Sunday, May 24, 2015

Oase-1: This man was 10% Neandertal

To Erik Trinkaus, the jaw of the oldest modern human found in Europe has always looked strange. Its huge wisdom teeth and hefty, buttressed lower jaw reminded him of Neandertals, and he argued that this fossil, 37,000 to 42,000 years old, was the product of generations of mixing between modern humans and our extinct cousins. “It wasn't a popular idea,” admits Trinkaus, a paleoanthropologist at Washington University in St. Louis. Other paleoanthropologists insisted that the young man whose remains were found in 2002 in Peştera cu Oase cave in Romania was just a chunky example of our own species.

Now, 15 years later, Trinkaus has been vindicated by ancient DNA. The young Oase man inherited as much as one-tenth of his DNA from a Neandertal ancestor, and that ancestor lived only 200 years or so previously, according to a talk this month at Cold Spring Harbor Laboratory in New York. “One of Oase's ancestors—its great-great-great-grandparent—is Neandertal,” reported Qiaomei Fu, a geneticist at the Chinese Academy of Sciences–Max Planck Society Joint Laboratory for Human Evolution in Beijing and a postdoc in the lab of population geneticist David Reich at Harvard Medical School. The finding is “important as the first direct evidence of a very recent admixture event in Europe,” says population geneticist Laurent Excoffier of the University of Bern.

Europe just after the arrival of modern humans has long seemed a likely setting for such close encounters, given that Neandertals and modern humans overlapped there about 45,000 to 39,000 years ago. But until now, ancient DNA pointed to a different time and place for such a liaison. By sequencing the genomes of fossil Neandertals and comparing them with today's human genomes, paleogeneticists had found that living Europeans and Asians—but not Africans—have inherited just 1% to 4% of their DNA from Neandertals. DNA from fossils of two modern humans from what is now Russia also suggested that their Neandertal heritage was faint (see So researchers proposed that modern humans and Neandertals had rare and relatively early encounters, perhaps in the Middle East, when moderns swept out of Africa 60,000 to 50,000 years ago.

The DNA from Oase 1, a lower jaw without a skull, complicates that picture, Fu reported at the Biology of Genomes meeting. Working in a team led by paleogeneticist Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, she and her colleagues captured 2.2 million base pairs of the fossil's DNA. Then, they sequenced 78,055 locations where the genomes of Neandertals and modern humans are known to differ. They found that the Oase man had far more Neandertal DNA—composing 4.8% to 11.3% of his genome—than either the ancient modern humans from Russia or living Europeans and Asians, Fu said.

What's more, the young man had inherited the Neandertal DNA in “large chunks,” including several segments more than 50 million base pairs long; one chunk spanned half the length of chromosome 12. Those unbroken stretches of Neandertal DNA suggest that the interbreeding must have been just four to six generations back. If the mixing had been more ancient, the long DNA segments would have been broken up by the reshuffling of chromosomes that takes place every generation. “This is quite amazing,” Fu said in her talk. “We're quite excited about that.”

If modern humans and Neandertals had several successful matings, why do living humans' genomes record only the earlier event?

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