Christopher Stringer [11.12.11]
Introduction by:



Christopher Stringer's Edge Bio Page

Rethinking "Out of Africa" from Edge Foundation on Vimeo.

Rethinking "Out of Africa"

[CHRISTOPHER STRINGER:] At the moment, I'm looking again at the whole question of a recent African origin for modern humans—the leading idea over the last 20 years. This argues  that we had a recent African origin, that we came out of Africa, and that we replaced all of the other human forms that were outside of Africa. But we're having to re-evaluate that now because genetic data suggest that the modern humans who came out of Africa about 60,000 years ago probably interbred with Neanderthals, first of all, and then some of them later on interbred with another group of people called the Denisovans, over in south eastern Asia.

If this is so, then we are not purely of recent African origin. We're mostly of recent African origin, but there was contact with these other so-called species. We're having to re-evaluate the Out-of-Africa theory, and we're having to re-evaluate the species concepts we apply, because in one view of thinking, species should be self-contained units. They don't interbreed with other species. However, for me, the whole idea of Neanderthals as a different species is really a recognition of their separate evolutionary history—the fact that we can show that they evolved through time in a particular direction, distinct from modern humans, and they separated maybe 400,000 years ago from our lineage. And morphologically we can distinguish a relatively complete Neanderthal fossil from any recent human.


You could argue that they're an extreme variant of Homo sapiens, but a very different 'race' from anyone alive today, or, as I prefer to argue, they're a separate species, with a separate evolutionary history. But I've never actually said that that meant they were completely reproductively isolated from us. We know that many closely related species in primates, for example, can interbreed. Various species of monkey can interbreed and have fertile offspring, and so can our closest living relatives, Bonobos and common chimpanzees. 

In my view the Neanderthals were closely related and probably potentially able to interbreed with modern humans, but until recently I considered that while there could have been interbreeding forty or fifty thousand years ago, it was on such a small scale that all trace of it vanished in the intervening years. But it now seems from Neanderthal genome studies that that was not so. We do have a bit of Neanderthal in us, you and I—it's a small amount, but certainly not negligible. 

Does that mean Neanderthals are a different species or does it mean we should include them in Homo sapiens? Well, they are still only a small part of our makeup now, reflecting something like a 2.5% input of their DNA. Physically, however, they went extinct about 30,000 years ago. They had distinct behavior and they evolved under different conditions from us, so I still think it's useful to keep them as a separate species, even if we remember that that doesn't necessarily preclude interbreeding.

Then there are these enigmatic people called the Denisovans, who we only know about because of DNA work that's gone on in the site of Denisova Cave in Siberia. The site has been known for a long time. There were some very fragmentary human fossils from there, a finger bone; a couple of teeth, a foot bone, and each of them have yielded significant DNA. The surprise was that while the foot bone DNA turned out to be Neanderthal, at the eastern limit of their known range, the other fossils had DNA that was quite distinct: it wasn't clearly Neanderthal, it wasn't modern human. It was something different. 

Svante Pääbo and his colleagues have dubbed these people the Denisovans. So we have this site in Siberia with Denisovans, and it looks like it was occupied in quite a short period of time by the Denisovans, by Neanderthals, and finally by modern humans. It's a remarkable site with three different kinds of humans living there in close proximity in time and space. However, the exact dating of these different occupations is still unclear.

Thus the Denisovans are only known from this one site, genetically. The fossils are too incomplete to tell us what these people were really like, except they've got big teeth. However, there are lots of ancient fossils from China, and one from India. We've known about the people in China for a long time, ones who didn't look Neanderthal, and didn't look modern human either. Fossils like from the ones from Dali, Jinniushan, Maba might well be Denisovans, but unfortunately we don't have DNA from them at the moment, and we have to hope that the DNA work will move on, and eventually we can unite the Denisovan DNA with more complete fossils, and say physically what these people looked like.

A further big surprise was that not only were there distinct humans in Siberia maybe 50,000 years ago, but when whole genome scans were done against modern humans, it turned out that there was one group of living humans that seemed to be related to the Denisovans, that had Denisovan DNA in them, and these people are down in Australasia. They're in New Guinea, Australia, and some neighbouring islands, so that's also very unexpected.

