Doubts over human-Neanderthal interbreeding
Tue, 14 Aug 2012 2:40p.m.
By Dan Satherley
Since it was discovered in 2010 that most modern humans outside of Africa share DNA with Neanderthals, it has been assumed early humans and the now-extinct species must have interbred.
But new research suggests that isn't necessarily the case at all, and that having a common ancestor would be enough to explain the 4 percent of DNA out-of-Africa Homo sapiens and Homo neanderthalensis both have.
Dr Andrea Manica of the University of Cambridge led a new study into the genomes of the two species, and found a common ancestor 500,000 years ago would be enough to account for the shared DNA.
“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 hybridisation," she says.
"So, if any hybridisation 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.”
Under current theories, early Homo sapiens left Africa and came across Neanderthals living in Eurasia. The two species interbred before the Neanderthals died out around 30,000 years ago, leaving modern humans with a trace of their DNA.
But according to the new research, a common ancestor of both species populated both Africa and Eurasia. Around about 300,000 years ago, the two groups became isolated from one another, and began to evolve differently – those in Africa became Homo sapiens, and those in Eurasia, Homo neanderthalensis.
Homo sapiens across Africa had quite a large genetic variation however, and those in the north would have had more DNA in common with Neanderthals from Europe and Asia than Homo sapiens in the south.
"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 (specifically, genetic variants) that is also shared with Neanderthals," says Dr Monica.
In other words, when Homo sapiens left Africa, heading north into Europe and Asia, it would have been Homo sapiens that had more DNA in common with Neandarthals already that left, rather than those in the south with less shared DNA.
"The idea is that our African ancestors would not have been a homogeneous, well-mixed population but made of several populations in Africa with some level of differentiation, in the way right now you can tell a northern and southern European from their looks," says Dr Manica.
“Based on common ancestry and geographic differences among populations within each continent, we would predict out of Africa populations to be more similar to Neanderthals than their African counterparts – exactly the patterns that were observed when the Neanderthal genome was sequenced, but this pattern was attributed to hybridisation.
"Hopefully, everyone will become more cautious before invoking hybridisation, and start taking into account that ancient populations differed from each other probably as much as modern populations do.”
The study was published in the Proceedings of the National Academy of Science.
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