Scientists have discovered that crossbreeding with Neanderthals, and another ancient hominid, called Denisovans, may have helped shape the human immune system that continues to help us fight off deadly viruses and bacteria, reports the BBC.

Last year, researchers discovered that Neanderthals and our human ancestors had mated, laying to rest the long-held belief that the two species never crossed paths sexually. Despite evidence of inter-species trysts, scientists thought that the Neanderthal DNA present in modern humans was basically junk, holding no real value.

Now, a new study from Stanford University researchers, which was published today in the journal Science, shows that key parts of human DNA likely came from humans breeding with both Neanderthals, and another cousin of humans, the now-extinct Denisovans.

Our entire base of knowledge about Denisovans comes from a single tooth and finger bone, which were found at a site in Russia.

At least one form of the DNA in question, known as human leukocyte antigen (HLA), commonly appears in humans from Europe and Asia, but rarely shows up in people from Africa, since their ancestors apparently didn’t come in contact with the Neanderthals and Denisovans. The study’s researchers believe that 1 to 4 percent of Eurasian DNA comes from crossbreeding with our long-gone brethren.

HLA genes are known to play a vital role in boosting the human immune system’s ability to ward off illness cause by viruses and malicious bacteria. According to the study’s author, Laurent bi-Rached of Stanford University’s school of medicine, HLA plays “a very profound functional impact in the immune systems of modern humans.”

“The HLA genes that the Neanderthals and Denisovans had, had been adapted to life in Europe and Asia for several hundred thousand years, whereas the recent migrants from Africa wouldn’t have had these genes,” said Peter Parham of Standford, who led the study. “So getting these genes by mating would have given an advantage to populations that acquired them.”

Some researchers disagree with the conclusion that interbreeding with other species of humans played such a significant role in forming our immune systems.

“I’m cautious about the conclusions because the HLA system is so variable in living people,” said John Hawks, assistant professor of anthropology at the University of Wisconsin-Madison. “It is difficult to align ancient genes in this part of the genome. Also, we don’t know what the value of these genes really was, although we can hypothesise that they are related to the disease environment in some way.”

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