The Denisovans are only known from their DNA in Siberia. Down in New Guinea and Australia, there is Denisovan DNA in living people. The best way to explain this at the moment is that modern humans were dispersing through southern Asia towards Australia and New Guinea, and Denisovans must also have been living in that region. So they weren't just in Siberia, they were actually right across eastern Asia and down into Southeast Asia, where there was another interbreeding with people whose descendants ended up in New Guinea and Australia. So those people have got a double archaic dose, if you like: they've got a bit of Neanderthal DNA that their ancestors picked up maybe in western Asia from encounters with some Neanderthals, and then coming through southeast Asia, they picked up some Denisovan DNA, and that gets added to the mix.         

We end up with a pretty complicated story of the interweaving of these lineages, which were separate for hundreds of thousands of years, but then when they overlapped, they exchanged genes. We don't know the circumstances of the interbreeding—we don't know if these were groups that came together peacefully, or maybe some modern humans were lacking mates and decided to capture some from a neighboring group. It can't have been that common a behavior, or there would be a lot more DNA from these archaic people. And it can't even have been a common behavior with the Neanderthals, because of course, if modern humans came out of Africa and spread gradually across Europe, we would expect if there was continuing interbreeding with Neanderthals, then Europeans would actually have a lot more Neanderthal DNA than someone in China or someone in New Guinea. 

The extraordinary thing is the level of DNA is about the same in a modern European, a modern Chinese and a modern New Guinean. One possibility is that an interbreeding event happened early on in southwest Asia. As modern humans first emerged from Africa, they met some Neanderthals—maybe only 25 Neanderthals and 1,000 modern humans. That would be enough. And then that DNA gets carried with those modern humans as they spread out from that area and diversify. 

Another possibility, which Mathias Currat and Laurent Excoffier have recently argued, is that the low level of interbreeding between Neanderthals and moderns was actually due to the unsuccessful nature of most of the interbreeding events. That actually the level of interbreeding in separate events was a measure of the low viability of those interbreeding events— which is why there isn't more Neanderthal DNA in people outside of Africa.


We can see the focus, the center of evolution, for modern humans in Africa apparently moving around from one place to another, driven by climate changes. 110,000 years ago the Sahara was not desert, it was well-watered, with extensive lakes and rivers. And we see evidence of human occupation in the form of stone tools right across the region. At other times those populations completely vanished, and we pick up the evidence of evolving modern humans in East Africa, or down in the south instead. And we have to remember that there are large parts of Africa where we have stone tools, but no fossil record to show us who was making those tools. We've got no ancient human fossils from central Africa or West Africa, none at all. So we have to bear in mind that our picture is still limited in terms of the sites that have been excavated and the information we've got from them. 

So for me, the exact processes involved in our African origin are still unclear. We don't know exactly when it happened, we don't know exactly where it happened. We have modern human fossils from Ethiopia at 160,000 years at Herto and 195,000 years from Omo Kibish. These do look physically like a more robust version of people today, but I think we're also learning that alongside those modern-looking people were surviving forms of more archaic humans, at sites like Omo Kibish, Ngaloba, Singa and Eyasi.

And there were further surprises from a specimen that I and collaborators published on a few months ago. It's the oldest fossil from Nigeria, from a site called Iwo Eleru. It's about 13,000 years old, and yet if you look at it, you would say from its shape that it's more than 100,000 years old. This reminds us that we have a very biased picture of African evolution, with many unknown areas, and there could be relics of human evolution hanging on not only outside of Africa in the form of the Neanderthals and the Denisovans, and over in Flores, this strange creature nicknamed the 'Hobbit'. In Africa itself, archaic humans could have lingered in parts of the continent as well. From some recent genetic analyses, there is evidence of an input of archaic DNA into some modern African populations as recently as 35,000 years ago. So even in Africa, the process was more complicated than we thought.

