It was the discovery that challenged what it is to be human. The Neanderthal genome revealed that our extinct cousin's genes live on in many modern humans, implying that the two species interbred. But a controversial new study casts doubt on those claims of interspecies hanky-panky.
In 2010, Svante P??bo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, and his colleagues sequenced the Neanderthal genome. Their analysis concluded that many modern humans carry a few Neanderthal genes. Only native Africans lack the Neanderthal genes, because Neanderthals did not live in Africa.
Right from the start, there was a problem. Neanderthals and modern humans ultimately evolved from the same ancestral population, so any genes shared by the two species might simply have been inherited from this common ancestor.
"We were very upfront in our papers that this was a possibility," says P??bo's colleague David Reich of the Harvard Medical School in Boston.
Drifted apart
Andrea Manica and Anders Eriksson at the University of Cambridge have now built a model to demonstrate a non-interbreeding explanation for the 2010 result.
They began with an ancestral hominin population throughout Africa and Europe. Because of their regional proximity, the hominins in Europe had more genes in common with those of northern Africa than those of southern Africa.
Africa and Europe then became genetically isolated from one another, perhaps triggered by changing climates, says Manica. This allowed the Europeans to evolve into Neanderthals and the Africans to evolve into modern humans. Crucially, though, the modern humans in northern Africa retained genetic similarities with Neanderthals that the southern Africans lacked. Northern Africans ultimately moved into Europe ? but they didn't need to interbreed with Neanderthals to share some genes in common with them (see diagram).
"You cannot prove there was never any hybridisation," says Manica. "But none of the evidence [for hybridisation] is convincing."
Shuffled up
Reich and P??bo disagree with the new formulation. They say recent analyses actually firm up the case for interbreeding. Most tellingly, Reich and his colleagues have an upcoming paper in PLoS Genetics that suggests non-Africans have shared genes in common with Neanderthals for only a few tens of thousands of years. If correct, this means these genes cannot predate the origin of Neanderthals, around 320,000 years ago.
His team examined the genomes of modern Europeans to work out how much the Neanderthal-like genes had been shuffled around. The greater the degree of shuffling, the longer they must have been a part of the genome. He found that there was limited gene movement, implying that they entered the European genome between 65,000 and 47,000 years ago ? around the time modern humans left Africa.
The evidence for interbreeding is growing, but Manica and Eriksson's study shows we should still consider the alternatives, says Joshua Akey of the University of Washington in Seattle.
Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.1200567109
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