Modern humans and Neanderthals interbred over a sustained period of around 7000 years, probably in the eastern Mediterranean. That is according to two studies that trace how these two hominins hybridised in unprecedented detail.
鈥淭he vast majority of the Neanderthal gene flow鈥� occurred in a single, shared, extended period,鈥� says at the University of California, Berkeley.
The studies confirm that modern humans acquired important gene variants by mixing with Neanderthals, including genes involved in immunity and metabolism. Furthermore, they show that several populations of modern humans, including some that had mixed with Neanderthals, died out tens of thousands of years ago and left no living descendants.
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Modern humans (Homo sapiens) evolved in Africa sometime around 300,000 years ago. Later, our species spread to every corner of the globe. Genetics tells us that all non-Africans today are descended from a single migration that happened sometime between 70,000 and 40,000 years ago.
However, archaeology has shown that there were earlier migrations. A jawbone in Misliya cave in Israel is at least 177,000 years old, and a skull from Greece is 210,000 years old. Humans also moved in and out of Arabia. What happened to those earlier out-of-Africa populations has been a mystery.
What is clear is that each time modern humans ventured outside Africa, they found other hominins: the descendants of earlier out-of-Africa migrations by other species. Crucially, in Europe and western Asia, they found the Neanderthals. Genetics shows that modern humans interbred with the Neanderthals. As a result, all non-Africans today have some Neanderthal DNA, typically 2 to 3 per cent of their genome.
The two new studies shed new light on the human-Neanderthal hybridisation: an event that shaped our species.
In one study, researchers led by at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, obtained the oldest modern human genomes to date.
They sequenced six genomes from bone fragments at Ilsenh枚hle in Ranis, Germany. Earlier this year, some of the same researchers showed that Ranis was inhabited by modern humans 45,000 years ago 鈥� confirming that our species had reached Europe by this time. The team also obtained the genome of one person from Zlat媒 k暖艌 in Czechia.
These genomes contained some Neanderthal DNA. Based on the length of the Neanderthal DNA segments, the team estimated the interbreeding happened between 45,000 and 49,000 years ago 鈥� and that it was the same mixing event that produced all non-Africans today. The interbreeding seems to have happened around 80 generations before the studied individuals lived.
In the second study, Moorjani and her colleagues set out to track how the level of Neanderthal DNA in modern humans has changed over time. They compiled the genomes of 59 ancient individuals from between 45,000 and 2200 years ago, plus a diverse group of 275 people from the present.
Most of the Neanderthal DNA segments are widely shared, suggesting a single hybridisation. The team estimates this took place between 50,500 and 43,500 years ago 鈥� in line with the estimate from Krause鈥檚 team.
鈥淚t seems that a single Neanderthal group, or a few closely related groups, contributed to interbreeding across all non-Africans alive today,鈥� says at the University of T眉bingen in Germany, who was not involved in the studies.
We cannot be sure where the interbreeding happened, but Krause says his team鈥檚 assumption is that it was somewhere around the eastern Mediterranean. This would have been one of the first places modern humans reached after leaving Africa, and we know Neanderthals lived there.
Given that all non-Africans today share Neanderthal DNA, it seems likely that the interbreeding happened as soon as modern humans got out of Africa, says Harvati-Papatheodorou. 鈥淚 think that the eastern Mediterranean in general is a likely candidate.鈥�
Furthermore, Moorjani鈥檚 team found that some of the Neanderthal DNA disappeared from modern human DNA within a small number of generations, leaving 鈥淣eanderthal deserts鈥� in our genomes. 鈥淭he fact that we have desert regions where no Neanderthal DNA is present in modern people, and that these regions were established very quickly after interbreeding, tells us that these genetic contributions were disadvantageous and were strongly selected against among early modern humans,鈥� says Harvati-Papatheodorou. 鈥淚ndividuals who carried these gene variants very often did not survive.鈥�
Local extinction
The studies also indicate that many early modern human populations did not survive outside Africa. Notably, there is no trace of earlier mixing with Neanderthals in modern human genomes: all the Neanderthal DNA comes from 47,000 to 40,000 years ago. The implication is that the earlier modern human migrations out of Africa did not succeed.
The genomics also shows that some of the modern human populations were shockingly small. To their surprise, Krause and his team found that the Ranis and Zlat媒 k暖艌 individuals were quite closely related, despite being hundreds of kilometres apart. The Zlat媒 k暖艌 individual was 鈥渆ither a distant cousin or a grandmother or grandchild鈥� of the Ranis individual, says , a member of Krause鈥檚 team at the Max Planck Institute for Evolutionary Anthropology.
Furthermore, the same type of stone tools found at Ranis 鈥� (LRJ), characterised by long, leaf-shaped points 鈥� are also found in Britain. Yet Krause鈥檚 team found that the effective population size was 鈥渁bout 200 individuals鈥�, despite them apparently spanning northern Europe.
This LRJ culture has left no living descendants. Neither has the Zlat媒 k暖艌 population.
鈥淚t seems that all human lineages that were present in Europe at the time, including Neanderthals, went extinct,鈥� says Krause. He speculates that a volcanic eruption around 39,000 years ago, in which the Campi Flegrei supervolcano under the Bay of Naples went off and covered much of eastern Europe in volcanic ash, may have played a role.
鈥淗uman history is really one of local extinctions and recolonisations,鈥� says , also at the Max Planck Institute for Evolutionary Anthropology and a member of Krause鈥檚 team. 鈥淚t鈥檚 almost the norm that populations don鈥檛 make it.鈥�
Journal references:
Nature ,
Science
