August 15, 2012
Two years ago the analysis of the Neanderthal genome revealed modern humans carry Neanderthal DNA, implying our ancestors mated with Neanderthals at some point in the past. Scientists only found genetic traces of Neanderthals in non-African people, leading to the conclusion that Neanderthal-human matings must have occurred as modern humans left Africa and populated the rest of the world. A new paper (PDF) posted on arXiv.org puts a date on those matings: 47,000 to 65,000 years ago—a time that does indeed correspond with human migrations out of Africa.
Sriram Sankararaman of Harvard Medical School and colleagues—including Svante Pääbo of Germany’s Max Planck Institute for Evolutionary Anthropology and Harvard’s David Reich—investigated the timing of the matings in part to verify that the trysts even happened at all. That’s because there’s an alternative explanation for why up to 4 percent of non-African human DNA looks like Neanderthal DNA. It’s possible, the researchers explain, that the ancestral species that gave rise to both humans and Neanderthals had a genetically subdivided population—in other words, genetic variation wasn’t evenly distributed across the species. Under that scenario, Neanderthals and the modern humans that left Africa might have independently inherited similar DNA from a part of the divided ancestral population that didn’t contribute genetic material to modern African populations. (Another paper published this week, in Proceedings of the National Academy of Sciences, considers this scenario.)
To determine what really happened, Sankararaman’s team looked at rates of genetic change to estimate when Neanderthals and humans last exchanged genes. If the shared DNA was due to interbreeding, the team expected to find a date less than 100,000 years ago—some time after humans left Africa. But if it was the result of sharing a common ancestor, they expected a date older than 230,000 years ago, approximately when Neanderthals and modern humans split from each other. The team’s findings support the interbreeding scenario: 47,000 to 65,000 years ago.
Neanderthals aren’t the only archaic species that may have contributed to the modern human gene pool. Denisovans, known from only a tooth and a finger bone, left a genetic mark in people living in Melanesia and Southeast Asia. And recent genetic evidence suggests that some ancient African populations mated with an unidentified, now-extinct hominid species that lived in Africa.
So far, our knowledge of Neanderthal and Denisovan genetics comes from only a few individuals, so our understanding of interspecies mating is likely to change as more Neanderthal and Denisovan DNA is analyzed.
(H/T John Hawks)