An international team of scientists has identified a previously shadowy human group known as the Denisovans as cousins to Neanderthals who lived in Asia from roughly 400,000 to 50,000 years ago and interbred with the ancestors of today’s inhabitants of New Guinea.
All the Denisovans have left behind are a broken finger bone and a wisdom tooth in a Siberian cave. But the scientists have succeeded in extracting the entire genome of the Denisovans from these scant remains. An analysis of this ancient DNA, published on Wednesday in Nature, reveals that the genomes of people from New Guinea contain 4.8 percent Denisovan DNA.
An earlier, incomplete analysis of Denisovan DNA had placed the group as more distant from both Neanderthals and humans. On the basis of the new findings, the scientists propose that the ancestors of Neanderthals and Denisovans emerged from Africa half a million years ago. The Neanderthals spread westward, settling in the Near East and Europe. The Denisovans headed east. Some 50,000 years ago, they interbred with humans expanding from Africa along the coast of South Asia, bequeathing some of their DNA to them. ...Next, the researchers looked for evidence of interbreeding. Nick Patterson, a Broad Institute geneticist, compared the Denisovan genome to the complete genomes of five people, from South Africa, Nigeria, China, France and Papua New Guinea. To his astonishment, a sizable chunk of the Denisova genome resembled parts of the New Guinea DNA.
“The correct reaction when you get a surprising result is, ‘What am I doing wrong?’ ” said Dr. Patterson. To see if the result was an error, he and his colleagues sequenced the genomes of seven more people, including another individual from New Guinea and one from the neighboring island of Bougainville. But even in the new analysis, the Denisovan DNA still turned up in the New Guinea and Bougainville genomes. ...
Dr. Bustamante also thinks that other cases of interbreeding are yet to be discovered. “There’s a lot of possibility out there,” he said. “But the only way to get at them is to sequence more of these ancient genomes.”
If the genomes of New Guineans come almost 5% from non-modern humans, then the obvious next step is to test the genomes of Australian Aborigines, who are last in line in the original Southern, Indian Ocean shoreline route Out of Africa. However, there are a lot of regulatory barriers against testing Aborigines, perhaps out of fear that scientists will find something like this. After all, Aborigines look a little archaic, so it wouldn't be terribly surprising if their genes turn out to be a little archaic.
It was lucky that the first findings of non-modern human ancestry involved Europeans, or it would have been hard to get up the political courage to publish this.
So, the Out of Africa model of evolution of the current human race turns out to be mostly, but not wholly, correct. Greg Cochran calls the new model "Out of Africa, with Benefits:" modern humans picked up useful genes from older human types, and not all of those inheritances spread equally to the entire current human race, probably in part because they aren't equally useful in all environments.
Here's an FAQ by John Hawks. And here are comments by Dienekes.
By the way, here's an interesting 2006 article on Nick Patterson, one of the scientists involved. He's had successful three careers, first as British and American government cryptologist, then as a quant for James H. Simons' hedge fund Renaissance Technologies, and now as a genome researcher.