In terms of modern humans, this means that in a sense some modern humans have got more archaic genes than others. That does seem to be so. So it leads us on to ask again: what is a modern human? Some of the most fascinating ongoing research topics in the next year or two will be homing in on the DNA that some of us have acquired from Neanderthals, that some people have acquired from the Denisovans, and that some African people have acquired, perhaps even from Homo heidelbergensis. 

Scientists will look at that DNA and ask, is it functional? Is it actually doing something in the bodies of those people? Is it affecting brains, anatomy, physiology, and so on? That's going to be a huge focus of research for the next few years because on the one hand, looking at these genes will help to really tell us what makes a Neanderthal a Neanderthal, what makes a modern human a modern human, what makes a Denisovan a Denisovan. But it might possibly also show that, as multiregionalists have argued in the past, robust fossils in regions like Australia could be a reflection of archaic gene flow.

We can say that the shared (specific) features of Homo sapiens (e.g. globular braincase, small brows, chin) evolved first, in Africa, while most of our regional ('racial') traits were added on to that modern template through the action of natural selection, sexual selection, founder effect and drift, as modern humans spread out to the regions where they are found today. But could archaic genes be responsible for some of them, at least?

Darwin was puzzled, of course, by the evolution of those features. If we read The Descent of Man, his favoured view for the evolution of many of the regional features was that they were sexually selected or, we might say, culturally selected. I think he was probably right, in some cases at least. We can see that skin colour generally has a relationship with ultraviolet light, with getting a balance between having enough UV getting into your skin to produce Vitamin D, but not too much of it that it will damage the skin or destroy folic acid. So there's a balancing act in the amount of skin pigmentation, and there's no doubt natural selection is at work on this. But even here, sexual selection in terms of mating preferences for lighter or darker skin could be playing a part. And when we look at other features such as, say, oriental eyes or the kind of hair we've got, Darwin may be have been right, and sexual selection is at work there. As populations spread out in small numbers, cultural preferences for attractiveness might have driven some of those differences. Not much DNA is involved, and some striking-looking differences between populations could have evolved quite rapidly.

There have been some remarkable advances in the time that I've been researching human evolution, which is 40-odd years now. When I began my PhD in 1970 and went on my doctoral research trip in 1971, the technology was very primitive. Basically, I went around Europe with a suitcase full of measuring instruments: calipers, tapes, protractors. I applied these to the fossil skulls of Neanderthals and modern humans that I was studying, spending four months doing that. It took half a day to study a single skull and collect that data, all put down by hand onto a paper sheet that couldn’t be backed up. There were not even any photocopy machines around so I could have lost all of my data quite easily. There were no pocket calculators, there were no photocopy machines—it was entirely non-digital recording.

When I got back to Bristol, I had to laboriously transcribe all of those measurements by hand onto punch cards, which were then fed into the massive mainframe computer for the whole of Bristol University. It was probably about four times the size of this room, but with less processing power than the digital watch that I'm wearing now. A day later I would come back and get the results of that particular analysis. Or if it didn't work because of some minor error in one of cards, I'd have to put them all in again, which happened often. 

Things were laboriously slow. It took me four months on that trip to gather the data. It took me another 18 months to analyze those data, to get the results for my PhD. But my conclusions were clear enough. I had cranial samples of modern humans from different regions, and they grouped with each other in cranial shape, rather than with their local predecessors. And the Neanderthals rarely fell into an intermediate position between ancient fossils and recent humans—they seemed to be heading off in their own evolutionary direction through time, rather than gradually approaching a modern cranial shape.

Now, of course, with the advent of scanning and digital technology, a good graduate student sitting at a computer console here or in Europe or the USA could conjure up an equivalent amount of data that I gathered, in fact probably more data than I gathered, on a series of skulls in a week or two, And they could do a more thorough computational analysis of that data than I managed, in a couple of weeks more. So what effectively took me nearly four years could be accomplished by a good student now in a few weeks!

