Humans can fight viral diseases as they interbred with this extinct species

Modern humans inherited genetic defences against viral diseases like hepatitis and influenza from Neanderthals when the two species interbred 50,000 years ago, a study has found.

Neanderthals mysteriously disappeared about 40,000 years ago, but before vanishing they interbred with another human species that was just beginning its global spread.

As a result of these ancient trysts, many modern Europeans and Asians today harbour about two per cent of Neanderthal DNA in their genomes.

"Our research shows that a substantial number of frequently occurring Neanderthal DNA snippets were adaptive for a very cool reason," said Dmitri Petrov, a professor at Stanford University in the US.

"Neanderthal genes likely gave us some protection against viruses that our ancestors encountered when they left Africa," said Petrov.

"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 at Petrov's lab.

The findings, published in the journal Cell, 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," said Enard, who is now an assistant professor at the University of Arizona.

"But that closeness also meant that Neanderthals could pass on protection against those viruses to us," Enard said.

In the study, scientists show that the genetic defences that Neanderthals passed to us were against RNA viruses, which encode their genes with RNA, a molecule that is chemically similar to DNA.

The scientists 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.

Enard then checked his 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.

From this, 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.

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.

The findings show that it is possible to comb through a species' genome and find evidence of ancient diseases that once afflicted it -- even when the viruses responsible for those diseases are long gone.

This technique would work especially well for RNA viruses, whose RNA-based genomes are more frail than their DNA counterparts, Enard said.