Episode 165      23 min 42 sec
Early dating techniques: Determining the age of Australopithecus sediba

Australopithecus sediba, discovered in 2008 in southern Africa, is the most significant paleo-archeological find in recent years. These fossilized specimens have anatomical features lying somewhere between those found in Australopithecus africanus and Homo erectus. Geochemist Dr Robyn Pickering discusses the significance of the find, and how the age of A. sediba was determined. With science host Dr Shane Huntington.

"There’s been a big revolution in the last five to ten years where the technology has improved very rapidly that we are able to do the uranium-lead dating and do much more precise palaeomagnetic dating." -- Dr Robyn Pickering




           



Dr Robyn Pickering
Dr Robyn Pickering

Dr Robyn Pickering is a McKenzie Post-Doctoral fellow in the School of Earth Sciences at the University of Melbourne, Australia. She is a geologist and geochemist who has dated the rocks surrounding the Australopithecus Sediba fossils from Malapa, South Africa.

Robyn has been working on the geology of the South African hominin bearing caves since 2002 and has provided new stratigraphic interpretations for the cave sediments at a number of sites, including Sterkfontein. She has also produced the first suite of direct U-Pb ages for the caves in the Cradle of Humankind region and shown how the flowstone layers can act as markers to provide a correlation tool between sites. Her other projects in South African include the human occupation sites of Pinnacle Points on the southern coast, Wonderwerk Cave, the fossiliferous dune deposits along the west coast and a large scale, country-wide stalagmite dating project.

Credits

Host: Dr Shane Huntington
Producers: Kelvin Param, Eric van Bemmel
Audio Engineer: Gavin Nebauer
Episode Research: Dr Dyani Lewis
Voiceover: Nerissa Hannink
Series Creators: Eric van Bemmel and Kelvin Param

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VOICEOVER 
Welcome to Up Close, the research talk show from the University of Melbourne, Australia. 

SHANE HUNTINGTON 
I’m Shane Huntington.  Thanks for joining us.  Human evolution has attracted researchers from a range of disciplines over recent decades.  Grappling with prehuman timelines requires not only expertise in locating, extracting and identifying the remains of distant ancestors, but also requires sophisticated techniques for dating and authenticating ancient remains.  Recent discoveries in innovative analytical methods have enabled researchers to paint a powerful and detailed picture of human evolution, albeit with many questions yet to be answered.  To learn more about these recent advances in our knowledge of human evolution, and in particular how the recently discovered Australopithecus sediba is being studied and dated by paleoanthropologists, we are joined in Up Close by geochemist, Dr Robyn Pickering, a McKenzie Postdoctoral Fellow in the School of Earth Sciences here at the University of Melbourne.  Welcome to Up Close, Robyn. 

ROBYN PICKERING
Thank you very much for having me. 

SHANE HUNTINGTON
Can you give us a quick lesson to start with about the evolution of modern humans - who we would find in our family tree, so to speak? 

ROBYN PICKERING
Well, the big question we need to answer here is looking at different timescales.  So if we go back around 4 million years this is where we find our very early ancestors, the various forms of Australopithecus, which we find mainly in East Africa.  Then as we move through time at around about 1.6 million years ago we have the first fossil remains of early homo, which is the beginnings of our own genus.  Then through a series of various evolutionary steps to around 60,000 years ago is where we see the beginnings of homo sapiens and our direct ancestors. 

SHANE HUNTINGTON
How different are we to these various groupings? 

ROBYN PICKERING
That's a good question, because from a biological point of view even our most distant ancestors at 4 million years ago walked upright and would have held themselves in a similar way to we do today.  It’s one of the main defining characteristics of us as humans, is that we walk upright on two legs, which is called bipedality.  So in terms of that, we are actually very similar.  Then in terms of our brains and our sophisticated culture, specifically we’re very different. 

SHANE HUNTINGTON
When we actually find fossils of these particular members of different groups, how do we go about identifying them as hominid remains?  How do we know? 

ROBYN PICKERING
Well, we have quite a large collection of these fossils already.  There are specialists in this field who have really studied human anatomy and fossil or other great ape anatomy, so we have a good understanding of what the bones should look like.  So when we find fossil material which we suspect to be hominid or early human, we can compare this with known material and there are some very distinctive characteristics on the actual bones. 

