Our Ancestors May Have Mated More Than Once

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.

There are a few potential explanations for how modern humans ended up with Denisovan genes, Reich says. It’s possible that Denisovans and modern humans mated only once, presumably while H. sapiens were on their way to Australia and Papua New Guinea. After that, Denisovan-related modern humans mated with modern humans whose ancestors had never met Denisovans, and different populations ended up with different proportions of the archaic genes. “On the other hand, it’s entirely possible, and I think plausible, that what you’re seeing in India is evidence of mixture of ancient South Asian ancestors with local Denisovans,” that is, an entirely separate mating event than the one that led to Denisovan ancestry in Oceania, Reich says. In fact, he explains, “the data could be consistent with as many as three independent mixture events with Denisovans.”

Although it’s difficult to determine which model is right without more data from both fossils and modern human genomes, “I don’t think it’s that hard to imagine that there were multiple admixture events,” says Joshua Akey, a population geneticist at the University of Washington, Seattle, who was not involved in the study. He led a team that recently identified as many as five mixing events between modern humans and Neandertals.

Another mystery is exactly when H. sapiens hooked up with Denisovans. Fossil evidence from Denisova Cave suggests that the ancient species lived there from as early as 170,000 years ago to at least 50,000 years ago, and who knows when they might have occupied the rest of Asia. That gives Denisovans a lot of time to run into other kinds of humans and potentially have babies with them.

Source: https://www.sciencemag.org

By: Lizzie Wade

Ragnar Larsen