Advances like CT technology give you access to far more, and far richer, data. I was limited to the craniometric points on skulls where I could put my measuring instruments. But with CT, you can capture the whole shape of a specimen, of course. You can look at the internal cranial morphology, the sinuses, the inner ear bones of Neanderthals, which we now know are differently shaped from our own. We only learned that through CT technology, so all of that has made a huge difference to what we can get out of our fossils. 

In one way I'm jealous of the new generation that can come in and do all of this in such a short period of time. On the other hand, by going around Europe for four months, I actually held the Neanderthal skull from Germany in my hands, and the Cro-Magnon skulls from France, and it was wonderful to have a hands-on approach to these important relics. So with only virtual access to the fossils, I think the people doing the digital stuff on their consoles are missing that special and even emotional contact with the actual fossils.

When I began my work in 1970, it's fair to say that people who believed in evolutionary continuity between Neanderthals and modern humans dominated the field. There was Loring Brace at Michigan, who certainly influenced me in my early studies. Loring firmly believed that human evolution passed through a Neanderthal stage all over the world. Everywhere we looked in the middle Pleistocene, there were 'Neanderthaloid' people, and these were the ancestors of modern humans in each region. Thus if we had a complete fossil record, we would see a gradual transition in each region through Neanderthal-like forms to modern humans. Around 1970, that was probably the dominant view.

Milford Wolpoff was one of Loring Brace's students and he came out of that tradition, but with collaborators he developed his own variant by going back to the views of Franz Weidenreich, the German anatomist. Weidenreich had developed a theory which is now known as Multiregional Evolution. In 1984 Milford, Alan Thorne and Wu Xinzhi published a paper that argued for multiregional evolution from fossil, archaeological and genetic data. Homo erectus, when it spread out around the Old World, started to evolve towards modern humans in each region. But these lines didn't diverge—they were glued together by gene flow. The populations were breeding with each other across the whole range of humans at the time, and so there was no single place where modern humans evolved. Basically modern humans evolved everywhere where ancient humans lived. Thus every fossil could potentially be placed in a lineage leading through to modern humans. And in one of the clearest distinctions from a Recent African Origin model, the establishment of regional features would often have preceded, rather than succeeded, the appearance of shared modern ones.

However there were also people who weren't part of the framework of regional continuity. For example there was William Howells from Harvard, who I spent a lot of time with in the 1970s. Bill was someone who didn't think the Neanderthals were our ancestors, and he exerted an increasing influence on my thinking. We didn't know where modern humans had evolved, but we both felt that it wasn't from the Neanderthals. But if not the Neanderthals, where were those ancestors? Were they in the Far East? Were they in Africa? In the 1970s, we couldn't say. However I followed Bill in arguing that there was probably a single center for modern human origins, given the similarities among humans all over the world, physically and genetically.

During the 1980s, data started to build up that the African record was significant. In terms of both morphology and archaeology, Africa wasn't the rather backward place it was often thought to be. First, modern humans and advanced tools were shown to be there as early as anywhere else in the world. Then as the data grew, it seemed that modern humans were indeed there earlier than they were anywhere else. This was the beginning, in the 1980s, of what we call "Out of Africa".

On the archeological side, Desmond Clark, was also very strong in that view. He had links with Tim White, and Desmond and Tim were people who went out in the field to find the fossils we needed to test our models. I haven't been so lucky on the excavations I've been on in places like Gibraltar, tending to find lots of archeology and fauna, but not the human fossils. But people like Desmond, Tim, Ofer Bar Yosef and Bernard Vandermeersch have invested many years in field work and were rewarded in finding those fossils. Clark Howell was another big influence on me, having written influential papers on Neanderthals during the 1950s and 1960s, and he was a pioneer of field work in many regions. He was someone who was meticulous in the anatomical details that he looked at in fossils and he taught me a lot about how to look at the morphology of fossils. And closer to home I learnt a lot from my Museum colleague Peter Andrews, who helped to sharpen my thoughts about an African origin for modern humans, co-authoring our influential 1988 paper in Science.       