SHANE HUNTINGTON
Even when we hear about homicide stories and the like, where remains of a person are found some 10 or even 15 years after they were sort of left in the bush, those bones are often scattered amongst kilometres of terrain.  Is this a similar scenario with what we’re finding with some of these very, very old remains or are they all localised? 

ROBYN PICKERING
Yes, exactly, it’s a good analogy; it’s a good way to understand it.  What we find with the old bones is that often they have been very scattered; they’ve often been buried or moved around form where the animal would have first died and so, yes, it’s a great challenge to be able to piece together the remains that we find. 

SHANE HUNTINGTON
Now, I understand there’s one particular World Heritage site which is an incredible locale for people like yourself and others called the Cradle of Humankind.  Tell us a bit about where that's located and why it’s so important. 

ROBYN PICKERING
So this World Heritage site, the Cradle of Humankind, is found about 60km northwest of Johannesburg, which is one of the largest cities in South Africa.  This region is a massive paleocast region, so we find a lot of caves, and in these ancient cave deposits we find the fossil remains of hominids or early humans. 

SHANE HUNTINGTON

When we look at this particular site, how do we know for sure that all the bones and the remains there are early humans?  How do we go about determining that? 

ROBYN PICKERING
Yeah, that's a good question.  You also have to understand that the majority of the fossils we find are, in fact, not early human fossils; the early human fossils are very rare.  The majority of the fossils we find are from extinct animals, so we have a massive collection of these animal fossils, and then about 1% of the fossils we find, or even less, are early human.  As I said before, they are quite distinctive in their morphologies and my colleagues who are real specialists in this can identify them as being early human. 

SHANE HUNTINGTON
When you're actually there - and I understand you've been to these sites many times - what does it look like?  Can you describe the scene?  Is this a series of caves or is this a very large, big excavation site?  What does this World Heritage site actually look like? 

ROBYN PICKERING
Well, this region, the Cradle of Humankind, today is a series of valleys and it’s lovely rolling grasslands landscape today with a number of sort of rocky ridges and some trees, and then within these grasslands we find the remains of caves.  So they don’t really look like caves anymore, there’s been a lot of surface erosion and the entire landscape has been lowered by about 30 metres.  So what we find today are the remains of caves - they’re not very impressive, actually, they’re mainly just sort of little holes in the ground - and in these we find the sediments, the ancient rocks which had formed inside the caves, and trapped in these the fossil remains.  At a number of these very well-known sites there have been enormous excavations which have been ongoing since the 1960s. 

SHANE HUNTINGTON
When you find one of these bones, how distinguishable is it from the surrounding rock itself? 

ROBYN PICKERING
So the rocks in these caves, the type of sediments are a kind of reddish-brown colour and then the fossil bones are often very white or kind of light cream, so there’s an amazing contrast between the fossils and the sediments, so there’s no mistaking when you find the fossils. 

SHANE HUNTINGTON
Now, Australopithecus sediba is a crucial find in this area.  Tell us a bit about why this is so important and how it’s changed our picture of human evolution. 

ROBYN PICKERING
Well, Australopithecus sediba is a remarkable find in two main areas that make it so special.  The one is that normally what we find in these caves are just small isolated remains; we might find a few teeth or, if we’re lucky, a little bit of a skull.  But in the case of Australopithecus sediba we have two relatively complete skeletons, which is just remarkable; it takes all the guesswork out of the analysis because we actually have most of the components of two skeletons, so the material is amazing complete.  Then the second aspect, which is equally remarkable, is that we’ve been able to date them very precisely.  So we know - we’re pretty confident that these fossils are 1.98 million years old and they fill in a very interesting gap in the early human record. 

SHANE HUNTINGTON
This is Up Close coming to you from the University of Melbourne, Australia.  I’m Shane Huntington.  Our guest today is geochemist, Dr Robyn Pickering, and we’re talking about determining timelines in human evolution.  Robyn, when was the discovery of Australopithecus sediba? 

ROBYN PICKERING
The discovery was part of a large project being run in South Africa to do exploration in this region, the Cradle of Humankind, and look for new sites.  And it was in late 2008 that Professor Lee Berger and his small son Matthew were looking for new sites and discovered these incredible fossils. 

SHANE HUNTINGTON
The site, I understand, is called the Malapa site.  Can you describe what is unique about it? 

