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Genetic Analysis Solves Human-Neanderthal Interbreeding Puzzle

Geneticists say they have finally settled the debate over when and where our ancestors had sex with Neanderthals

Human history more than matches the best soap operas and Hollywood thrillers. The latest blockbuster is a tale of sex and war on an epic scale but from an era that pre-dates conventional historical analysis.  

It comes from the world of anthropology where researchers are piecing together the twists and turns using the latest genome sequencing technologies and data analysis techniques.

The story began in 2010 when anthropologists announced that they had sequenced the Neanderthal genome for the first time. The announcement came with the bombshell that humans and Neanderthal share small fraction of their genes, between 1 and 4 per cent. 

There was also a curious twist: not all humans have these Neanderthal genes. In fact, you can divide humanity in two by this measure. On the one hand there are humans from sub-Saharan Africa who have little Neanderthal DNA and on the other, there is the rest of us who do have it. 

Why the difference? Neanderthals arrived in Europe about 400,000 years ago and disappeared about 30,000 years ago. Modern humans came later, appearing in Africa some 200,000 years ago and expanding into Europe about 80,000 years ago.   

So one idea is that the modern humans who invaded Europe must have enjoyed a little hanky panky with the local Neanderthals who were already lived there. By contrast, the humans who stayed in Africa missed out on the party, hence the difference in DNA.

But there is an alternative hypothesis. Humans and Neanderthals have a common ancestor who lived in Africa about 500,000 years ago. Some of these guys clearly went off and evolved into Neanderthals. 

But of the rest that evolved into modern humans, some must have been more closely related to Neanderthals than others. The alternative theory is that this distinction was somehow preserved in the structure of early modern human society in Africa. 

When modern humans finally left Africa and moved to Europe, it was the group more closely related to Neanderthals that made the journey. By contrast, the modern humans less strongly related to Neanderthals stayed behind, hence the genetic difference we see today. 

Today, Sriram Sankararaman at Harvard Medical School and a few pals say they’ve found a way to to distinguish between these scenarios. 

Their trick is based on the way genetic material shuffles around after each generation. This shuffling ensures that contiguous chunks become smaller as time goes on. So the size of the contiguous chunks is a handy measure of their age. 

Sankararaman and co say that by this measure, humans and Neanderthals must have shared their genes between 47,000 and 65,000 years ago, well after the exodus from Africa. 

That’s good evidence for the first theory that humans and Neanderthals enjoyed one almighty love-in about 50,000 years ago in Europe, although the real story is probably one of rape and pillage rather than of peace and love. 

That’s a fascinating episode from an area of science that is changing the way we think about ourselves and our relationship to the world around us. 

Bring on the next instalment–anyone for a Denosivan love triangle? 

Ref: arxiv.org/abs/1208.2238: The Date Of Interbreeding Between Neandertals And Modern Humans

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