Relationships are complex – and none more so than the sex-laced, migratory mystery of Neanderthals and modern humans.
A startling new discovery begins with a Neanderthal femur bone found 80 years ago and ends with a change to the timeline of human history, published today in the journal Nature Communications.
A 124,000-year-old Neanderthal fossil found in Germany has turned up modern human DNA. This suggests that hominin migration out of Africa occurred much earlier than expected – more than 270,000 years ago.
The story as we previously knew it went as follows: The common ancestor of modern humans, Neanderthals, and Denisovans lived between 765,000 and 550,000 years ago. While modern humans remained in Africa until about 60,000 years, Neanderthals and Denisovans spread throughout Eurasia. Breeding between humans and Neanderthals became more common roughly 50,000 years ago.
This, however, seems to be incorrect. It turns out that some Homo sapiens (or close relatives of them) walked out of Africa more than 270,000 years ago and subsequently interbred with Neanderthals in Europe. It’s likely that these ancient migrants then vanished from the fossil record for some reason or another, but not before their genetic legacy left an imprint on generations of Neanderthals to come.
This is a huge discovery. Paleontologists have struggled for decades to reconcile piecemeal fossil data into a coherent timeline of ancient human history.
"We are realizing more and more that the evolutionary history of modern and archaic humans was a lot more reticulated than we would have thought 10 years ago," co-author Fernando Racimo of the New York Genome Center told New Scientist. "This and previous findings are lending support to models with frequent interbreeding events."
The current evidence comes from the mitochondrial DNA (mtDNA) of a Neanderthal femur bone discovered in 1937 near the entrance of Hohlenstein-Stadel cave in southwest Germany.
"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 lead author Cosimo Posth of the Max Planck Institute in a statement. However, it also contains elements of early human mtDNA.
Mitochondria are the energy powerhouses of our cells and carry DNA separate from nuclear DNA, which is inherited from both parents. mtDNA, on the other hand, is inherited via maternal lineages and can trace when populations split from one another.
Neanderthals and modern humans were believed to have split more than 550,000 years ago, however recent mtDNA evidence suggests this occurred around 400,000 years ago.
By comparing the differences between the mtDNA in this ancient femur bone and other Neanderthal fossils, the team used mutational rates to calculate that these Neanderthals split from each other between 316,000 and 219,000 years ago.
This find is one of a slew of recent discoveries changing the timeline of human history. Last month, researchers reported the discovery of Homo sapien fossils in Morocco that pushed the origin of humans back by a whopping 100,000 years.
For his part, Dr. Posth remains reticent to rule out other possibilities. It is possible that these ancient hominins belonged to a different species closely related to modern humans, but not yet known.
The team are now working to retrieve nuclear DNA from the femur bone to clarify the hypothesis even further.