Julia Lee-Thorp

Isotope biogeochemistry and early hominin diets

This talk will look back at the genesis of isotope chemistry applied to the hominin record, examine what we have learned, and pick out some opportunities for future progress.

FULL TRANSCRIPT 

Thank you very much, Marin. I was reminded as Greg was speaking and dancing, I'm not going to dance. One of the interesting things about this meeting is that there are lineages of researchers, and I think Greg is probably kind of the grandchild stage compared to Thure because he was Ben Passi’s student. So we have the whole sort of lineage of isotope chemists here amongst other kinds of researchers, which is really, really interesting. So what I would like to talk about today is half looking back and talking about how we got to where we are and stable isotope applications to hominin diets, hominin diets and environments. I'm not going to deal with the environments, just the hominin isotopes. And so it's looking back and then I would like to spend a little bit of time doing that and then I'll move on and talk about where some of the ideas that we may have for how we deal with what we've produced over the last 40 something years now. 

And I would just mentioned before I carry on that a lot of the work that I'm going to talk about later on has been produced in collaboration with Scott Blumenthal. So he is my co-author on this and we've discussed the contents. Okay, so what I want to ask really is where have we come from and then where are we going? Because we have a huge amount of data now and we have to move out of the phase of collecting lots of data and do something really sensible with it. Design questions and methods in which we can use all of this material to answer some of the problems that we would like to see resolved. So that's really what I want to do with the talk. And then, so this is the first part is where have we come from? Because I think Thure alluded to this earlier, it's taken a very, very long time to get here, nothing in terms of paleo years of course, but it's taken us about 40 to 50 years to get to the point where we are now. And it's been hard work because as Thure also alluded, he's kind of given both our talks already. 

It's taken a very long time to convince the community that these methods could be usefully applied and that we needed permission to analyze precious samples. So, in the first part of my talk, what I'd like to show is that our focus has been on diet-genesis, which is an issue that we always have to be interested and engaged with, but it's, for some time it was perhaps the overriding factor. So, it wasn't just diet-genesis, but it was the specter of diet-genesis that smeared over everything else and we've had hard work getting through that sort of veil. 

So, there was a question about, the questions are all about diet-genesis, not about what are the good questions that if we can overcome this, what are the good questions that we can answer that we can address? And some of those have got to do with our sort of rather lessened state of our knowledge earlier on, and I'm talking about the late seventies, early eighties. In the last century, we didn't understand a lot of things very well and they include, include the nature of the crystal chemistry. We had to learn about calcified tissue chemistry again. The chemical and physiological pathways, including digestive pathways for animals and people. We didn't understand all of the pathways from the environment to calcified tissues. We didn't understand what the behavioral influences are. So all of these things were poorly understood and we didn't always know that we didn't know this, which made it a little bit more difficult. So I'm going to start at this point here, which is in 1981, Sullivan and Harold Krueger published this seminal paper which showed that a direct relationship between the carbon isotope values in bone collagen and bone appetite. And they showed that there's a simple relationship between the two of these. Now there was some things that one has to note about this. 

Harold Krueger ran a radiocarbon lab and he did a lot of work in East Africa. So he knew that he knew that there was a problem with appetite measurements on radiocarbon. So, he had some experience with the problem of diet-genesis and that he also knew that there was usually no collagen left in bones from tropical environments. So he also understood that it would be very useful to get around this problem. So, he found a simple relationship and he concluded with pretreatment that he designed in the radiocarbon lab, he concluded that bone apatite, carbon isotope values were reliable as long as one got rid of, so to speak, the diagenesis. Apart from Russia just one point out, one other thing is that most of these points in this straight-line relationship of animals, not of humans. So that was followed absolutely immediately in 1982 by a paper saying quite categorically that you absolutely cannot do this bone apatite values of carbonized type values are useless. 

