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Our earliest ancestors

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Matt Smith:

Welcome to a La Trobe University podcast. I would be your host Matt Smith and with me today is Dr Andy Herries from the archaeology program at La Trobe University. Thank you for joining me Andy.

Andy Herries:

G'day Matt.

Matt Smith:

So you're here today to talk to me about the dating of human ancestors, not dating as you take them out for dinner, but dating as in working out how old they are. You've been working with some fossils that are from South Africa. Can you tell me about the finds that have been coming from that area.

Andy Herries:

The area just outside of Johannesburg in South Africa is one of the richest fossil sites for early humans anywhere in the world, partly that's because you get lots and lots of cave deposits outside of Johannesburg. These preserve the fossils really well. I spend a lot of time trying to work out how old all the various different fossils are in South Africa, because until recently we didn't know how old most of the fossils were, it was very difficult to date them. In East Africa they can date them very easily because they have volcanic material – volcanic material is easy to date. In South Africa there isn't any volcanic material in the caves. We've really struggled in knowing firstly how old the different species of fossils that have been designated in South Africa relate to each other, but also how they relate to fossils in other parts of Africa, and unless we know how old they are, it makes it more difficult to see what evolved to what, and what the progression was through from the apes to humans. And the fossils that we recently discovered, we named a new species which is Australopithecus sediba. It is the most amazing fossil I have ever seen. You don't find complete skeletons of early humans very often. There's a few, but certainly not two partial skeletons and what may eventually turn out to be maybe five or six partial skeletons in the same deposit. Normally we jump up in the air and get really excited if we find a bit of a tooth, or a bit of a leg bone. But finding partial skeletons where everything is articulated with each other…

Matt Smith:

You had to sit down for a while?

Andy Herries:

Yeah, I think everyone had to sit down for a while. Everyone is going to argue over what it all means and what it is, how it fits into human evolution for the next forty, fifty, a hundred years or something, I don't know, but no-one will disagree about is that the fossils are unprecedented as far as the number of specimens that we found, and also their preservation is amazing.

Matt Smith:

OK. So, how are these fossils dated then?

Andy Herries:

In South Africa, caves are always in notoriously difficult places to work, not very many people like working in them, which is why I get so much work, because I've been caving since I was at school. So I've always had a fascination with them and that's been my specialty. So, the different ways that we've sort of attempted it over the years, you can try and date them directly using the teeth. The problem with that is two-fold – firstly they're so rare, the last thing you want to do is grind rare teeth up into a powder. Palaeoanthropologists don't generally like it when geologists try to destroy their teeth, and also there's lots of complexity how certain isotopes are taken up into the teeth. So for my PhD I started doing a method known as palaeomagnetism, which is what I do here at La Trobe. Every now and then the earth's magnetic field actually flips itself around 180 degrees. At the moment, as we all know, the north pole is where the north pole is, but every now and then it actually flips round and so your compass would point 180 degrees the other way. We know when these periods of reversal occur because of lots of other studies of sea floor spreading zones and things like that. So, what we do is, we go to the caves and the direction of the field is preserved in the caves because when sediment is washed into the caves, as all of the sediment drops out of suspension in water, magnetic minerals in the sediment orient themselves with the earth's magnetic field. So when it then gets to the bottom, and it then gets depressed and de-watered and becomes like a rock, that's then fossilized. So we basically go to the cave, take little samples, and we see if the direction's pointing north or pointing south. So that's the method I use and then my colleague at the University of Melbourne, does another method known as uranium lead dating which, until recently, we couldn't really date anything younger than about ten million years, because it just takes such a long time for uranium to decay to lead. We just couldn't measure it. So if we're really lucky, we find stalagmites in the caves which have high enough uranium in some of the layers that there's enough lead for us to actually measure and we can sometimes get an age, at the moment. Hopefully, things will get better as time goes by. So she dated the stalagmites that are underneath the fossils. That told us that the fossils couldn't be older than 2.02 million years. But obviously the fossils aren't in the stalagmites. So that's where my work comes in, that we then use the palaeomagnetism, which we can do on the stalagmites but we can also do it on the sediments. And we know that there's a number of magnetic reversals just after two million years, and so we were able therefore to pinpoint the fossils to about 1.98 million, so it just helped us refine it that little bit more.

Matt Smith:

So the fossils are all from ancestral humans 1.8 million years ago…

Andy Herries:

1.98 million. If you round it all up, it's 2 million years. So both methods correlate with that. What most palaeoanthropologists are interested in is – there's a number of different phases in evolution I suppose, broad phases, and we used to think that tool using was something that defined us from apes. But apes use tools. Chimps use rocks to crack nuts. So they even use stone tools. We then thought, well a big brain is something and what we actually find now is that we don't really get larger brains until around the two million year mark. Before that, our ancestors still had relatively small brains like chimpanzees, and the first defining character that moves us from apes to being more like humans, which happened somewhere in the four million to seven million year mark, depending on who you believe, is that we become bi-pedal, we start walking upright. And then all the way through to somewhere in the 2.5 to 1.8 million year mark, the human ancestors are essentially still bi-pedal apes, they're still quite short in stature, they still have relatively small brains… They're slowly evolving and they're slowly getting certain traits in the teeth and various other things that are more like us.

Matt Smith:

Well, what about this fossil? What characteristics does it have?

