leaving a lasting legacy in our DNA. The DNA of non-Africans is made up of roughly 1 to 2 percent Neanderthal DNA, and some Asian and Oceanic island populations have as much as 6 percent Denisovan DNA.

Over the last few years, scientists have dug deeper into the Neanderthal and Denisovan sections of our genomes and come to a surprising conclusion. Certain Neanderthal and Denisovan genes seem to have swept through the modern human population — one variant, for example, is present in 70 percent of Europeans — suggesting that these genes brought great advantage to their bearers and spread rapidly.

“In some spots of our genome, we are more Neanderthal than human,” said Joshua Akey, a geneticist at the University of Washington. “It seems pretty clear that at least some of the sequences we inherited from archaic hominins were adaptive, that they helped us survive and reproduce.”

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.” [Continue reading…]