How It Works: First, could you tell us why you decided to write The Origin Of Our Species?
Chris Stringer: I actually decided to write it about three years ago as there had been a lot of new developments in human evolution and I wanted to explore them. I also wanted to bring together different approaches; you need the archaeology, the environmental evidence, the dating methods and genetic evidence, etc, [to get a full picture].
HIW: How robust is the ‘Out of Africa’ theory today?
CS: I think the Eighties was a time of turmoil where it wasn’t clear what our origins were. On the contrary I would say it was around 2000 when the mainstream view was that we had a single origin in Africa. In the last 10-12 years we have seen in some ways a confirmation of that pattern as more data has been collected, but also that it is far more complicated; it is not simply an African origin. We have clearly got DNA outside of Africa coming from Neanderthals and even you or I will have 2-2.5 per cent of that DNA, while if you were from Australia or New Guinea you would have DNA from the Denisovan people. So the story now is ‘mostly out of Africa’, but not entirely.
HIW: Speaking of the Denisovans, why are they causing such a stir in the human evolution field?
CS: Well, no one had known about them until the last two years, apart from a few people who knew about this cave in Siberia called Denisova. But then, we had an extraction of DNA from a finger bone and a tooth that was published and suggested that this was a distinct human form; not a modern human, not a Neanderthal, but an ancient human that – for want of a species name, which it is not – was called ‘Denisovan’ after the cave.
So that was the first stage, but just as I was finishing my book Nature published that scientists had now got most of the Denisovan genome sequenced and it turned out that this was actually a form more closely related to Neanderthals than to modern humans. It was perhaps an offshoot of the Neanderthal line or – you can take the other view – that Neanderthals were an offshoot of the Denisovan line. So the argument went then that you had a three-way split of humans roughly 500,000 years ago, rather than a two-way split.
HIW: What relationship did modern humans have with Neanderthals/Denisovans?
CS: In my view they evolved separately for about 100,000 years and then about 60,000 years ago a small group of modern humans came out of Africa into the Middle East. From this we estimate that there was some interbreeding with Neanderthals, presumably in the Middle East, and this is where the Neanderthal DNA was picked up. Then modern humans started to spread in all directions, eastwards to China and Australia and westwards into Europe.
The only place we have any solid evidence of interaction comes from Europe and western Asia. We think the groups must have overlapped for 10,000-20,000 years until the Neanderthals died out roughly 30,000 years ago. Of course, the big question is: why did they die out and were modern humans responsible? I think the old views were quite polarised. There was the idea that we basically blended with them and the other view that we completely replaced them. Today I think the view is more that we did replace them, but not totally, and there was a bit of interbreeding. Why did they physically die out? It’s probably a combination of factors: war, disease, climate change, tool use and many more.
HIW: What can we expect from future research?
CS: In terms of the big picture of our origins, I think the genetic data suggests that we could be in for more revelations. For example, it was only in the last two years that we discovered who the Denisovans were! The same is true for the ‘hobbit’ species on the island of Flores [in Indonesia], which we only discovered ten years ago.
So I think we have to be aware that there will be areas where we have big gaps in the evidence. But in terms of fleshing out the main bits of the story, we have quite a lot of very good data in a lot of regions, but we need to start filling in data for the areas in between.
Chris Stringer’s new book The Origin Of Our Species is out in hardback and paperback now. For more information, visit www.penguin.co.uk.