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Sex with our evolutionary cousins? What’s not to love?

So what’s it to be, buddy, my cave or yours? Kaptain Kobold

We humans had sex with Neandertals; we bonked the relatives of Neandertals; we got down and dirty with members of an as-yet unrecognised African population; and we, of course, got jiggy with each other.

Yes, we really got around, as shown by papers such as the one published today in the American Journal of Human Genetics.

It’s a remarkable story, within an even more remarkable framework.

That’s because the 150-year-old study of human evolution is in the midst of a revolution. It’s a transformation with implications as immense as the “discovery” by Darwin of Natural Selection.

Yet it’s a revolution just beginning, and one that has barely touched the thinking of most anthropologists who, like me, are specialists in the fossilised remains of our ancestors.

Geneticists now have the capacity to extract and compare the DNA of our long-extinct cousins with our own genes, and to understand not just our biological relationships to them, but also differences in function between our genes, and their impact on our genome through hybridisation.

As already said, this shows our ancestors were “shagging” the Neandertals, with each of us carrying around the genetic legacy of a rather unique love affair!

The closest image in my mind is a fling between Captain Kirk and a female Klingon.

We humans are the product of hybrid mating between early modern humans (who looked just like people alive today around the globe) and archaic humans, including the Neandertals.

And also the “Denisovans”, a 30-50 thousand year old relative of Neandertals found in a cave in Central Asia – known from a single tooth and a finger bone.

We were also doing the love-thang with an archaic African population, as yet unrecognised from the fossil record, although its DNA signature is clear among living Africans.

Talk about sowing the seeds of love! Keep your woolly mammoth cloaks on, boys!

The American Journal of Human Genetics paper, by Harvard geneticist David Reich and his international colleagues, has compared the genome of the extinct Denisovans to those of a wide range of living Asians and Oceanians.

Over the last 30 years, a large body of DNA evidence has accumulated supporting the peopling of Asia in a single dispersal of modern humans from Africa 60,000 to 70,000 years ago.

It was, after hundreds of confirmatory studies, cut and dry.

Modern humans got into the region (actually right across Eurasia) either after archaic humans had long gone extinct, or were pushed to extinction by invading moderns.

This was the earliest example of megafaunal extinction, and one with a rather unsavory twist.

But until last year, when the fossilised Neandertal genome was published by US anthropologist Richard Green and colleagues, there was no evidence in human DNA for any hanky panky.

We were genetically untarnished by our archaic cousins. A number of possible hybrid skeletons were hotly debated, but largely disbelieved by anthropologists.

The Neandertal genome from 2010 showed that 1-4% of the DNA of living non-Africans had its origins through interbreeding with this archaic population.

In December 2010, the same geneticists published the genome of the Denisovans and argued that there was evidence for up to 6% of the DNA of living Papua New Guineans having originated from interbreeding.

Yet, Papua New Guinea is a hell of a long way from Central Asia (i.e. Siberia). The study raised many more questions than it answered and left some anthropologists deeply troubled.

This new work builds dramatically on the earlier Denisova work. It suggests modern humans colonised East Asia/Oceania in two waves: one from Africa, following a southern route into Southeast Asia, and a much later one, perhaps from the north.

All of these people carried Neandertal DNA.

Moreover, the earliest immigrants into East Asia/Oceania, the ancestors of indigenous people living today in the southern Philippines, eastern Indonesia, Papua New Guinea and Australia, apparently also interbred with the Denisovans in Southeast Asia before settling distant parts of our region. That’s two interbreeding events!

But how can this be? Southeast Asia, the geographic source of most modern humans in our region tens of thousands of years ago, is a long way south and east of Siberia, the location of the cave where the Denisovan bone was found.

The only solution is that the Denisovan population must have had a very broad geographical distribution, one that spanned Siberia to the tropics of Southeast Asia.

Were the Denisovans Neandertals on a tropical holiday? Or a hitherto unknown archaic population from Southeast Asia?

While answering some of the questions I had from the last Denisova installment, this new study raises a whole set of new ones. I guess only more fossils and fossil DNA will provide the answers.

There has never been a more exciting time to be a student of human evolution. I’m sure Darwin would have been astonished.

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