It’s believed that Homo sapiens evolved hundreds of thousands of years ago in Africa, but none of them actually left the continent until about 70,000 years ago. Why wait so long?
As Carl Zimmer reports for The New York Times, they may not have. A new study published in the journal Nature Communications suggests that a small group of early humans left Africa 270,000 years ago and headed to Eurasia where they interbred with Neanderthals, leaving signs of their migration in that species' DNA.
Scientists have long believed that humans and Neanderthals split from a common ancestor, later mixing their DNA through interbreeding when they met hundreds of thousands of years later. But researchers have struggled to pin down the timeline.
As Zimmer reports, scientists have collected DNA samples from Neanderthal bones across western Eurasia, analyzing both the mitochondrial DNA or mDNA (which is genetic material contained in the cell's powerhouse passed down from mother to child) and nuclear DNA (genetic material in the cell's control center inherited from both mother and father). But these two types of DNA tell seemingly conflicting tales.
As Rachel Becker at The Verge reports, the nuclear DNA of Neanderthals suggests that the group split off from a common ancestor with humans between 765,000 and 550,000 years ago. But the mDNA suggests that split didn't happen until 400,000 years ago. What’s more, while the nuclear DNA from Neanderthals is similar to their close cousins the Denisovans, their mDNA looks closer to humans. This latest research suggests that an early period of intermingling could explain some of these discrepancies.
Analysis of a 124,000-year-old Neanderthal femur found in the Hohlenstein-Stadel Cave in southwestern Germany yielded some of the oldest mDNA yet found. This sample showed an mDNA lineage that differed from the mDNA found in other Neanderthals so far. And further analysis suggested that the mDNA of this sample diverged from that of other Neanderthal's roughly 270,000 years ago.
The researchers suggest that this divergence could reflects an early wave of intermingling between humans and Neanderthals. When this took place is challenging to say with precision, but it happened sometime after the two species split and before the 270,000-year mark when the genetic changes occurred. It's also challenging to pin down where the interbreeding took place or how many times. Zimmer reports that it could have been just a single encounter between a female human or proto-human and a male Neanderthal that introduced the mitochondria into the gene pool.
Over time, this human-like nuclear DNA disappeared in the Neanderthal genes, but the human mDNA dominated the gene pool, pushing out the original Neanderthal mitochondria and eventually completely replaced it. This period of early interbreeding could explain the closer affinity of Neanderthal mDNA to modern humans than their Denisovan cousins.
This idea of early intermixing of species isn't out of the question, Johannes Krause, the director of the Max Planck Institute for Human History and an author of the study, tells Zimmer. He points out that Africa is physically linked to the near East. “You could have just walked out,” he tells Zimmer.
Not everyone is completely convinced by the study. As Joshua Schraiber, a population geneticist at Temple University not involved in the research, tells Becker, if Neanderthals had a small population living in groups stretched between Spain and Siberia, as researchers believe, it makes the uniform spread of the mitochondria seem less likely. “It’s hard for genes to move when they don’t have cars and airplanes,” he says.
The researchers state in the press release that more study is needed and they hope to gather high-quality nuclear DNA from the Hohlenstein-Stadel femur or other Neanderthal samples to see if there is more genetic evidence for an early wave of humans interbreeding with Neanderthals.