Robert Foley

Turkana, Eastern Africa and the Afrotropical model of hominin evolution

Turkana and Eastern Africa are well known for their fossiliferous environments, hominin record, and the many field projects that have exploited its potential. However, equally important is their biogeographical context, and in this paper we explore ideas from biogeography and ecology and their implications for human evolution.

FULL TRANSCRIPT 

Thank you very much for inviting me to be here. It's an honor, particularly to have a chance to remember Richard who I met when I was an undergraduate and when I first went to Kenya in 1973. And as everybody who ever met him, especially when they were young, would know he was extraordinarily generous with his time, with his ability to infuse people and give people the opportunities. He was also generous with some of the museum vehicles, which was perhaps less of a gift. If you've ever tried to drive one of those old vehicles of the museum back then. After this morning's talks and yesterday's talks, I've come away with one very strong image of Richard.

For the younger people here this is a pipe despite what it says. And I think what you have to know is that Richard was by no means alone pipe smoking was a kind of tribal badge amongst paleontologists and paleoanthropologists at this time. Indeed, I smoked a pipe whether I did it just to be cool or because I liked it, I'm not sure. I think there are one or two other people in this room who used to smoke a pipe as well, but they probably wouldn't admit it anymore more. But apart from the pipe, I think the thing we most associate Richard with of course is Africa and Africa's role in evolution. And that's really what I want to talk about here. Less from the point of view of looking for fossils and discovering fossils, which others know far more about and are much better talking about than I am, but to talk about what are we learning from thinking of Africa as the center of human evolution. 

And here's a phylogeny because I love phylogenies and of course the root of the phylogeny is Charles Darwin. And he was the first person to suggest that Africa was the most likely place we would find hominin origins being Darwin of course, he promptly followed up the sentence with one of his usual qualifications and doubts and saying, well of course it's a long time ago and we don't really know, but he was right. The right hand branch is the extinct branch that's Dubois and Haeckel, who thought that Asia was the origins. And then on the left hand branch are the range of people who have contributed to the idea that Africa is the center. And as we're a multidisciplinary group, I hope somebody will be able to recognize every single person on that in the pictures here. And of course, principle among the ones we wanted to talk about today is Richard and Meave who've done so much to expand that fossil record and to demonstrate it. 

Now I'm very grateful we're all very grateful for that. I think it's probably fair to say that our students are not very grateful. When I learned human evolution, I only had to learn about three names and it was very easy. They need chat GPT now there are so many wretched names to learn, so we should sympathize with them. And the legacy of exams are much harder than the ones we used to take. But the evidence that Africa is the center of human evolution comes from a whole range of sources and is clearly, I think, irrefutable at the moment. One says that with great caution like Darwin, obviously the genetics, the comparison with apes and the fact that we are closest living relative to the chimpanzees and that the African clade is quite clearly established and it's a close relationship going back 7 million years or so. 

But we also know it's not just the origins of the clade that's important, but what happens afterwards, and the genomics again is showing that the modern human lineage, the Homo sapiens lineage also has an African origin. I'm not going to talk about any of this, but just to establish the fact that it's a continuous thing. Now it's not just genomics and I know we're mostly paleontologists here so the paleontology matters. And if we look at the range of taxa that have been identified, and I please don't want to get into an argument about whether these are real species or not, but the taxa that have broadly recognized in hominin evolution today are primarily African in origin. And on the left column are the non-Homo forms, which are not just have African origins but are entirely African in their distribution. We then have a number of Homo species which are African, entirely endemic to Africa and only found there. 

And then we have a few species which are found earliest appearances in Africa, but then they're found in other continent as well, our own species being the best example of that. And then we have this rather slim list on the right hand side, which is the non-African ones, some of which are rather isolated and poorly known. And of course it's not just, those are the first appearances of the fossils. We can turn to the archeological record and have a look at that. I could have a gut Bernard here and say, Bernard, that thing at the bottom right is a stone tool. 

But again, the major technological modes, the Lomekwian, the Oldowan, the Acheulian, these all have their origins again in Africa, their earliest appearances in Africa, and they're listed there. But it's not just the stone tools. Many of the markers that we associate with the long-term evolution of Homo sapiens, we're taking a kind of rather gradualistic view that it's not a sudden revolution a 100,000 years ago. It's a gradual evolution stretching back 2 or 300,000 years. Again, we find the markers of modernity, if you want to call it that fairly clearly earlier in Africa, the use of ochre, evidence for hafting and then things to do with the symbolic patterns. I've put ritual behavior of the dead here at 150,000. I might have to change  that of course after yesterday. 