The preceding year of 1987 was a real watershed, with the publication of the 'Mitochondrial Eve' paper in Nature by Rebecca Cann, Mark Stoneking and Allan Wilson. A few of us had been advocating a recent African origin for modern humans before then, but it wasn't until '87 that this topic suddenly made the front pages of journals and newspapers. Suddenly modern human origins became very sexy, and more money became available for research and for field work on recent human evolution.        

Before that, the sexy areas for human evolution were in the much older African record. People working in the Rift Valley and in South Africa were the focus of attention and funding. But after 1987, people started to pick up on the evolution of modern humans as a significant topic, and we started to get more conferences, more fieldwork, and more public interest in our own evolution. Of course I was delighted to ride on that wave of increasing public interest in modern human origins.   

Until 2004, we thought that only modern humans had got across the Wallace line. The Wallace line was named after the zoologist Alfred Russel Wallace, who recognised significant changes in the fauna and flora in Southeast Asia as we move from places like Java across into the islands leading to New Guinea and Australia. The view was that ancient humans like Homo erectus got as far as Java, but they didn't get any further—the assumption was that only modern humans with boats were able to get onto the islands leading to New Guinea and Australia.

Then the find known as Homo floresiensis was made in Liang Bua Cave on the island of Flores, and was quickly nicknamed "The Hobbit," because the Lord of the Rings films were popular at that time. The excavators who described this material argued that they had found a new species of human, small-bodied at about a meter tall, with primitive features in the skeleton, and a brain the size of that of a chimpanzee. And this creature was living on the island of Flores, way over the Wallace Line, five-hundred kilometers beyond Java. Not only that, it was still around 17,000 years ago, long after the Neanderthals had died out. It was an extraordinary claim from a partial skeleton and some more fragmentary material dug up from just this one site on Flores.

I was at the Nature press conference where these findings were announced, and commentated on the discovery, which did impress me. I took this seriously as a distinct human-like species, which had somehow got to Flores and had evolved separately in isolation for a long period of time. The leading view in 2004 was that this creature represented a dwarf form of Homo erectusHomo erectus had somehow headed eastwards, arrived on Flores, and under the conditions of this relatively small island, the species had dwarfed down in size (a process called insular dwarfism, which happens to medium-to-large-sized mammals on islands with reduced resources, when evolution favors a reduced body size). The argument was that this was a dwarfed Homo erectus, explaining the smaller body and brain size.

However, some researchers refused to accept that. They felt that this was such a bizarre find, under bizarre circumstances, and they actually favored the view that they were some kind of pathological modern human, perhaps suffering from cretinism, microcephaly or something called Laron Syndrome. These conditions can produce small brains and small bodies in modern humans, so some people have argued that these findings are not a distinct species at all.        

That view is a minority view, but it continues up to this day. However I'm not convinced by these counter-arguments. We've got now over 100 fossils from Liang Bua, not just that one skeleton—there are a number of other individuals. There's a second jaw bone, which to me looks every bit as primitive and archaic as the first jaw bone that's with the skeleton. And there are two sets of primitive-looking wrist bones. These finds were made in levels from about 17,000 right down to about 90,000 years in the cave, and there's archeology right through those levels, archaeology which in some respects resembles much older stone tools found elsewhere on the island.

So for me, it remains a convincing distinct form of human, and one that may be even more primitive than was originally considered because recent research on the material, more detailed research, has found a number of features that seem to be more primitive than even the ones we find in Homo erectus. The suggestion is now that this might represent an even earlier stage of human evolution, one that's closer to Homo habilis or even to Australopithecus, creatures that lived two million years ago or more in Africa. Although we've got no evidence of it happening yet, the argument is that one of those more primitive forms got out of Africa more than two million years ago, somehow found its way over to southeast Asia, and survived in isolation on the island of Flores until 17,000 years ago, when it went extinct. That would be an even more extraordinary story than a Homo erectus getting there and dwarfing, that you've actually got a relic of an earlier stage of human evolution that got all the way over there.

