Two new DNA studies this week contribute to a possible route map of modern human migration from the Horn of Africa around 60,000 years ago, and their first sexual encounters with Neanderthals.
The first of the studies shows that, along with European and Asian people, modern North Africans carry genetic traces of Neanderthal ancestry. This suggests that Sub-Saharan Africans are the only modern humans whose ancestors did not directly interbreed with Neanderthals. The study team, led by palaeontologist Federico Sánchez-Quinto of Barcelona’s Institute of Evolutionary Biology, analysed the genomes of 125 people from 7 North African locations. Their research, published in the journal Plos One found that North African populations had similar levels of Neanderthal DNA in their genomes as Europeans and Asians (1 to 4%), even in cases where the North Africans were genetically isolated from Europeans and Asians for the last 12–40,000 years, such as the Tunisian Berbers and Saharawi. This suggests that the traces came directly from ancient interbreeding with Neanderthals around 50-60,000 years ago, and not with more recent interbreeding with the descendants of modern human/Neanderthal hybrids.
It was the groundbreaking 2010 study of ancient DNA by the Max Planck Institute in Germany which revealed that everyone with European and Asian ancestry can trace between 1 and 4% of their genes directly back to the Neanderthals. The genes can even be found in people living in areas where no Neanderthal fossils have ever been found, such as China and Papua New Guinea, which also points to interbreeding with modern humans at the point before they began to colonise the rest of the planet. DNA analysis can’t (yet) show exactly when or where the first sexual encounter between Neanderthals and modern humans took place, but the Middle East is the geographical nexus.
We also don’t know whether or not sex between them was common, but it is likely that the successful production of hybrid children was a rare event over the thousands of years that the 2 groups coexisted. Despite sharing a small percentage of nuclear DNA, modern humans and Neanderthals do not share the same mitochondrial DNA. As mitochondrial DNA is exclusively maternally inherited, it is likely that only male Neanderthals were able to mate successfully with female modern humans and that, in line with Haldane’s rule, only their female hybrid offspring survived – male hybrids would be absent, very rare, or sterile. However, these female hybrids, carrying their Neanderthal fathers’ mitochondrial DNA, did not produce a lineage that survived into the present day. This scenario would account for the presence of Neanderthal nuclear DNA, but the lack of Neanderthal Y-chromosome and mitochondrial DNA in modern human populations.
Given that modern Europeans appear to average a bit more Neanderthal DNA in their genomes than Asians, they probably acquired further small doses of Neanderthal genes as they moved through their ranges into Europe. Asians acquired a further mix of archaic human genes from the Denisovans who ranged widely over the continent from Siberia to Southeast Asia.
Which leads to the second DNA study reported this week by the Wellcome Trust Sangster Institute, revealing that a previously unknown modern human population explosion occurred around 40-50,000 years ago. It is believed that the population growth occurred over a very short period when modern humans left their traditional coastal migration routes and began to explore inland territories as they genetically adapted to living in the mountains and forests of Europe and Asia where there was an abundance of space and food.
Was it their Neanderthal genes which gave modern humans the genetic boost to thrive in these new inland environments? Even though only a very small percentage of the Neanderthal genome made its way into modern humans, that fact that it has persisted in all populations outside Sub-Saharan Africa to the present day may hint that their genes gave us something of significant value. As well as red hair, freckles and pronounced brow ridges, they may have conferred survival advantages such as greater resistance to cold weather, the ability to withstand new diseases or to digest different foods. Who knows, they may even have conferred certain cognitive advantages such as the ability to navigate seas, lakes and rivers.
The problem is that the quality of Neanderthal (and Denisovan) DNA data is just not good enough yet for scientists to make reliable conclusions and, until it is, our specific genetic heritages remain murky. However, just as the Middle East is a geographical nexus, so the period ranging 40-50,000 years ago is clearly a temporal nexus in our complex development.