Modern humans may have been wiped out by flu if they had not mated with Neanderthals, a new study suggests.
Scientists at Stanford University have discovered that ancient trysts led to the swapping of important DNA which protected humans from diseases after they left Africa.
Neanderthals became extinct around 40,000 years ago, but most modern Europeans still carry about two percent of their DNA in their genomes.
The researchers found that the 152 genes we inherited from Neanderthals interact with modern day influenza A and hepatitis C, and helped our ancestors fend off the diseases when they encountered them.
"Our research shows that a substantial number of frequently occurring Neanderthal DNA snippets were adaptive for a very cool reason," said Dr 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."
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. But our newly emigrated ancestors, by comparison, would have been much more vulnerable.
For the study scientists compiled a list of more than 4,500 genes in modern humans that are known to interact in some way with viruses. They 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.
"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," added David Enard, a former postdoctoral fellow in Petrov's lab.
"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."
The new study was published in the journal 'Cell'.