ROBYN PICKERING
Well, at Malapa we have this remarkable sort of combination of features.  We have a relatively small site which has this incredible preservation of fossil bone, so it’s not only the early human fossils but it’s the other animals we find there as well are all incredibly preserved.  What this means is that instead of just having a few isolated pieces of bone, we have these relatively complete skeletons.  We have examples where the bones are in articulation, so that means that, for example, the fossil hand we have all the bones from the hand and we found them all lying together, so they hadn't been disturbed or moved around and almost nothing had been lost; this is actually the first time in the fossil record that we found a complete hand of an early human. 

SHANE HUNTINGTON
Do we have an idea of why they’re so complete in this particular case? 

ROBYN PICKERING
Well, we studied this very carefully; this was one of our major research questions.  From looking at the bones themselves and the sediments around them in the whole setting - today it’s on the surface - but we believe that 2 million years ago this cave would have been about 30 metres below the ground and the entrance of the cave would have just been a vertical sinkhole, so it wasn’t the sort of cave you could walk into.  So we believe that the two individuals of Australopithecus sediba would have fallen into the cave by accident and then not been able to get out, and the bodies in the bottom of the cave were them buried very rapidly, which is why we find their bones in articulation and why the skeletons are so relatively complete. 

SHANE HUNTINGTON
Typically in many of these areas of geology and palaeontology and so forth, the deeper you go, the further back in time you tend to go.  Is that the case with this particular site? 

ROBYN PICKERING
No, unfortunately not.  The South African cave sites are very complicated and there has been very cycles of erosion and burial in the caves, so we can't interpret them as simple layer cakes; that the deeper you go, the older you get.  On a short scale, on a scale of a few metres, then yes, the material towards the bottom probably is older.  But it’s been one of the major challenges of this research as understanding the stratigraphy, so the layers of rock and how they form in the caves. 

SHANE HUNTINGTON
Now, a lot of your work is based around the requirement to actually date these fossils.  I understand you date the rocks rather than the fossils.  Can you explain why you take that approach? 

ROBYN PICKERING
Yes, well, knowing how old the fossils are is such a critical piece of information in order to fit them into our family tree, so it’s something which we’ve also doing a lot of research into.  In this case we can't date the actual fossils themselves, they’re too old for something like radiocarbon dating, which only goes back to around 50,000 years.  We knew at this site of Malapa from the types of the other fossils animals that the site was somewhere between 2.5 and about 1.9 million, so we knew it was quite old and that radiocarbon wouldn't be an option.  So then our next option is to try and date the actual sediments around the fossils and if we can work out how old these are then we can go on to calculate how old the fossils must be. 

SHANE HUNTINGTON
Why do we normally use carbon dating for fossils?  I mean, how does that work?  What’s happening in terms of the carbon in the bones and so forth that we find? 

ROBYN PICKERING
Well, while all animals and plants, all living organisms, while we’re alive one of the major building blocks of our body is carbon.  And carbon exists as various different isotopes - it’s still carbon but the atoms are just slightly different - so while we’re alive we all have some carbon-14 in us and this remains at a constant level.  But once you die the amount of carbon in you begins to decay away, so if we find a fossil we can measure the amount of carbon-14 left in the bones and then work out how long it is that that animal has been dead.  The problem is that this carbon, carbon-14, is radioactive and does decay away, so around 50,000 years there’s none of this carbon-14 left which is why we can't date any material older than that. 

SHANE HUNTINGTON
So we can't use carbon because it doesn't give us a long enough timeframe.  I’m assuming when you talk about dating the materials around the fossils, the rocks are not absorbing the carbon-14 in the way living entities do on the planet, so what do you use to date the rocks? 

ROBYN PICKERING
Well, what we can do is we use a method called uranium-lead dating.  So this, in many ways, is the same concept as carbon dating but we’re using the element uranium instead of carbon.  The advantage here is that uranium has an amazing long half-life which is much longer than carbon, so we can date the rocks which form in the caves.  So in caves you get specific rocks which form in caves such as stalagmites and stalactites, and we also get a type of the same material, which is calcium carbonate which is known as flowstones which form layers on the floors of the cave.  As these rocks form they trap uranium in them and this trapped uranium acts as a clock, basically, from the time the rocks are formed, and this uranium is naturally radioactive and it decays to form lead.  What we can do at the University of Melbourne is measure the amount of uranium and the amount of lead in the rocks today and calculate how old they must be for that amount of uranium to have produced that amount of lead. 