And again, the other thing to think about is that when we talk about Africa as the center of hominin evolution, and this I'll return to, there's very much an emphasis on out of Africa. Ironically, the African model is usually referred as the out of Africa model as if it only becomes interesting once hominins leave Africa. And really I want to say no, it's the other way around. What we really want to know about is why so much happens in Africa. So what I wanted to do is explore some comparative models and ways of thinking about Africa as the source of hominin evolution. I refer to this as the Afro tropical model simply because that's the zoological region that hominin species belong to along with many other taxa. And of course, that means that hominins evolved in a specific evolutionary biogeographical context and that means it's the context that matters. 

These are the conditions that promoted the characteristics of becoming human and the evolutionary dynamics that are involved. And I think what I want to suggest, we have to look at that in a slightly broader context than we usually do and think a bit about the whole evolutionary patterns. So the question really is why Africa? Why should Africa be the homeland? Now of course, one could say immediately, well, it's a very big continent, so it's more likely to happen there. But I think there are other and more interesting answers. This is just showing the distribution of mammals through the main biogeographical realms. And you can see there's one would expect there's a variation in the number of families that there are, but there's also enormous variation in the number of the percentage of those that are endemic to the region in which they're found. Australia is the obvious extreme. It's pretty difficult not to be endemic in Australia. But we also notice if you go to the two tropical ones, the Neotropics and the Afrotropical, they have a higher percentage of endemic species than you find in the more temperate regions. And so, sorry, I was trying to use the, oh yes, this is a very large pointer. 

So that we here, the percentage of endemics is really quite high. And the question is, why should that be? What are the reasons for such endemism? So just to remind you, the major regions of the world, so the Afrotropical and Africa, when I'm talking about Africa, I'm actually talking of course about sub-Saharan Africa, south of Sahara because the Northern Africa biologically belongs to either its own region, the Sahara Arabian one or is often part of the Paleo arctic as well. So why Africa? I also emphasize that when I talk about Africa and I suggest it's not the whole of Africa, neither is it a constant it's a dynamic process. And I would distinguish between geological Africa, which is a lump of rocks floating around in the plate, tectonics and biological Africa, which fluctuates and changes. And we know for the Pleistocene, when we get this sequence of major climatic changes here and the fluctuations, we know that at some points when it's very, very arid, the Sahara is extreme and becomes a real barrier. 

And the Afro tropical zone is restricted to sub-Saharan region much it is now, but when it's very wet, such as it was at the beginning of the present interglacial, the Afro zone, Afro tropical zone expands much more broadly. So, when I talk about the Afrotropical model and not necessarily confining to Africa, geologically speaking. Now what I want to use as a framework is to talk a little bit about endemism and cosmopolitanism and sort of the extreme endemism or an endemic species is one that has, is found in one local area, one small area. It's generally thought of as a restricted distribution as we'll see, there are a number of different meanings which have been interpreted and most species are endemic. So here's a very scientific graph which I put together. This floats around terribly that just points out that most species are relatively restricted in their distribution and very few are really cosmopolitan and spread over larger and larger areas. And the map and graph below shows that that endemism creates the uniqueness of biological regions and ecological regions. And the darker the colors here, the more unique those regions are in terms of the taxa that are represented there. So that creates zones of animals that are distantly related to their own clades in other parts of the world and share regions with other taxa. 

But that's a very static view of endemism. And of course we know that over time organisms change their distribution. Indeed our own lineage is one of major changes in distribution. So, we have to think how does this concept of endemism and cosmopolitanism map onto a more dynamic view? So, this is something that Marta and I published a long time ago, and it's a simple model and pretty much describes probably the evolution of any lineage and all that will vary is how wide it is. So geographical range along the X axis time on the Y, and most species almost certainly have a small local origin. And then if they are successful, they will expand in their distribution and then life gets miserable for them, climate changes, whatever happens, something bigger and nastier comes along and they gradually decline in distribution and you end up, (I keep pressing the wrong button), you end up here with a final extinction point and all these extubation points along the way. 

