Last year, an international team of researchers announced that they had completed the first draft of the Neanderthal genome–more than three billion nucleotides, sequenced from three minute samples of bone powder. Now, in two papers published today in Science, the group reveals in-depth analyses of the data and provides an unparalleled glimpse at human evolution. By comparing Neanderthal DNA with that of living humans from around the world, the scientists have found evidence that–sometime between 50,000 and 80,000 years ago–modern humans and Neanderthals interbred.
Archeologists’ traditional view of human evolution, gleaned from carbon dating, skeletal structure, fossil location, and, more recently, Neanderthal mitochondrial DNA, posits that humans today are descended from a small group of individuals that migrated out of Africa and dispersed throughout the world. But the new research shows that modern humans in Africa have a lower percentage of the Neanderthal genome than non-Africans do–implying that the founder group that left Africa interbred with Neanderthals before moving on to populate the other continents. “Likely, it took place somewhere in the Middle East or in northern Africa, perhaps at the gateway as they were migrating out for the first time,” says Harvard University geneticist David Reich, who performed the population genetics analyses.
The scientists used just half a gram of bone powder, collected from the bones of three individual Neanderthals excavated from the Vindija Cave in Croatia. The data they provide tells a story not just of migration but physical evolution, and allows researchers to isolate what makes humans unique. “The Neanderthals are our closest evolutionary relative,” says Svante Pääbo, the project’s leader and director of genetics at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. He notes that now scientists can begin to ask what genetically differentiates us from our closest living relative–the chimpanzee–and our closest extinct one.
So far, the answer to this question appears to be “not much.” According to the researchers, humans inherited anywhere from 1 to 4 percent of their genomes from Neanderthals; not only does the amount vary among individuals, but the actual location varies, too. “There’s no indication at this point that there was any particular gene with selective advantage that came in from Neanderthals,” says Richard E. Green, a biomolecular engineer at the University of California at Santa Cruz who was responsible for much of the genetic sequencing. “The signal appears to be widely distributed across the genome, and differs from individual to individual.”