Lots of questions arise from this very challenging find in explaining where it came from and what happened to it. Did it die out because of the impact of modern humans, which is an argument that's been used for the extinction of Neanderthals? Well, according to the excavators on Flores, there's no evidence of modern humans there 17,000 years ago. Supposedly the modern humans came later. But there is evidence of a massive volcanic eruption about 17,000 years ago, which produced very thick ash in the Liang Bua cave and elsewhere on the island. It may well be that this eruption was so enormous that it devastated the vegetation on the island and led to the extinction of the hobbit, which would be a very sad end after maybe two million years of evolution in a remote region, at the edge of the inhabited world at that time.

Where did it come from? Well, that's also still a mystery. On the one hand, was it from Homo erectus? Mike Morwood has recently argued that it's more likely that the ancestors of the Hobbits came from the north, because the currents of water in that region actually run from Sulawesi southwards, down to places like Timor, and then westwards. That's the opposite direction from a transit from Java to Flores. So Morwood argues that the Hobbit's ancestors will be found further north. Remarkably, he and his colleagues have found stone tools on Flores that are a million years old, which might have been made by the ancestors of the Hobbit. Reportedy he's even found tools which are a million years old on the island of Sulawesi, and that island is also over the Wallace Line. So there may actually be many more populations related to the hobbit waiting to be found on the islands of the region.

We've got a whole unknown history there for the hobbit, just as we've got an unknown history for the Denisovans in East Asia.

Changing topics, I think one of the most remarkable recent finds is the material from the site of Malapa in South Africa. This is material that's been found in the last few years, and we've seen a series of papers published in Science in the last few months. This is a species of Australopithecus called Australopithecus sediba, and it's clearly related to the previously known and possibly ancestral species Australopithecus africanus. The Taung specimen and the 'Mrs Ples' fossil are two famous examples of Australopithecus africanus, a species that lived in South Africa more than two million years ago. 

It's true to say that for most experts, the South African australopithecines have been side-lined from the mainstream of human evolution. The mainstream view has been that East Africa was where the first humans evolved, with Homo habilis coming out of a species like Lucy’s, Australopithecus afarensis. From there, in turn, the species Homo erectus supposedly evolved about 1.8 million years ago.

What’s new is that sediba is close to two million years old and has many more human features than Australopithecus africanus. So we've got these strange fossil skeletons of sediba down in South Africa, on the one hand looking like Australopithecus africanus, but with more human features in the teeth, pelvis, legs and hands. This suggests for people like Lee Berger (the discoverer of sediba), that the transition to Homo occurred in South Africa, not east Africa. You could then turn things around and sideline all of those east African fossil.

I tend to the view that it will be more complex than that. We know there were australopithecines living in Ethiopia, Kenya, Tanzania and Malawi, down into South Africa about 2.5 million years ago. Then if in a number of areas we get parallel evolution in adapting to environmental change, these different species start to use tools to an increasing extent, they start to eat meat to an increasing extent, they start to travel longer distances on two legs to get their food, this could have driven parallel human-like changes in the body, the hands, the brain, even. That is maybe what we're seeing in both East Africa and South Africa. And an even more radical possibility is that hybridization events which we can now map from ancient and modern DNA were also occurring in Africa two million years ago and might have produced some of the mosaic morphologies that we observe there. So which area will eventually turn out to be the place of origin of the genus Homo is still an open question, but sediba reminds us that South Africa could be part of that story, and that perhaps Australopithecus africanus didn't die out. Maybe it carried on evolving, and even started to evolve some human-like features. So this material is important in evolutionary terms, but also important because of the completeness of the several skeletons discovered so far.  

Published so far are two fairly complete skeletons of what are probably a boy, perhaps nine or ten when he died, and an adult female. Still unpublished are at least three more individuals, all from this one site. It looks like these individuals fell one after another into a death trap. They may have fallen into anoxic water, where there was very slow decay of the tissue, and they were mummified before they were fossilized, with even the possibility, according to Lee Berger, of soft tissue preservation. Between the bones and the sediments around them there could be layers of fossilized skin that might have preservation of skin, pores, hair and even pigments. Even more extraordinary if that's true. But just for their completeness, these are really important specimens.