SHANE HUNTINGTON
Has this type of dating technology been used widely to date or is this something that we’re just applying now to these sorts of samples? 

ROBYN PICKERING
Well, the technique of uranium-lead dating is very well-established and has been used for many decades now, and is, in fact, the dating technique which was used to date the earth.  It is better on very long timescales, so on billions of years.  So the challenge in recent years has been to apply this technique to relatively young rocks - so by young I mean around 2 million years old - because it’s only been recently that the technology that we use, the laboratory techniques and technologies have become sophisticated enough to because able to measure uranium in relatively young rocks. 

SHANE HUNTINGTON
Robyn, when you do the uranium dating, how does this process actually work in the lab?  Can you describe what’s involved? 

ROBYN PICKERING
Well, it’s quite an involved, long process - I spend long hours standing in the laboratory standing here in Melbourne - and we collect the rocks in the field in South Africa. We have to make sure that we collect the right ones, so we spend several days onsite understanding the different layers in the cave and deciding which rocks to date and how those relate back to the fossils.  So then we bring these precious samples back to Australia, to the University of Melbourne.  Then what we need to do is extract the uranium and the lead out of the rocks, but because there’s so much lead in the environment around us - mainly from car exhausts and things like this - these samples could get contaminated very easily. So we work in a clean lab environment where the air has been filtered around a million times and we wear special clean suits and gloves and everything.  We have a series of laboratory procedures to extract the uranium and the lead from our rocks and then we measure these elements on a fantastic, big machine called a mass spectrometer which we can measure the amount and the various isotopes of uranium and lead and then put all that back together and calculate the ages. 

SHANE HUNTINGTON
This is Up Close coming to you from the University of Melbourne, Australia.  I’m Shane Huntington.  Our guest today is geochemist, Dr Robyn Pickering, and we’re talking about determining timelines in human evolution.  Robyn, I understand that in addition to uranium dating, some of the material was later examined using a technique based on paleomagnetism.  What is palaeomagnetism and how do you use that for dating? 


ROBYN PICKERING
Well, palaeomagnetism is another technique, a complimentary technique, which looks at the actual sediments around the fossils.  So with the uranium lead dating we were able to date the flowstone layers which we have in this case above and below the fossils, but we couldn't actually date the sediments around the fossils.  So what we were able to do is to use this technique, palaeomagnetic dating.  Today the earth’s magnetic field points to the North Pole, as we all know, but in times in the past this entire magnetic field has flipped around 180º and any rocks forming at the time record the magnetic signal, all the little magnetic grains in the rocks line up to align to the magnetic field at the time.  So if this magnetic field is reversed then the rocks actually preserve this.  So what my colleague, Dr Andy Herries, from La Trobe University also here in Melbourne, Australia, was able to do was to collect samples from this site - so he started at the base and collected a series of samples going up through the layer which had the fossils - and he was able to measure the magnetic signal preserved in these rocks.  What he found was sort of a barcode of different signals where he found a normal signal, which is the same as the field is today, and then he found rocks which have a reverse signal, which means that at that time the earth’s magnetic field was reversed.  And from other sites we have a huge body of data and we have a known stratigraphy of these various reversals, so what we were able to do was to combine the uranium‑lead ages with the signal of reversed and normal palaeomagnetic data.  Andy was able to identify this very small normal event in a large reversed time period which we know formed at 1.977 million years ago and this, fantastically, was the layer which had the fossils in it.  So we were able to zoom in and really narrow down the age of the fossils to 1.98 million years. 

SHANE HUNTINGTON
Now you're so specific about that timeframe, how does the palaeomagnetism sort of get calibrated?  I mean, people listening to our programme would be familiar with things like tree rings and ice core samples, but how do you do that in terms of the magnetic field flipping of the earth?  What’s the standard of measurement? 

