Alasdair Wilkins — The evidence has been mounting for years that early humans and Neanderthals interbred, but now it's pretty much a certainty. Part of the X chromosome found in people from outside Africa originally comes from our Neanderthal cousins.
It's kind of amazing to think that, as recently as just a few years ago, the scientific consensus was that humans and Neanderthals were completely separate species and probably didn't interbreed. Since then, a ton of new evidence has come to light to change that position, and now new research from Damian Labuda of the University of Montreal more or less completes this big reversal.
Neanderthals, one of the last extant hominid species other than our own, left Africa somewhere between 400,000 and 800,000 years ago and settled mostly in Europe until they went extinct 30,000 years ago. Early modern humans left Africa about 80,000 to 50,000 years ago, meaning they overlapped with Neanderthals in time and place for at least 20,000 years. On an evolutionary time scale, that's not a ton of time, but could it be enough to leave lasting evidence of human/Neanderthal interbreeding?
According to Dr. Labuda, the answer is an emphatic "yes." Back in the early '00s, he and his team had identified a particular piece of DNA in the human X chromosome that seemed out of place with everything else, and they wondered whether it might have originated from a non-human source.
That answer came with the first sequencing of the Neanderthal genome last year. Dr. Labuda compared 6,000 chromosomes from all over the world to the corresponding part of the Neanderthal sequence. With the exception of people from sub-Saharan Africa - whose ancestors would have been unlikely to come into contact with Neanderthals, since their territories didn't overlap - every chromosome featured evidence of the Neanderthal sequence.
That even includes particularly far-flung groups of humans like native Australians, who are thought to have reached the island continent by as far back as 40,000 years ago. For that sequence to show up even in such geographically isolated groups, it suggests that there was a lot of interbreeding between the two hominid species, and that pretty much all ancient humans that left Africa passed through Neanderthal territory and had close interaction (read: a ton of sex) with their evolutionary cousins.
Here are some reactions from other scientists to Dr. Labuda's breakthrough:
Dr. Nick Patterson, MIT and Harvard:
"There is little doubt that this haplotype is present because of mating with our ancestors and Neanderthals. This is a very nice result, and further analysis may help determine more details."
Dr. David Reich, Harvard Medical School:
"Dr. Labuda and his colleagues were the first to identify a genetic variation in non-Africans that was likely to have come from an archaic population. This was done entirely without the Neanderthal genome sequence, but in light of the Neanderthal sequence, it is now clear that they were absolutely right!"