So actually an endemic species can be thought of in three different ways. It can be thought of as it's endemic and this is its place of origin, this is where it evolved. We can think of it as this is its place of origin it's also the point from which it disperses out. And we think of many examples of dispersals as a major process of evolution, or it could be the complete opposite and it could be where it ends up just before it becomes extinct. Now of course, in some cases one and three might be the same area, but it by no means follows automatically. So, we've actually got these three different notions of what endemism is, which we can then think about how this operates in terms of introducing us to or helping us understand why and how hominins evolved in Africa. Well, we know then that there are a lot of hominin taxa are about, I think this is out of date now because they keep adding new ones but broadly speaking, we've got lots of taxa.

And what I've done here is just to plot them into four categories, whether they're local, which pretty much means they find them in one site only, and there are single or maybe two sites that represent them, whether they're regional, which in this case would be whether it perhaps they're found only in our East Africa, only in South Africa, whether they're continental. So in this case across the whole of Africa or the whole of Eurasia. And then intercontinental, there's us basically who are found across more than one continent. I'm being slightly unfair to the Neanderthals here because of course they do Europe and Asia are actually two different continents, so they probably deserve to be promoted slightly higher up the range. However, what we can see from this, the main thing is that most hominin taxa that we know are very local. 

Now of course that might just be a sampling issue. That's simply where we found them and they could have been much more widespread or it could indicate particularly in the context that there's quite high levels of local diversity that they were pretty much like other taxa not getting very far. And if they expanded out, they speciated and performed different clades relatively easily. And we can see that that's not just a question of the early ones are all very endemic. And certainly the later ones, it's only the later ones that become widely dispersed and cosmopolitan in their distribution. But some quite clearly are endemic all the way through. We would currently think of say naledi or floresiensis as being characterized as highly endemic species. 

So what I want to do, sounds going to it, is just run through in a sense this notion. We've got three different ways of thinking about in endemism and we've got three different scales at which we might want to think about. And I just wonder, I can't talk about too much of these, but I just want to mention some examples that may be useful if we take simply the idea of endemism as the point of origin. That certainly makes quite a lot of sense. And there's a wonderful paper by Roland in 2014, and lots of people have written about this, but this expresses it very clearly that if you compare tropical species or taxa and temperate taxa, what you see very clearly is that tropical species have a much higher speciation rate than temperate species. That's the top left hand box there. And if you also then go across to dispersal rate in the far right hand here, we also find that the, (sorry I keep pressing the wrong button here). 

Here we can see that the dispersal out here is much though they disperse much further in pattern. And so this suggests that the tropics, and obviously it has to be the Afro tropics or the oriental tropics given the ancestry, are more likely to be places where we're going to see high diversity. And the Eurasian temperate continents are going to be receivers of dispersals rather than sources of dispersals. And that of course is simply something we see when we look at this number of species in Africa and Eurasia as we see markedly different rates of endemism in that sense. 

We can also look at this at the regional scale in East Africa and ask the question, well, Africa is a very variable place, it's large place and it's not really the level. And Kay was pointed this out early today about the scale is so important here that different things go on in different parts of Africa and we can't treat it as a whole. And again, this is a paper by Peter Linda published a long time ago now, which shows the sort of phylogeny of the biological realms of Africa very clearly and it divides up as you would be not surprisingly into northeast, west and south. And what I would point out here is if you look here, the southern clade doesn't even fit onto the figure. It's rather more distant, so evolutionary, it's much more isolated and more distant from the others and the north and the east. 

In other words, the top right hand quadrant of Africa is the area that is most closely related. And we see that in this attempt here to the regionalization with this corner here as a quite clearly identifiable region. As I'm running out of time, I won't talk about that. And when we then simply look at the African record, the number of species we only find in Eastern Africa is much substantially greater than species we find either in central Africa or in southern Africa. And where we do find species across Eastern and southern Africa, they're more likely to have earlier points of origins in the east than they are in the south. And that goes back to this idea of the tropicality as being the driver of diversity, the driver of higher speciation rates. And what we see in East Africa fits in probably with what if we could do this the same analysis with other animals we'd see a very similar pattern.

