Scientists Say Humans Did not Interbreed with Neanderthals ...

Scientists Say Humans Did not Interbreed with Neanderthals
Cambridge researchers have raised questions about the theory that Neanderthals and modern humans at some point interbred. Their findings show that common ancestry, not hybridization, better explains the average 1-4 per cent DNA that those of European and Asian descent share with Neanderthals.
Early humans in a cave, left, and Neanderthal family, right (Charles Knight / AMNH / The Natural History Museum)
In the last two years, a number of studies have suggested that modern humans and Neanderthals had at some point interbred. Genetic evidence shows that on average Eurasians and Neanderthals share between 1-4 per cent of their DNA. In contrast, Africans have almost none of the Neanderthal genome. The previous studies concluded that these differences could be explained by hybridization, which occurred as modern humans exited Africa and bred with the Neanderthals who already inhabited Europe.
The scientists found that common ancestry, without any hybridization, explains the genetic similarities between Neanderthals and modern humans. In other words, the DNA that Neanderthal and modern humans share can all be attributed to their common origin, without any recent influx of Neanderthal DNA into modern humans.
“Our work shows clearly that the patterns currently seen in the Neanderthal genome are not exceptional, and are in line with our expectations of what we would see without hybridization,” said lead author Dr Andrea Manica of the University of Cambridge. “So, if any hybridization happened – it’s difficult to conclusively prove it never happened – then it would have been minimal and much less than what people are claiming now.”
Neanderthals and modern humans once shared a common ancestor who is thought to have spanned Africa and Europe about half a million years ago. Just as there are very different populations across Europe today, populations of that common ancestor would not have been completely mixed across continents, but rather closer populations would have been more genetically similar to each other than populations further apart.
Then, about 350-300 thousand years ago, the European range and the African range became separated. The European range evolved into Neanderthal, the African range eventually turned into modern humans. However, because the populations within each continent were not freely mixing, the DNA of the modern human population in Africa that were ancestrally closer to Europe would have retained more of the ancestral DNA that is also shared with Neanderthals.
On this basis, the scientists created a model to determine whether the differences in genetic similarities with Neanderthal among modern human populations, which had been attributed to hybridization, could be down to the proximity of modern humans in northern Africa to Neanderthals.
By examining the different genetic makeup among modern human populations, the scientists’ model was able to infer how much genetic similarity there would have been between distinct populations within a continent. The researchers then simulated a large number of populations representing Africa and Eurasia over the last half a million years, and estimated how much similarity would be expected between a random Neanderthal individual and modern humans in Africa and Eurasia.
The scientists concluded that when modern humans expanded out of Africa 60-70K years ago, they would have brought out that additional genetic similarity with them, making Europeans and Asians more similar to Neanderthals than Africans are on average – undermining the theory that hybridization, and not common ancestry, explained these differences.
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Bibliographic information: Manica et al. 2012. Effect of ancient population structure on the degree of polymorphism shared between modern human populations and ancient hominins. PNAS, accepted for publication.