Questions over human and Neanderthal interbreeding

THIS WEEK

Michael Marshall

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 Pääbo 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 Pääbo’s colleague David Reich of the Harvard Medical School in Boston. Andrea Manica and Anders Eriksson at the University of Cambridge have now built a model to demonstrate a noninterbreeding explanation for the 2010 result. They began with ancestral hominin populations throughout Africa and Europe (see diagram). 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. The Europeans evolved into

Nikola Solic/Reuters/Corbis

Neanderthal notch on the bedpost lost?

–Fancy furs not so seductive–

Neanderthals and the Africans evolved 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 with them (Proceedings of the National

Mixed up genes, minus the interbreeding Hominins in Europe have more genes in common with those in northern Africa than those in southern Africa

Hominins in Europe become genetically isolated. Neanderthals evolve in Europe, Homo sapiens in Africa

Homo sapiens from northern Africa replace Neanderthals. No interbreeding required for Neanderthal-like signature

HOMO SAPIENS WITH NEANDERTHALLIKE GENES

NEANDERTHAL GENES

NATURAL VARIATION IN ARCHAIC GENES HOMO SAPIENS WITH NEANDERTHAL-LIKE GENES

HOMO SAPIENS GENES

500,000 years ago 12 | NewScientist | 18 August 2012

320,000 years ago

HOMO SAPIENS SAPIEN GENES GENESS

Modern era

Academy of Sciences, DOI: 10.1073/pnas.1200567109). “You cannot prove there was never any hybridisation,” says Manica. “But none of the evidence [for hybridisation] is convincing.” Reich and Pääbo disagree. 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 the crucial genes with Neanderthals for only a few tens of thousands of years, not the 320,000 years suggested by Manica and Eriksson. Reich’s team studied the genomes of modern Europeans to work out how much the Neanderthal-like genes had been shuffled. The greater the degree of shuffling, the longer they must have been a part of the genome. He found limited gene movement, implying that they entered the European genome 65,000 to 47,000 years ago – around the time modern humans left Africa. The evidence for interbreeding is growing, says Joshua Akey of the University of Washington in Seattle, but Manica and Eriksson’s study shows we should still consider the alternatives. n