A group of early human ancestors may have migrated out of Africa much earlier than thought and bred with Neanderthals in Europe, say scientists who have studied DNA from an ancient hominin bone found in Germany.
The genetic data recovered by scientists from the Max Planck Institute for the Science of Human History and the University of Tubingen in Germany, provides a timeline for a proposed hominin migration out of Africa that occurred after the ancestors of Neanderthals arrived in Europe by a lineage more closely related to modern humans.
These hominins interbred with Neanderthals already present in Europe, leaving their mark on the Neanderthals' mitochondrial DNA.
The study, published in the journal Nature Communications, pushes back the possible date of this event to between 470,000 and 220,000 years ago.
Prior research analysing nuclear DNA from Neanderthals and modern humans estimated the split of the two groups at about 765,000 to 550,000 years ago.
However, studies looking at mitochondrial DNA showed a much more recent split of around 400,000 years ago.
Moreover, the mitochondrial DNA of Neanderthals is more similar to that of modern humans, and thus indicates a more recent common ancestor, than to that of their close nuclear relatives the Denisovans.
There has been debate about the cause of these discrepancies, and it has been proposed that a hominin migration out of Africa might have occurred prior to the major dispersal of modern humans.
This human group, more closely related to modern humans than to Neanderthals, could have introduced their mitochondrial DNA to the Neanderthal population in Europe through genetic admixture, as well as contributing a small amount of nuclear DNA to Neanderthals but not to Denisovans as recently detected.
However, more data was needed to evaluate the feasibility of this scenario and to define the temporal limits of the proposed event.
The femur of a Neanderthal excavated from the Hohlenstein-Stadel Cave in southwestern Germany provided just such an opportunity.
"The bone, which shows evidence of being gnawed on by a large carnivore, provided mitochondrial genetic data that showed it belongs to the Neanderthal branch," said Cosimo Posth of the Max Planck Institute.
The proposed scenario is that after the divergence of Neanderthals and modern human mitochondrial DNA, but before the Neanderthal specimen, designated HST by the researchers, and the other Neanderthals diverged, a group of hominins moved from Africa to Europe, introducing their mitochondrial DNA to the Neanderthal population.
Thus this intermediate migration out of Africa would have occurred between 470,000 and 220,000 years ago.
"Despite the large interval, these dates provide a temporal window for possible hominin connectivity and interaction across the two continents in the past," said Posth.
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