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Web edition: October 5, 2012
Print edition: November 3, 2012; Vol.182 #9 (p. 8)
A new study suggests that present-day Europeans share more genes with now-extinct Neandertals than do living Africans, at least partly because of interbreeding that took place between 37,000 and 86,000 years ago.
Cross-species mating occurred when Stone Age humans left Africa and encountered Neandertals, or possibly a close Neandertal relative, upon reaching the Middle East and Europe in the latter part of the Stone Age, says a team led by geneticist Sriram Sankararaman of Harvard Medical School.
The new study, published online October 4 in PLOS Genetics, indicates that at least some interbreeding must have occurred between Homo sapiens and Neandertals, Sankararaman says. But it’s not yet possible to estimate how much of the Neandertal DNA found in modern humans comes from that interbreeding and how much derives from ancient African hominid populations ancestral to both groups.
A separate analysis of gene variants in Neandertals and in people from different parts of the world also found signs of Stone Age interbreeding outside Africa. That study, published online April 18 in Molecular Biology and Evolution, was led by evolutionary geneticist Melinda Yang of the University of California, Berkeley.
Results from Sankararaman and Yang’s groups “convincingly show that the finding of a higher proportion of Neandertal DNA in non-Africans compared to Africans can be best explained by gene flow from Neandertals into modern humans,” says evolutionary geneticist Johannes Krause of the University of Tübingen in Germany.
Other studies have found that ancient interbreeding may not be necessary to explain the presence of Neandertal DNA in modern humans. It may be possible that African populations ancestral to both H. sapiens and Neandertals possessed some genes that became part of both species’ genomes. Evolutionary ecologists Anders Eriksson and Andrea Manica of the University of Cambridge recently demonstrated the plausibility of this scenario using a model based on more than 100 populations of human-Neandertal ancestors spread across Africa, Europe and West Asia.
Sankararaman’s analysis assumes that the common ancestors of humans and Neandertals more than 230,000 years ago consisted of two African populations and one population outside Africa. It’s not clear whether a more complex model that includes 100 or more populations of human-Neandertal ancestors would yield any signs of late Stone Age interbreeding, says Cambridge’s Manica.
Sankararaman and his colleagues measured the lengths of DNA segments shared by Neandertals and present-day Europeans. Since genetic reshuffling via sexual reproduction reduces the size of such segments over time, lengths of Neandertal-related chunks of DNA in people today can be used to calculate the time since those chunks entered the human genome.
The researchers say that mating between European Neandertals and modern humans most likely occurred between 47,000 and 65,000 years ago.
Other evidence finds slightly more Neandertal DNA in present-day East Asians and South Americans than in Europeans (SN: 9/22/12, p. 5). Interbreeding might have occurred separately in Asia and Europe, Sankararaman says. Or large numbers of people migrating into Europe after interbreeding took place could have diluted the Neandertal genetic contribution to populations already living there.
Citations
S. Sankararaman et al. The date of interbreeding between Neandertals and modern humans. PLoS Genetics. doi:10.1371/journal.pgen.1002947
Suggested Reading
B. Bower. DNA unveils enigmatic Denisovans. Science News, Vol. 182, September 22, 2012, p. 5. Available online: [Go to]
A. Eriksson and A. Manica. Effect of ancient population structure on the degree of polymorphism shared between modern human populations and ancient hominins. Proceedings of the National Academy of Sciences, Vol. 109, Aug. 28, 2012, p. 13956. doi:10.1073/pnas.1200567109. Abstract available: [Go to]
Sriram Sankararaman website: [Go to]
M. Yang et al. Ancient structure in Africa unlikely to explain Neandertal and non-African genetic similarity. Molecular Biology and Evolution. Doi:10.1093/molbev/mss117. Abstract available: [Go to]
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If these studies, as it seems, are saying that indigenous Africans can have no connection with the Neanderthals, how is it that many of them have an almost "residual occipital bun"? Their heads seem to be bigger at the back than is normal in Cromagnon heads in general. Sometimes the rear of the head seems to be actually hanging downward just a bit. Where might they have obtained this feature if not from Neanderthals? Not every indigenous African has the feature, but a great many do.
Is there any reason to think the Neanderthals (or their precursors) would never have moved south into Africa? The climate alone is an argument in favor that they might have, or would have wanted to. There's even more reason to think they emigrated north FROM those warmer climes. There doesn't seem to be any evidence of Neanderthals in the African anthropologic bone collections, though. Or might I be wrong? Might there BE some skeletal evidences of Neanderthals in Africa? Or might some exist, but which have gone unrecognized as such?
It seems odd that such a trait, in a diminished, residual form, could be found in people in Africa, yet nowhere else EXCEPT in the anthropological records of Neanderthals.
This tends to hint to me that the studies are not comprehensive, but are focused on one aspect of genetics. That's not their fault, of course, because you have to be in a very tight focus on genetics if you want to learn anything at all. What it does tell me, though, is that there is a whole LOT more we don't know yet because we haven't yet studied it. Or even looked for it, because we assume it isn't there. We tend to associate the Neanderthal with the cold temperate climates, and maybe nobody is looking for evidence that they were in a warmer one earlier, because they think they were all in the north. So if they DO find some evidence, it might easily be misinterpreted. We are only human, after all. Even scientists.
I also find it odd to think that the race of Neanderthals came into being and spent their entire history in cold climates. To do so, they'd have HAD to have mastered both fire and the use of skins and furs to protect them against the elements. They couldn't have learned it IN those cold climates, because they'd have died off from exposure before they could establish themselves. Therefore, they HAD to have learned those things in a benign climate, before venturing into a climate that can kill them with ease. Where else might that have been but in Africa?
So is it just possible that some bones dug up have been identified as something else, when they actually were Neanderthal - or their precursors?
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