The findings, led by researchers from the Universities of Arizona and Stanford, showed that while Neanderthals became extinct about 40,000 years ago, many modern Europeans and Asians today carry about 2 per cent of Neanderthal DNA in their genomes.
Early humans inherited 152 genes from Neanderthals that helped them fight off modern day HIV, influenza A and hepatitis C whenever they encountered them.
"It's not a stretch to imagine that when modern humans met up with Neanderthals, they infected each other with pathogens that came from their respective environments," said lead author David Enard, Assistant Professor in ecology and evolutionary biology at the University of Arizona.
"By inter-breeding with each other, they also passed along genetic adaptations to cope with some of those pathogens," he added.
According to studies, 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.
In the study, published in the journal Cell, the team showed that the genetic defences that Neanderthals passed to humans were against RNA viruses, which encode their genes with RNA, a molecule that is chemically similar to DNA.
The team examined 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.
In addition, the findings also demonstrate 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 noted.
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