If we just think now about endemism in terms of dispersals in a different way. This is best thought of in terms of range size. Now haven't range estimating range size of hominins is a thing that only the foolish would do if I told you with range sizes of different hominins somebody would put their hand up and say, ah, but it's all sampling and it's absolutely true. So instead, this is just looking at catarrhines instead and this is really to point out that if you look at range size in relation to a number of evolutionary and ecological variables, there are some strong and interesting relationships. So range size decreases with diversity. So areas that are highly diverse have lots of species that are quite, have small distributions. Range size increases, the more habitat tolerance that an organism has. Again, this is all fairly common sense when you think about it with dietary specialization as well. 

So what this implies is that when we are finding cosmopolitan species as we are here, we are really beginning to pick up either decreased habitat specificity or increased habitat tolerance. We are moving out of zones of high speciation rates, diversity into areas that perhaps have larger geographical ranges. And again, I think I might've said this the wrong way around, that highly endemic restricted species will be highly specialized. And of course, one thinks here of some of the Parathropenes or the early Australopithecines that might've been very restricted in their distribution because they're also restricted in their diet. 

If we drop down to the third scale, which we could take any part of Africa if you wanted, but it would be rude not to talk about Turkana in this conference. I'll use Turkana as an example and we can say the question, well, what is Turkana in terms of its endemism and case and others have given talks about the fossil record here. And I think again, we come back to the notion of what does endemism mean? And one option is that endemism means that it's a species factor, as Mikael has suggested, sorry, that it's refugium as Josie Joordens has suggested, and that it's the area where animals survive in periods of high aridity or periods of extreme climatic stress. But the opposite argument which Fortelius has placed is that actually Turkana Basin is a species factory and that it is driving novelty and so on. I don't want to get into the arguments particularly in this context as to which is right, but it raises the question, when we find these isolated species, are they the last gasp of a relic species hiding out in difficult conditions or is this a species factory generating more and more diversity? 

My final point takes us to the fieldwork that Marta and I have been doing in Turkana in the last 13 years, something that which and Marta’s erc projects that have funded it. And just to interest the idea that of course it's not just it's local regionally, it's also varies through time and what might be the Turkana evolutionary drivers in the Plio-Pleistocene. And we've heard about the Oligocene and the Miocene earlier today could be very different once we get into the Pleistocene with its very structured and patented cyclic variations in climate. And we've been working down in the Kerio Valley where we've discovered a whole bunch of sites, both of Holocene and middle Pleistocene sites, multiple localities from which we now have something like 17,000 fossil specimens. And this is telling us about the different landscape of the Basin. A few pictures just to show I do get into the field occasionally it's not just building models. 

And I think Marta will talk more about this in her talk, I'm sure. The key thing is that what we're finding in these middle Pleistocene deposits, and this is the middle Pleistocene record of Turkana, which is relatively unknown, is the apparent late survival of many species, multiple species of crocodile and pigs, late surviving pig lineages and elephants. And this, I think it's just about bits, Kay’s stromatolites because this one has an Elephus recki tooth inside. It's beautiful, isn't it? So it could be that in the Pleistocene as opposed to in the Plio-Pleistocene, the ecological landscape, the evolutionary landscape of Turkana is rather different. So just to wrap this up, what I've been trying to look at it is to stand back from fossils as the process of discovery and thinking a bit more perhaps possibly in what Curtis was getting at early with his question, thinking more about the evolutionary framework and context in which we should be asking questions about the record that Richard, Meave and many of you here, many others have founded when thinking about at every level, every scale geographically in terms of origins, points of origins, the sources of dispersals and the refugia, and the sorts of processes that are involved. 

I think a lot of this is to do with the bottom right hand corner of Kay's triangle, the biological processes that underlie the evolution that we're looking at. So in conclusion, I would encourage us to think more about the comparative models that are available to us to think about. In this case, I chose endemism and cosmopolitanism as a framework for thinking, but there are plenty of other ones, ways one could find avenues into this problem that Africa is a natural source of high evolutionary diversity, and the hominins are part of that. A perfectly natural part of this high evolutionary diversity, evolutionary dynamics that's important that Eastern Africa and I think possibly Turkana in particular are evolutionary hotspots. And we can see this in current distributions of species, that that would certainly be consistent. And then finally is the last bit that the nature, the changes that have taken place as we've moved into the last million years in the landscape, the ecological and climatic landscape of Turkana could have changed those evolutionary dynamics. So we have to think about issues like endemism, points of origin, sources of dispersal in a different way than if we were working in the Plio-Pleistocene. So thank you very much.

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