And this area over here was pointed out because these values all stood out outside of that straight line relationship. But this is where we begin to see the nuances of understanding the pathways is that the diet collagen, carbon isotope values are not a very good test for humans. And the reason is that the pathways, the collagen, , carbon isotope values change for different diet types. So humans are going to have all sorts of different diet types and we cannot use this test if we have humans as samples. So that was pointed out quite quickly by Hal Krueger and a couple of subsequent papers and I demonstrated that he was correct. The pre-treatment methods used in this study were completely inappropriate. What they did was recrystallize the bone apatite into something else. So it's just not, wasn't very useful. So this wasn't a very good test and one had to figure out another way of doing it. 

So let's skip that one. So the method that, let me step back [00:09:00] a sec. So I started my PhD in 1982 when Margaret Schoeninger and Michael De Niro had come out and said, you absolutely cannot do this. And when I went around asking for samples from museums, modern samples even, or archeological samples that was met with quite a wall saying, “Well, this came out in nature saying you absolutely can't do it. Who do you think you are coming from a lab in Cape Town thinking that you can do something with this?” But the design of this part of the project was to take a series of animals with known diet and specifically browsers. So using this well-known relationship between C3 plants and C4 plants, and the fact that that translates into the browsers and grazers, we can design a test over a long period of time and a much longer period of time that had been done before. 

So this was a construction from a whole series of sites in Southern Africa. There are some younger sites, oops, this is very hard to work. It goes all over the place. These sites are from middle stone age sites. Klasies River Mouth were choosing animals which could be shown to be reliable browsers. And then these sites over here from here onwards 150,000 years ago, up to 3 million, there's a break over here that I need to point out. So those sites were Sterkfontein, and Makapansgat. And I hope we're not going to going on what Darryl was talking about earlier, it may be that actually some of those are much older than those I have put them here. So the point of what I'd like to point out here is that if we have this long sequence of 3 million years, what we can see is that the browsers here down in this area over here do not shift very much. And if there was all of this amount of diet-genesis around, we would expect them to be shifting up towards the matrix values, which were up around here somewhere in this region. The matrix values of all of these sites are quite positive, and they do shift a little bit, but not very much. And so even by the end of this sequence over here, we can see that the browsers and the grazers are easily separable. They're quite distinct. And there was one other thing that appeared from this is that there was one or two samples where we had the bone and the tooth enamel, and they're quite clearly absolutely distinct.

So, there's a very good example over here. This is bone and one of these three dots over here is the enamel of the same animal. It was a little mandible. And what we can deduce from that is that enamel is pretty stable over a really long period of time, of millions of years. We now know this to be the case in many other situations. 

That was one of the things that comes out of this information. There's just one sort of addendum after this, is that when this test was designed, what we didn't really understand was that quite recently the carbon isotopes in modern atmosphere had changed, had become more negative over the last couple of decades. And this information was just peering in the literature at the time. And so this test is probably a lot of what looks like a small shift is actually the fact that our modern samples were by that stage already a little shifted to the negative. So the green line is probably not quite right in other words, the green line representing modern samples. 

So the next thing was we approached Bob Brain who was to which I owe a great deal and he was really interested and saw the potential. And he suggested this test, which was to analyze Papio and Theropithecus from Swartkrans and he promised that he would be convinced if we could show that they remain separate and in fact, indeed they do. So is we have modern Papio ursinus, Papio robinsoni, and Theropithecus all from Swartkrans Member I, I remember correctly, and a couple of browsers and grazers from the same layers. And indeed they separate out very nicely. So this is the paper that Terry referred to earlier. So very, very distinct and shows that this is a useful approach. 

So this is all taking, maybe I should try to draw your attention. So we started out in 1981. This is now a decade later, and we've got to the point where we might now be allowed to apply this method. So the next after that, we were allowed to analyze some hominins. And the first ones to be analyzed were Paranthropus robustus from Swartkrans and a little bit, that was period in 1994, and then a little bit later we could analyze three Homo specimens. So that's shown here in addition to the baboons over here. Oops. Okay. So, what it showed was that those Paranthropus robustus were primarily participating in a C3 diet, but their diets were shifted by roughly one third towards the C4 end of the spectrum, suggesting that they were not eating just fruits and nuts was some of the earlier prevailing paradigms, but they're actually participating in the C4 food web. So in other words, they were more generalist than what we thought earlier on. 

