As recently as 2008, scientists thought that Neanderthals and modern humans had never mated.

Then, last year, they said that the two species had, but that the few Neanderthal genes that survived in modern human DNA were not functional.

Now researchers believe that key versions of immune system genes in modern humans appear to have been passed down by archaic relatives, including Neanderthals, after all.

Indeed, DNA inherited from Neanderthals and newly discovered hominids dubbed the Denisovans has contributed to key types of immune genes still present among populations in Europe, Asia and Oceania. And scientists speculate that these gene variants must have been highly beneficial to modern humans, helping them thrive as they migrated throughout the world.

This DNA has had ‘a very profound functional impact in the immune systems of modern humans,’ said study first author Laurent Abi-Rached, a postdoctoral researcher in the lab of senior author Peter Parham of the Stanford University School of Medicine.

Neanderthals were stocky hunter-gatherers who populated Europe and parts of Asia until about 30,000 years ago. In 2010, a team of biologists led by Svante Paabo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, sequenced the Neanderthal genome via DNA extracted from ancient bones.

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