Our ancestors may have mated more than once with mysterious ancient humans

Lizzie Wade in Science:

It looked like an ordinary finger bone. But when researchers sequenced its DNA in 2010, they uncovered the existence of a group of ancient humans no one had seen before: the Denisovans. Then came an even bigger surprise. Some modern humans also carry Denisovan DNA, meaning that at some point in the ancient past, Denisovans and modern humans mated and had children. Now, a new study concludes that all that free love had some dark consequences, including male offspring that were likely sterile.

In the absence of much fossil evidence, the best way to study Denisovans is through the genes they left behind in modern humans. So population geneticists Sriram Sankararaman at the University of California (UC), Los Angeles, and David Reich at Harvard University sifted through 257 genomes of present-day people from 120 non-African populations around the world. (Africans, whose ancestors didn’t leave Homo sapiens’s original home, do not have any Denisovan heritage.) They confirmed an earlier finding that among humans living today, people from Papua New Guinea, Australia, and other parts of Oceania have the most Denisovan ancestry, between 3% and 6% of their genomes. This compares with about 2% from Neandertals for all non-African genomes. Sankararaman and Reich found another hot spot of Denisovan ancestry in an unexpected place: South Asia. “It’s about 10% of what we see in the Oceanians,” Sankararaman explains. That’s quite a small contribution—which allowed it fly under the radar in previous studies—but it’s more than researchers expected to find based on their best models of population mixing. East Asians, in turn, have more Denisovan ancestry than Europeans but less than South Asians, the team reports today in Current Biology.

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