So that was kind of a little part of this long sequence of learning how best to deal with questions about diet-genesis and how to interpret the results. It was subsequent to those series of analysis and publications that Thure began to analyze material from East Africa. He was eventually given permission and he and his students produced quite a lot of this work. And then after that, the floodgates opened, and I mean the word floodgates purposefully, because what I want to put out here is that these are the carbon isotope values. This is a compilation of carbon isotope and oxygen isotope values for hominins in Eastern Africa that's been produced over the last 10 to 15 years I would say. So what we have now is a huge plethora of data for hominins, which is very nice but what I want to suggest is that we need to think about new ways of looking at this data and analyzing it and using it to model hominin behavior and hominin ecology in particular. So what we can see is that there's, if we just look at the carbon isotopes, that there's this kind of messy shift from values that are more depleted in carbon 13 to values which are much more enriched in carbon 13. 

And the reason I said it's messy is because it's highly variable. It's highly variable at every level, at every period. And so we need to think about our way, how are we going to see our way through all of this data? And then the other thing I would like to point out is that the oxygen isotope values are also highly variable, but they do not change in the same way as the carbon isotope values do. So what we see is this very large scale shift towards engaging with the C4 component of the landscape, and we don't see any similar shift in the same direction for oxygen, which maybe we should not expect. 

Okay, so the last section, I would like to say something about some of the ideas that we've been experimenting with lately in quantifying and finding ways of quantify niche breadth and overlap, niche overlap between hominin individuals and more recently within hominin individuals. So we can use several methods to do this, one of them being inter-individual variability. So we have that data already so we can compare between individuals for different taxa and for different sites using the existing data. And we can compare for oxygen as well. So we can do that in two dimensions or one dimension or two dimensions. And then I want to end up by saying something about, oops saying something about intra-individual variability because we've just done a little, some work of the last couple of years based on laser ablation of individual teeth. 

So one of the inspirations for this work was basically derived from modern isotope ecologists who've been applying some of these methods for a while. And we were particularly impressed with the Vander Zanden model, which was actually, this is designed to look at turtles. And what they pointed out was that if you really want to understand the ecology of a species, you need to look at what's happening as the group, as the population as they call it. But you also need to look within individuals. So it's individual behavior over time and intra-individual behavior, no inter-individual behavior over time. So that's a way in which we can perhaps derive a more holistic view of the ecological as a topic, ecological niche of that animal. So they divided it into three types, which one could divide it up into slightly different ways if one wanted to. So the first one at the top is this has all been converted into carbon isotopes. 

They did this for nitrogen isotopes, but specialist population, specialist individuals. So in other words, that would be a population where they're all doing the same thing and they're all doing the same thing all the time. And then let's just go to the other extreme over here. Here we have a generalist population, which may be picking out different dietary components in the environment, but they're doing it differently between individuals. But each individual is doing the same thing, but inter-individual variation is distinct. And in the middle one has this generous population generalist individuals, in other words, they're all generalists and they're all doing their own thing. 

So part of the inspiration for looking at the Van Zandan model in particular, because what they're saying is you need to not know just what one population is doing. You need to know that what's happening over time. You need some kind of information about the trajectory for each individual. So part of the inspiration came from this early work, this was the paper published by Matt Sponheimer et al., in 2006, so quite a long time ago. But one of the, the surprising bits of information to come out of this paper was that these Paranthropus robustus individuals from Swartkrans again, there were five, was it 4? 5?, 4 individuals in this case. What it showed was this a surprising amount of change within each individual. And this was based on these little trajectories on tooth enamel on outside of tooth enamel obtained by laser ablation mass spectrometry. Okay, so now we've been experimenting with some of those of that existing data and then thinking about this variability using some published data. And then we've generated some more laser ablation profiles. And the importance of the laser ablation profiles on teeth is that that's only method, the only means to look at a trajectory for individuals in the past. That's the only way of doing it. 