Andy Herries:

Then we get into the… what is often termed the muddle in the middle. So from about 2.6 million years ago we begin to see the first evidence of tool-using. And we begin to see the first few fossils that have characters that look a little bit more like us in what we call the genus homo, so we're Homo sapiens, and so what we're interested in is the beginning of things that look more like Homo sapiens, the beginnings of the genus Homo, and we have specific characters that define that as an entity. So we get a number of very fragmentary fossils before 2 million, in East Africa, that have features that people have said, this is the beginning of Homo, but the problem is they're still very fragmentary. It's not until about 1.8 million years ago that we see complete skulls of fossils that everyone would define as being Homo, or the species you would call Homo ergaster and that has a bigger brain, it has a taller stature – it's about 1.8 metres like us, so it looks a lot more like us, whereas the ones before that are probably only about 1.4, 1.2 metres. So with sediba, sediba is interesting because it has features that look exactly like the earlier genus Australopithecus but it has features that are exactly like later Homo. Now, this is where it gets all very complicated, and what everyone would argue over. Because there are these fossils in East Africa that are 2.3 million years old that people have defined as Homo, and sediba is only 2 million years, people have said, well, sediba, even though it's got features of both Australopithecus and Homo, and looks exactly like a transitional species, it can't be because we've got older fossils that people have defined as being of Homo, but one of the problems is that they're fragmentary fossils. We haven't the complete skeletons, almost. And what we see is that if you found one little bit of sediba, you might classify it as one thing, and another little bit of it you might classify as another. Because it has both of these traits. The complexity for us now is to understand when we've only got fragmentary fossils, exactly what are they, and how do they fit in with the picture. Whereas when we have complete fossils, we can really understand the full anatomy of that species. My colleagues would argue, this is the best evidence we have for the transitional species, or what a transitional species between one and the other should look like. It may very well be that we find specimens of sediba that are older than this at some point. There's sort of a big argument over exactly how it fits in. But it may in the end help us to define what the beginnings of Homo is, you know, what are the characters in actual fact, how do we actually define what is the beginning of that genus?

Matt Smith:

Isn't that the whole point of evolution though, that it's a gradual change over a long amount of time.

Andy Herries:

Yeah, I mean, people often have a very old, Linnaean view of evolution where they all follow each other in a linear progression. Evolution doesn't really work like that. It's far more complicated which makes our life really difficult. Evolution happens because you get these mutations in the genes in particular populations. If those genes are advantageous, then that population will reproduce and they will go on. But that doesn't mean that the population that they came from suddenly die out, because they're still adapted to a relatively good way of living, so you end up with a situation where a new species can form and you end up with two species in tandem. And also, evolution is partly driven by environmental change, because as environments change, species will adapt to these little changes in environment and that's the other reason you get different species occurring in different areas. So the stuff in South Africa is all different species from the stuff in East Africa – there's a very different environment. You also have complexities of things where you may get the adaptation of particular features, identical in two different lineages but one doesn't come from the other, they essentially like, evolve in parallel. So that's one of the other things that you could say, is that during this period it's an intense period of climate change in Africa, and so all of a sudden, the variation that we see – we have a more sort of linear progression, for three million years. That might just be because we haven't got so many fossils, but certainly around two and a half to two million years, we suddenly see this explosion of different species of human running around the landscape, all trying out different methods of surviving, essentially, and responding to these climatic changes in different ways. And the real task is to understand which one of those branches is the one that leads through to us. We're trying to understand our origins in that respect.

Matt Smith:

So there's a chance that the fossils that you've now dated aren't exactly our ancestors?

Andy Herries:

There are plenty of people out there who would argue that they're a side branch in evolution. The problem is we have one site. We have one site that's dated to one time period. We may find when we look into things more that there's other sites that do have sediba in them, it's just that they've not been classified as such, because the fossils were so fragmentary. Two million years is a really time in South Africa because you get this transition from Australopithecus africanus which is this earlier species and in actual fact you may have three species of Australopithecus now running around the landscape, whereas previously we only knew about one. And then you have the beginnings of things we considered to be Homo and this other species Paranthropus, which is this vegetarian side branch that evolved and died out. It's incredibly complicated and no matter what I think, sediba will help us re-define our understanding of human evolution and how it works. Everyone is going to sit there and argue over whether it's a human ancestor direct line or not but because we've got male, female, juvenile, adult, babies maybe, from the site, we can actually understand the entire developmental history of an early hominid. And because all these various species were around at similar times, they'll be at similar types of levels and the same developmental characteristics we see will probably be similar in the other species as well, so that no matter what, it helps us understand really what a human ancestor looked like, running around the landscape at that time. If sediba isn't our direct human ancestor, it's very similar to it. If sediba is the direct human ancestor it raises interesting questions because it means that human evolution can happen really quickly because sediba is still relatively primitive in a number of features, and you know, a hundred thousand years later we have things that look a lot more human-like. I think my colleagues are expecting that we'll find older specimens of sediba and this specimen that we found may just be one of the ones that didn't evolve into Homo and kept on its merry way and eventually died out. Time will tell as they say.

Matt Smith:

That's all the time we've got for the La Trobe University podcast today. If you have any questions, comments or feedback about this podcast, or any other, then send us an email at podcast@latrobe.edu.au. Dr Andy Herries, thank you for your time today.

Andy Herries:

No worries.