Ancestral interbreeding: our Neanderthal relatives may be closer than we thought

Neanderthals, our enigmatic and extinct relatives, may have been even more closely related to us then we thought, new research suggests.

It is well established that modern humans interbred with Neanderthals in Europe around 50,000 years ago, leaving Eurasians with around 1-3% of our genome containing Neanderthal DNA. However, it now appears that this was not a one-way exchange.

A paper published in Nature last month suggests that a small band of modern human ancestors may have interbred with Neanderthals as far back as 270,000 years ago. This almost triples the 100,000 year lower limit of this interbreeding event that was reported in another Nature paper just last year.

The most recent evidence comes from mitochondrial DNA (mtDNA); a small ring of DNA found in the cell’s power plants and passed down the female line. The mtDNA showed that the earliest Neanderthals were more closely related to our other extinct cousins, the Denisovans, than to modern humans. This implies that the common ancestor for both Neanderthals and Denisovans left Africa before the two populations split and evolved in different directions. Younger Neanderthal skeletons, dated to around 50,000 years ago, have mtDNA which is more closely related to modern humans than to Denisovans. One of the most likely explanations for this change is an interbreeding event between modern humans and Neanderthals.

Scientists have been attempting to narrow down the dates that such an event may have occurred, so that future research efforts may begin in the right place. To do this, scientists sequenced mtDNA from a Neanderthal femur found in a cave in Hohlenstein–Stadel, Germany. The mtDNA in this sample was more like that of modern humans. The sample was dated using a technique known as the molecular clock, which assumes that mutations accumulated between generations occurs at a steady rate. This technique dated the sample to around 124,000 years ago. It is descended from a Neanderthal lineage that split from other Neanderthal lines 270,000 years ago, making it one of the oldest Neanderthal samples from one of the oldest lineages, found thus far.

Whilst further genetic analysis looking at the rest of the genome from the Hohlenstein–Stadel sample would be useful, this is a significant result. It reveals that our history and presence in Eurasia is much more complicated than we thought, showing that with a greater understanding of other species and our relationship to them comes a greater understanding of ourselves.

This article was written by Michael Mabbott and edited by Sam Stanfield.

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