Current thinking is that modern humans began moving out of Africa and into Eurasia about 70,000 years ago.
When they arrived, they met up with Neanderthals who, along with their own ancestors, had been adapting to that geographic area for hundreds of thousands of years.
The Eurasian environment shaped Neanderthals’ evolution, including the development of adaptations to viruses and other pathogens that were present there but not in Africa.
The new study provides details about the role of adaptive introgression, or hybridization between species, in human evolution.
“Our research shows that a substantial number of frequently occurring Neanderthal DNA snippets were adaptive for a very cool reason,” said study co-author Dr. Dmitri Petrov, from the Department of Biology at Stanford University.
“Neanderthal genes likely gave us some protection against viruses that our ancestors encountered when they left Africa.”
“It made much more sense for modern humans to just borrow the already adapted genetic defenses from Neanderthals rather than waiting for their own adaptive mutations to develop, which would have taken much more time,” added lead author Dr. David Enard, from the Department of Ecology and Evolutionary Biology at the University of Arizona.
The findings are consistent with a ‘poison-antidote’ model of gene swapping between two species.
In this scenario, Neanderthals bequeathed to modern humans not only infectious viruses but also the genetic tools to combat the invaders.
“Modern humans and Neanderthals are so closely related that it really wasn’t much of a genetic barrier for these viruses to jump. But that closeness also meant that Neanderthals could pass on protections against those viruses to us,” Dr. Enard said.
The genetic defenses that Neanderthals passed to us were against RNA viruses, the researchers found.
They reached their conclusions after compiling a list of more than 4,500 genes in modern humans that are known to interact in some way with viruses.
They then checked their list against a database of sequenced Neanderthal DNA and identified 152 fragments of those genes from modern humans that were also present in Neanderthals.
They showed that in modern humans, the 152 genes humans inherited from Neanderthals interact with modern day HIV, influenza A and hepatitis C — all types of RNA virus.
From this, the scientists concluded that these genes helped our ancestors fend off ancient RNA viruses that they encountered upon leaving Africa.
Interestingly, the Neanderthal genes they identified are present only in modern Europeans, suggesting that different viruses influenced genetic swapping between Neanderthals and the ancient ancestors of today’s Asians.
“This makes sense since interbreeding between Neanderthals and modern humans is thought to have occurred multiple times and in multiple locales throughout prehistory, and different viruses were likely involved in each instance,” Dr. Enard said.
The new results also show that retained segments of Neanderthal ancestry can be used to detect ancient epidemics.
“It’s similar to paleontology,” Dr. Enard said.
“You can find hints of dinosaurs in different ways. Sometimes you’ll discover actual bones, but sometimes you find only footprints in fossilized mud.”
“Our method is similarly indirect: because we know which genes interact with which viruses, we can infer the types of viruses responsible for ancient disease outbreaks.”
David Enard Dmitri A. Petrov. 2018. Evidence that RNA Viruses Drove Adaptive Introgression between Neanderthals and Modern Humans. Cell 175 (2): 360-371; doi: 10.1016/j.cell.2018.08.034