A comment on ‘Ancient gene flow from early modern humans into Eastern Neanderthals’ paper published in Nature
Prof Chris Stringer, Research Leader in Human Origins at the Natural History Museum, London.
It’s generally agreed that modern humans evolved in Africa by 200,000 years ago, and a small group dispersed from there about 60,000 years ago to form the populations of the rest of the world. This exit date from Africa is often calculated using data from extant mitochondrial DNA (inherited from mothers to daughters) and Y-chromosome DNA (passed from fathers to sons), and is supported by the presence of early modern fossils in western and eastern Asia dating from 55,000 years or later.
This late exit from Africa also seems to be confirmed by the evidence that interbreeding between modern humans and Neanderthals only started to occur after about 60,000 years ago. However, there are much older early modern fossils in Asia, from the Israeli sites of Skhul and Qafzeh, dated to around 100,000 years, which are usually regarded as representing a ‘failed dispersal’ from Africa, one which never established itself properly in Asia and promptly became extinct without issue. This is because there is only disputed evidence of a modern human migration beyond Israel prior to 55,000 years. Such claims have generally been based on stone tools in Arabia and India older than 70,000 years, which may have been made by modern humans, but which have no associated fossil evidence to confirm this. However, these ideas were strengthened recently by the dating of 47 modern human teeth found in Daoxian, southern China, to at least 80,000 years.
Now, ancient DNA studies by Sergi Castellano and colleagues have provided further evidence of a pre-60,000 year old modern human presence in Asia via a surprising source – traces of modern human DNA in the genome of a Neanderthal woman who lived in the Altai region of Siberia. Her genome was reconstructed and published in 2014, showing that she was part of a small and inbred Neanderthal population. The latest comparisons have shown that parts of her genome are closer to those of modern humans than are ones from Neanderthals in Spain and Croatia, something best explained by her ancestors having interbred with modern humans at an estimated date of 100,000 years ago. Not only is this the first evidence of modern human DNA entering Neanderthal populations, rather than the reverse process, but this is also a separate and much earlier interbreeding than the one placed at 60,000 years onwards.
The modern humans concerned might have been populations in Israel like Skhul or Qafzeh, but given the new evidence from China, I think that anywhere in southern Asia could theoretically have been the location of this early interbreeding, since we really don’t know how widespread Neanderthals and early modern humans might have been in the regions between Arabia and China at this time.
At the moment we simply don’t know how these matings happened and the possibilities range from relatively peaceful exchanges of partners, to one group raiding another and stealing females (which happens in chimps and some modern hunter-gatherers), through to adopting abandoned or orphaned babies. Eventually, geneticists should be able to show if the transfer of DNA in either direction was mainly via males, females, or about equal in proportion, but it will need a lot more data before that becomes possible.
Does this change the view that the pre-60,000 year old modern human dispersals failed? If both the early modern human groups and the early intermixed Neanderthals went completely extinct, then this was still ultimately a failed expansion. But the increasing resolving power of genetic studies means that the search is now on for further traces of these mysterious early moderns, and their Neanderthal relatives in Asia. We must also hope for discoveries of significant human fossils from the many ‘empty’ areas of southern Asia, in order to properly to map the people who were there before modern humans made their ‘successful’ dispersal across the region about 55,000 years ago.
Notes for editors
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- ‘Ancient gene flow from early modern humans into Eastern Neanderthals’. DOI:10.1038/nature16544
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