It's only relatively recently that the scientific community has begun to accept that early humans interbred with our Neanderthal cousins. Now it looks as if it was not only possible, it was essential for providing us with immunity from strange diseases.
When the Neanderthal genome was published last year, it offered the first conclusive evidence that humans had swapped genes with our closest hominid cousins. This was seen as another possible way in which humans had driven Neanderthals to extinction - when we couldn't out-compete them, we simply assimilated them into our bloodlines. But the question of whether we actually gained something tangible from sexual contact with Neanderthals went unanswered.
That's the focus of new research by Stanford's Peter Parham. He examined the human leukocyte antigens, or HLAs, which are 200 or so genes that are intimately involved with the proper maintenance of the immune system. They also happen to be some of the most flexible genes, with dozens of different possible permutations that allow humans in different locations to deal with vastly different disease threats.
Our earliest human ancestors probably only carried a small number of HLAs because of their limited contact with other groups, and these would only really be able to deal with diseases present in Africa. The question then is where they picked up new HLA variants, or alleles, that would have allowed them to cope with moving outwards to Eurasia.
One such allele is HLA-C*0702, which most people of European and Asian ancestry possess but almost nobody of African ancestry does. That particular variant can be found in the Neanderthal genome, suggesting humans picked up the gene from them. Another example is HLA-A*11, which is generally only found in people of Asian ancestry. This particular variant probably comes from Denisovians, a third race of ancient hominids that we discovered last year.
The case for all this becomes particularly compelling when you look at the bigger picture. Only about 6% non-African human genome came from contact with other hominids, but over half of all European HLA-A alleles come from Neanderthals and other evolutionary cousins. That number only increases the further from Africa you get, says, Parham, with it reaching 72% in China and 90% in Papua New Guinea. This might be because the tropics of southeast Asia contained far more diseases than the frigid north of Europe, necessitating a greater transmission of HLA genes.