Neanderthals had been living outside of Africa for hundreds of thousands of years, giving them time to evolve defences against viruses.
Boston: Neanderthal genes likely gave modern humans protection against viruses that our ancestors encountered when they left Africa, according to a study.
The research, published in the journal Cell, shows that the genetic defences that Neanderthals passed to us were against RNA viruses, which encode their genes with RNA, a molecule that's chemically similar to DNA.
Neanderthals mysteriously disappeared about 40,000 years ago, but before vanishing they interbred with another human species that was just beginning its global spread, said researchers from the Stanford University in the US.
As a result of these ancient trysts, many modern Europeans and Asians today harbour about two per cent of Neanderthal DNA in their genomes, they said.
"Our research shows that a substantial number of frequently occurring Neanderthal DNA snippets were adaptive for a very cool reason," said Dmitri Petrov, an evolutionary biologist at Stanford's School of Humanities and Sciences.
"Neanderthal genes likely gave us some protection against viruses that our ancestors encountered when they left Africa," Petrov said.
When first contact occurred between the two species, Neanderthals had been living outside of Africa for hundreds of thousands of years, giving their immune systems ample time to evolve defences against infectious viruses in Europe and Asia.
Our newly emigrated ancestors, by comparison, would have been much more vulnerable, researchers said.
"It made much more sense for modern humans to just borrow the already adapted genetic defences from Neanderthals rather than waiting for their own adaptive mutations to develop, which would have taken much more time," said David Enard, a former postdoctoral fellow in Petrov's lab.
The findings are consistent with a "poison-antidote" model of gene swapping between two species, researchers said.
In this scenario, Neanderthals bequeathed to modern humans not only infectious viruses but also the genetic tools to combat the invaders, they said.
"Modern humans and Neanderthals are so closely related that it really wasn't much of a genetic barrier for these viruses to jump," said Enard, who is now an assistant professor at the University of Arizona in the US.
"But that closeness also meant that Neanderthals could pass on protection against those viruses to us," Enard said.
The scientists compiled a list of more than 4,500 genes in modern humans that are known to interact in some way with viruses.
They checked the list against a database of sequenced Neanderthal DNA and identified 152 fragments of those genes from modern humans that were also present in Neanderthals.
The scientists showed that in modern humans, the 152 genes we inherited from Neanderthals interact with modern day HIV, influenza A and hepatitis C -- all types of RNA virus.
The researchers concluded that these genes helped our ancestors fend off ancient RNA viruses that they encountered upon leaving Africa.
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, researchers said.
This makes sense, Enard said, 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.