On the move
Palaeoanthropologists agree that the garden of Eden - if it ever existed - would have been in Africa. That's where we all originated. What's less clear is how people have moved about since within Africa, and at what point, if at all, the groups that had left the continent to populate the rest of the world, came back. Now, using DNA from thousands of individuals across - and outside of - Africa, George Busby is beginning to compile some of the as yet unwritten recent history of the African population, as he explained to Chris Smith...
George - Various studies have shown that humans left Africa something like 50,000 to 70,000 years ago and populated the world. Most of the ancestry in people alive today outside of Africa is the result of this main exodus. What we were interested in looking at was what the population structure within Africa looks like now. And in particular, how much ancestry might be the result of back migrations into Africa much more recently, so within the last 5,000 to 10,000 years.
Chris - How do we know, George, that there was this mass exodus 55,000, 60,000 years ago in the first place?
George - There have been several genetic studies that have looked at this and they've shown that the diversity that you see in Africa is much greater than the diversity outside of Africa, and the sort of most obvious reason that that would be is that there was a bottleneck which means that there was a reduction in population size as people left Africa.
Chris - And that fits with the archaeology, does it?
George - It fits with the archaeology as well and we're now starting to get ancient DNA. So DNA from long extinct hominems in fact like Neanderthals and all of this information is reiterating the idea that the majority of the ancestry from people outside of Africa is a result of this big exodus.
Chris - So, where does the idea that people went out and then appreciable numbers came back again come from?
George - Actually, for a long time, there wasn't any particular thought this is what happened. Much more recently within the last 3 or 4 years, there's been genetic evidence. What we really tried to do in this study was to try and systematically characterise this non-African ancestry and see if it is ubiquitous across Africa and where, and really try and work out when we think this ancestry might have come in to Africa.
Chris - And you're using modern genetic techniques to do that.
George - Yeah. We collected a whole load of samples from 48 different African populations. So, we've got these individuals of which there were about 2,500 in total. We compared them to 1,200 individuals from outside of Africa. We sequenced some of their DNA. So we looked at about 330,000 different places in the DNA. So the idea here is that if two people share identical stretches of DNA then these two people are likely to be closely related to each other.
Chris - This means that you can trace sections or segments of genetic information coming into Africa from European populations. But equally, you can see where individuals or groups in Africa have been going because there will be chunks of DNA which have moved as populations have moved within Africa as well as you can look at both influx and intra-African migration.
George - That's right. Most DNA that comes from Europe is efficiently different from African DNA that it's actually quite easy to spot the differences. But even within Africa, you can start seeing differences in relationships between people from eastern and western Africa and eastern and southern Africa, and western and southern Africa as well. So, we can begin to understand how these chunks of DNA have been moving around.
Chris - Presumably, you can also get a temporal relationship there because the more dilute the signature becomes and the more diverse within the whole of Africa it becomes, the longer it must have been there.
George - Yeah, that's right. So if you share a very long chunk of your DNA that really implies that our common ancestor was very recent. So, we can use that property of DNA to understand when groups might have been related in the past.
Chris - Very hard question to put to you because this is a detailed paper which looks at a huge number of people and a huge continent. What were the real standout kind of things that you began to find when you did this analysis?
George - One of the really big results from this is that we're really piecing together the different migrationary events that have affected different parts of Africa. European ancestry coming into west Africa, we see Asian ancestry coming in to east Africa at a slightly different time, around a thousand years ago. Also, we see movement of genes within Africa and in particular, these movements seem to be associated with the expansion of agriculture in Africa. Prior to about 5,000 years ago, the vast majority of people in Africa were hunter gatherers and it's only within the last 2,000 to 5,000 years that people have really started being agriculturists, and agriculture technologies were likely to have originated in central West Africa like where Cameroon is now. What we see is that there was certainly a movement of genes from central West Africa into southern and eastern Africa within the last 2,000 years. So what we're seeing here is real concrete evidence that this expansion really did involve the movement of people as well. So the vast majority of Africans, south of line between Cameroon and Somalia today can trace their ancestry back to this movement of people from central West Africa.