ROBYN PICKERING
Well, we’re very fortunate in that we have an amazingly well dated calibration of these reversals and that forms in the mid-Atlantic ridge, so in the middle of the Atlantic Ocean where the process of plate tectonics is very active.  We actually have new rocks being formed in this mid-Atlantic ridge and it’s where the ocean is spreading apart and these new rocks that have formed there take on the magnetic signal of the earth at the time and these rocks can also be very well dated.  So we have this incredible series of millions of years old different rocks which we’ve been able to date very well and have this incredible palaeomagnetic calibration, basically.  So we have this timescale which we can use and then go to apply to other sites, such as the caves in South Africa. 

SHANE HUNTINGTON
You have a number of amazing things going on here: one is these incredible specimens that have been located, in addition to the combination of the two techniques that you've spoken of. Putting all this today, how does our knowledge of the timeframes involved here compare to what it was, say, 20 years ago? 

ROBYN PICKERING
Well, 20 years ago in South Africa - because there’s a whole series of caves with early human fossils - it wasn’t possible to date these caves directly, we just didn't have the laboratory techniques and knowledge to be able to date these deposits so the caves were given sort of estimates of age.  So there’s been a big revolution in the last five to ten years where the technology has improved very rapidly that we are able to do the uranium-lead dating and do much more precise palaeomagnetic dating.  So it’s been quite a revolution in being able to date these sites. 

SHANE HUNTINGTON
Australopithecus sediba - is it our direct ancestor? 

ROBYN PICKERING
Well, that’s a really good question and the best answer we can give at this stage is that this time period of around 2 million years has always been fascinating to paleoanthropologists.  Because it’s around this time we believe that our genus, homo - as in we’re homo sapiens - and that the beginnings of this genus, we believe, appears around 2 million years ago but there’s never been very complete or very well dated fossils from this time period.  There are some fossils but it’s never been completely clear how they fitted in. So the sediba fossils are incredible in that they’re so complete; we have very human-like characteristics as well as some more primitive Australopithecine-like characteristics in the fossils and they’re so well dated.  So what we believe is that because Australopithecus sediba we have at 1.98 million years, and we believe that this fossil is the best candidate to be the ancestor of our genus homo, so if it the ancestor of the beginnings of our genus then, yes, it is our most distant ancestor. 

SHANE HUNTINGTON
When we look at some of the new work coming out in areas such as dinosaurs and so forth and just the explosion of knowledge there, especially when we start looking in areas like China and so forth, are we to expect a similar explosion of knowledge as you start to look in other parts of the world that we haven't previously sort of examined in detail? 

ROBYN PICKERING
Well, yes and no.  We know from genetic evidence that humankind originated in Africa and, so far, all the oldest early human fossils we’ve found are exclusively in Africa, so I don't think we’re going to find our very early human remains anywhere else.  But then, the example in South Africa is a good case where for many years research had been undertaken at the caves which we knew had fossils, but then there was a big project to go and look for other new caves in this region and that was how they found the site of Malapa and then went on to find the Australopithecus sediba fossils.  So, yes, by doing more exploration and actually going to look for new things it’s really incredible what we can find. 

SHANE HUNTINGTON
Robyn, just finally, what big technological advances are sort of coming up in this area that you think will again enhance our understanding of our evolutionary process? 

ROBYN PICKERING
Yeah, that’s also a good question.  I think what I would like to see, from my point of view as someone who does the dating, is more precise dating and we basically get better and better at what we do in the laboratory and we’re able to date the rocks in more detail and, for my colleagues who work on the actual human fossils, the more we study these fossils the more we learn. 

SHANE HUNTINGTON
Dr Robyn Pickering form the School of Earth Sciences here at the University of Melbourne, thank you for being our guest on Up Close today and telling us about how we go about mapping the timelines of our ancestral history. 

ROBYN PICKERING
It was an absolute pleasure.  I enjoying doing it, thank you. 

SHANE HUNTINGTON 
Relevant links, a full transcript and more info on this episode can be found at our website at upclose.unimelb.edu.au.  Up Close is a production of the University of Melbourne Australia.  This episode was recorded on 6 October, 2011.  Our producers for this episode were Kelvin Param and Eric van Bemmel.  Audio engineering by Russell Evans.  Background research by Dyani Lewis.  Up Close is created by Kelvin Param and Eric van Bemmel.  I'm Shane Huntington.  Until next time, goodbye.  

VOICEOVER
You’ve been listening to Up Close. We’re also on Twitter and Facebook.  For more info visit upclose.unimelb.edu.au.  Copyright 2011, the University of Melbourne.


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