Am I running out of time? Okay, maybe I should skip this one. I just want to skip through here because what I wanted to show was that we can do this in different dimensions. So, this is one measure of variability, very simple one being the IQR for the variability for which turns out to be quite a good measure of variability between individuals. And there's one example over here. This includes East African Paranthropus, Southern African Paranthropus, and what's called other hominin groups. And what you might see is that there's this kind of relationship here in which we, with greater engagement with the C4 part of the biome, we get this increasing in variability. And then we get these couple of these points over here which are decreasing again. And it turns out that those two are, this is for instance, Paranthropus boisei from East Africa, as you can see that over here expressed in another way. So with increasing C4 and ending up with Paranthropus boisei, we get this highly reduced variability. So these individuals don't show much difference between each other. We can also do that in two dimensions using the carbon isotopes and the oxygen isotopes that are already published. And this allows us to also look not just at the variability within the niche, but also to think about the overlap between different populations or different taxa. And that's shown over here as these Standard Area Ellipse models over here, which is perhaps a little bit easier to see if you look over here at the summaries. So, what we can see here is Paranthropus aethiopicus and boisei are shown to be pretty variable, but when you get down to Paranthropus boisei and Paranthropus robustus, in fact the later boisei are highly invariable, they're very similar. Each individual is similar and the whole group is similar. 

Oops, just not working. 

It is working now. Okay. Alright. So I just wanted to end with this. The last little piece, which is some new work that we've just more or less almost fully completed now, which is to look at intra-individual variability based on tooth profiles. So, this is similar to the work that Sponheimer et al published, but in this case, we are not using the outside; We haven't laser ablated the outside of teeth. We've used only fractured already fractured teeth and laser ablated down the inside of the tooth enamel profile. So quite near the enamel dentine junction. And this, just to compare, in this example over here, these, we had about between 7 and 14 pits for each tooth depending on the size. But this one over here is the bulk sample that had been taken previously. So the idea was that we minimize any damage as much as possible, minimize any destruction, so they're not sectioned. So, this is some of the raw data, two each of a whole sequence of hominins from all from the Turkana Basin, starting with Australopithecus anamensis and then ending up with Homo over here. And I hope you can see that in spite of the fact that we saw Inter group or inter population variability in some, there's less or vice versa.

So, what we believe we are seeing and still busy analyzing this is that there's a slightly different relationship between those two kinds of variability. And the other thing I wanted to point out here, because completely running out of time, is that in the carbon isotope profiles, we see a certain amount of variability change through time, but we don't see any relationship with oxygen isotope profiles. 

I'll skip over that. So here's a summary. I think I'm going to stop with this. Over here we have a summary of intra-tooth carbon isotope ranges. So, in other words, that's the IQR of each of those individual little profiles. And we've constructed a sequence of or series of similar kind of information from hair profiles of modern primates. And what I wanted to point out here in particular was that the range of variability, inter-individual range for hominin and in particular for Homo, is far, far greater than it is for any of the primates and with the possible exception of modern baboons. So, it's only Savana, not just modern baboons, but Savana baboons. So only Savana baboons approach the same level of variability. Hominins are incredibly variable. I think there's something wrong with this goodie it doesn't give us, okay, since I've got back here, can I just point out that these are Paranthropus individuals over here and Paranthropus robustus and Paranthropus boisei changes from the earlier forms to the related forms so that these are the inter-tissue variability in Paranthropus boisei. So it's the lowest of any of the hominin, and we didn't really expect this. We expected to see that anamensis might be the lowest.

And even in the same period of time, Paranthropus boisei and Homo are absolutely distinct in their intra-individual variation. Each one is not responding until I sort of hit it. So those are the conclusions I'm going to have to skip now because I've run out of time. But I just wanted to suggest that there are ways that we can begin to deal with that variability and we can extract good ecologically meaningful information from all of the isotope data that we have already that's already published, and that we can generate more if we think about what happens within individual lifetimes. And maybe this is running out of battery. So I wanted to end by just putting up this picture, which I think you've seen before, is that I personally owe a great deal to Bob Brain, and I was incredibly sad to hear that he just died. I think he was great. He was the most amazing person, in addition to being an amazing scientist. And he was the inspiration for a lot of the work that I did. Yeah, yeah. Okay. So just a series of acknowledgements and I'd like to thank the organizers for this amazing meeting in honor of Richard Leakey. I think it's been a splendid idea and I've had, I think there's been a lot of inspirational round. 

Thank you.

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