The impact of genetic work on our field is enormous, and growing. When you think back to 1997, a tiny bit of mitochondrial DNA was recovered from the original Neanderthal skeleton found in Germany. I was at the press conference with Svante Pääbo, and it was undoubtedly a pioneering achievement, and a breakthrough. But no one could have believed that ten years later, we'd be talking about most of a genome of a Neanderthal being reconstructed. So the technical and computational advances have been huge.

The ability to recover the DNA, massive computing power, huge databases of comparative DNA samples have allowed us to map most of the genome of a Neanderthal, in fact several Neanderthals, and also recover the DNA of these enigmatic people called the Denisovans. I think wherever there are suitably cold conditions, and just as importantly, where it was predominantly cold in the past there should be good DNA preservation. So in northern Asia and Europe and in sites at high altitude outside of those areas, there should be more DNA to come from the fossils, and we will see increasing amounts from modern human fossils as well, which has been slow to come through because of the problems of contamination. We may find there are other people than the Denisovans and the Neanderthals to be recognized from their DNA in these regions— there may well be more surprises to come. For example there is evidence both from fossils and recent DNA that even Africa had an overlap of modern and archaic humans, with the possibility in a continent so large that there were other descendants of heidelbergensis living there alongside Homo sapiens. These populations could have exchanged DNA too, evidence of which might be found in the genomes of living Africans. We will also get the first good look at functional DNA in the genomes of ancient individuals. For the first time, we can make a comparison, not just between the chimp genome and the modern human genome, but we can now add in the Neanderthal genome and the Denisovan genome. We can start to see what unites those three human genomes compared with the chimpanzees. What evolved along the modern human line to make us what we are? And then individually, what made the Neanderthals what they were? What made the Denisovans what they were? This will have an impact, of course, on our own nature, what makes a modern human a modern human. Already a number of bits of DNA have been identified that are distinct among humans, where the Neanderthals are like chimpanzees. Some of these are concerned with the brain, some are concerned with the skin and physiology, some are concerned with how the skeleton grows, and some are concerned with things like the motility of sperm. These things really are going to help us tell what makes a Neanderthal, what makes a Denisovan, and what makes a modern human. Equally we will see studies of the function of Neanderthal-derived and Denisovan-derived DNA in the modern populations that show this from previous interbreeding. So we will find out whether we picked up short or longer-term advantages from those interbreeding events in terms of local adaptation, resistance to new pathogens etc.

Here's a somewhat simple representation of my current thinking now about human evolution over the last two million years:

We've got the lineage of the hobbit, 'Homo floresiensis' (in quotation marks because its human status in not yet clear), perhaps diverging more than two million years ago, evolving in isolation in southeast Asia, and apparently going extinct about 17,000 years ago.

We've got Homo erectus, most likely originating in Africa, giving rise to lineages which continue in the Far East in China and Java, but which eventually go extinct. In Europe, it perhaps gave rise to the species Homo antecessor, "Pioneer Man," known from the site of Atapuerca in Spain. Again, going extinct.

In the western part of the Old World, we get the development of a new species, Homo heidelbergensis, present in Europe, Asia and Africa. We knew heidelbergensis had gone two ways, to modern humans and the Neanderthals. But we now know because of the Denisovans that actually heidelbergensis went three ways—in fact the Denisovans seem to represent an off-shoot of the Neanderthal lineage.

North of the Mediterranean, heidelbergensis gave rise to the Neanderthals, over in the Far East, it gave rise to the Denisovans. In Africa heidelbergensis evolved into modern humans, who eventually spread from Africa about 60,000 years ago, but as I mentioned, there's evidence that heidelbergensis populations carried on in Africa for a period of time. But we now know that the Neanderthals and the Denisovans did not go genetically extinct. They went physically extinct, but their genes were input into modern humans, perhaps in western Asia in the case of the Neanderthals. And then a smaller group of modern humans picked up DNA from the Denisovans in south east Asia. 

We end up with quite a complex story, with even some of this ancient DNA coming back into modern humans within Africa. So our evolutionary story is mostly, but not absolutely, a Recent African Origin.