According to a new study, we owe our ability to fight off certain viruses, such as the flu, hepatitis, and HIV, to ancient bits of DNA inherited from our Neanderthal ancestors.

The paper, published today in the journal Cell, argues that the interbreeding between Neanderthals and modern humans has left us with 152 gene fragments that interact with specific viruses and which have toughened our immune systems, giving us protection against a host of dangerous diseases.

The conclusion comes from two researchers at Stanford University in California, Dmitri Petrov and David Enard (a former postdoctoral fellow in Petrov’s lab currently affiliated with the University of Arizona).

The pair identified the gene fragments after combing through scientific literature in search of modern human genes known to interact with viruses. Their effort led to a comprehensive list of 4,500 genes, 152 of which have been passed down from Neanderthals, reports Phys.org, citing Stanford University.

Why Some People Still Are 2 Percent Neanderthal

The genomes of modern-day Europeans and Asians consist of about 2 percent Neanderthal DNA. While much of the genetic material originally passed on has been “removed by purifying selection,” these particular snippets stuck around for a reason, the authors debate in their study.

“Our research shows that a substantial number of frequently occurring Neanderthal DNA snippets were adaptive for a very cool reason,” said Petrov, who is 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.”

Furthermore, the scientists suggest that by studying these DNA fragments leftover from ancient humans, we might get a clear picture of the viruses and diseases that ailed Neanderthals during their time on Earth.

“It’s similar to paleontology,” said Enard, referring to the way in which trace fossils, such as fossilized footprints, can help scientists find out details about long-extinct creatures without actually looking at their bones. “Because we know which genes interact with which viruses, we can infer the types of viruses responsible for ancient disease outbreaks.”

Ancient Trysts

To explain their theory, the scientists take us back in time some 70,000 years ago, when Homo sapiens migrated from Africa to Eurasia, where they encountered the Neanderthals.

By that time, these earlier ancestors had had hundreds of thousands of years to adapt to the harsh climate of these new lands and had developed a different immune system, designed to fend off specific viruses depending on the region, notes the Independent.

Neanderthal man reconstruction at the Neanderthal Museum in Mettmann, Germany. Featured image credit: Einsamer Schütze Wikimedia Commons/Resized(CC BY-SA 4.0 )

This meant that Neanderthals carried unfamiliar viruses to which Homo sapiens wouldn’t have been exposed until the two species met in Europe and Asia and interbred.

But the intermingling between modern humans and Neanderthals did more than introduce new microbes in the mix. When the two species swapped viruses, the Neanderthals also passed on to our newly emigrated ancestors the genetic tools to fend off the pathogens.

“We call it the poison-antidote model,” Enard told The Atlantic.

“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.”

As Enard explained, the poison-antidote model is actually more efficient from an evolutionary standpoint.

“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,” he pointed out.

Thank you Neanderthals https://t.co/Otr9NXbgWU — David Rewhorn (@David_Rewhorn) October 4, 2018

In the study, Enard and Petrov show that the 152 Neanderthal DNA snippets we still carry today are specifically designed to interact with specific RNA viruses, such as modern-day HIV, influenza A, and hepatitis C. All these viruses have RNA (ribonucleic acid, chemically similar to DNA) as their genetic material.

Furthermore, these genes are only found in modern-day Europeans, which suggests that Asians populations benefitted from a different host of DNA fragments targeting completely separate viruses.

According to Enard, this is all perfectly valid, as evidence suggests Neanderthals interbred with modern humans at least twice in the past 100,000 years and in different prehistoric locales. These instances likely saw different viruses being exchanged between the two species, he said.