Examining Ancient Fossils for Clues to Human Origins

Several papers published this week in the journal Science look at fossilized bones from the hands, feet, pelvis, and other parts of the prehistoric hominid Australopithecus sediba. Paleoanthropologists Lee Berger and Bernard Wood discuss what the research means for our understanding of the human family tree.

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IRA FLATOW, Host:

You're listening to SCIENCE FRIDAY. I'm Ira Flatow. A couple of years ago, a nine-year-old boy literally tripped and stumbled over something on the ground, and it turned out to be a collection of very significant fossil fragments in South Africa.

He was and is the son of a well-known paleoanthropologist prospecting for potential dig sites, and so the find got immediate attention, and this week the analysis of the bones from the hands, the feet, the pelvis and other parts of the fossil find was - the analysis was published in the journal Science.

And the scientists say that the find is unusually complete and well-dated, and it has the potential to really shake up the human evolutionary family tree. Joining me now to talk about what was found or what it might mean is Lee Berger, he is professor and reader in human evolution and the public understanding of science at the Institute for Human Evolution in the University of the Witwatersrand, that's in Johannesburg, South Africa.

And it was his son that made the initial find, and he's co-author on a number papers this week in science. Welcome back to SCIENCE FRIDAY.

LEE BERGER: Well, thank you very much. It - my son always has me remind everyone that the dog was instrumental in the discovery, as well.

(SOUNDBITE OF LAUGHTER)

FLATOW: You mean it was the dog that was - he was chasing the dog or something?

BERGER: Absolutely and said he tripped over the rock and saw the fossil sticking out of the side of it and then called me over. So we have to give Tau(ph), the dog who is sitting right next to me here in the study, credit in the find, as well.

FLATOW: Does he get an asterisk in there, in the paper, or something?

(SOUNDBITE OF LAUGHTER)

FLATOW: (Unintelligible) footnote. Also with us is Bernard Wood. He is the GW University Professor of Human Origins and a professor of human evolutionary anatomy at George Washington University and also an adjunct senior scientist at the Smithsonian's National Museum of Natural History in Washington. Welcome to the program.

BERNARD WOOD: It's very good to be with you both.

FLATOW: Thank you. Lee Berger, let me ask you first: You've called this a bona-fide transitional species. That's pretty heavy.

BERGER: Well, and I'll tell you why. I think one of the reason is because the find is so very complete. And I know extraordinary claims require extraordinary evidence, but I do think it's fair to say in this case where, you know, our field of paleoanthropology is often justifiably accused of being a science of fragments due to the nature of the fossil records, that these two skeletons are (unintelligible) areas of anatomy that really we haven't seen in this sort of condition before.

In saying that, as we have analyzed in these five papers, more extensively for critical areas of anatomy that are based on largely new material that's come out since we described the species last April. There are parts of it that do appear to tell a different story than the one we were interpreting from the fossil record we had before.

Again, granted, the fossil record around this time period is incredibly small. It's based on literally just a few dozens of fossils of - and most of them incredibly fragmentary.

FLATOW: And so you found the complete parts. Instead of bone fragments scattered around a site, they were basically together. So you could get a better idea of what some of the parts really were like instead of speculating on them.

BERGER: That's right, and there are many individuals coming from this site. The two that we are dealing with specifically are an adult female skeleton that is in articulation within the site, as well as a young male, probably around sort of 11 to 13 years of age in human developmental terms that's also largely in articulation within the site.

And that allows us to look at variation within these single skeletons, looking at proportions and looking at different parts of the anatomy. It also allowed us to compare between two individuals, which - that are definitely from one species and one time range, and that in and of itself is a special thing.

FLATOW: And you've called this Australopithecus sediba?

BERGER: That's right. We named last April, 2010, a new species, Australopithecus sediba, because the holistic anatomy of these, we were able to separate them from the fossil collections of those species and in particular the most likely candidates, which would be the southern African species of (unintelligible).

FLATOW: So where would this fit in the human lineage? If Lucy is 3.2 million years old, where would yours fit in there?

BERGER: We're more than a million years younger than that. The date that we have published this week in Science, or in fact today at Science, is 1.977 millions, plus or minus about 3,000 years. And so about 1.2 million years and then the date you just cited.

And we don't know where they fit in the family tree. We put forward four possibilities in our papers. One is that this is a dead end, but if it's a dead end, it's one that shows many characters that we feel are associated with later members of the genus homo. Or it is an ancestral form of one of the several potential candidates of early homo. People would know them as homo habilis or homo (unintelligible) or that it might be potentially considered as a direct ancestor of what is broadly considered as our immediate ancestor, homo erectus.

We put those four ideas. One of the issues is that this fossil is, in many ways, so different, particularly in its post-cranea but also in parts of its craneodental and brain anatomy, that it doesn't necessarily tell the same kind of story that we had understood from previous discoveries.

Again, one of the difficulties - again one of the difficulties is that in this time period, largely from east Africa, there's a very poor fossil record in this time period from southern Africa.

FLATOW: Doctor wood, what's your reaction to this?

WOOD: Well, my reaction is that Lee I think has correctly emphasized the importance of the completeness of these skeletons. You know, if you use an analogy of a motor car, the difficulty, if you have a lot of fragments, is that, you know, you find, you know, a bit of a clutch and a bit of a brake pad and a bit of a steering wheel and a bit of the offside wing, but you - because you've never seen the whole car, you don't know whether they all belong to the same car or not.

The advantage of the completeness of these skeletons is that you can see what the combination of traits is in this creature, and although I don't think anybody would have correctly - necessarily correctly predicted this particular combination of traits, it's clear that - or at least it's clear to me that, you know, we are likely to find creatures with combinations of traits that we haven't yet seen before.

The difficulty about working out where this fits in the tree is that, you know, a few resemblances do not an ancestor make. And I think it's really difficult to identify creatures as ancestors, and I think this interesting organism, this interesting early hominid to my mind probably is not an ancestor of later homo because I think it is a bit too much evolution to do in a little, you know, in very little time.

But you know, I can't be sure. It's just that my guess is that it's not. But does this make this a less interesting discovery and these papers less significant? Not at all. I mean, you know, I think the combination of morphology which we see in sediba is one that we've not seen before, and that makes a whole lot of problems for reconstructing ancestry because we can't be sure that this - you can't really specify a little bit of (unintelligible) morphology that, you know, if the creature has this morphology, then it has to be ancestral.

So we're now finding that these morphologies, they occur in different, in different early hominids in different combinations.

FLATOW: Well, let's talk about some of the unusual combinations of what the structure of the creature was. And one of the problems - I'll call it a problem just because I need a word - is that this creature had a smaller brain size, a smaller cranium than we would normally think in evolutionary process, right, Lee? We...?

BERGER: That's absolutely true. I think if you were - if you were going to put this in a - in the model that we had before sediba, the way that (unintelligible) or brain size increase had worked, he'd gone from very small brains that are slightly larger than chimpanzee brains, in the 400 to 500-plus cubic centimeter range in early Australopithecenes like Lucy or Mrs. Plez, things like that.

And then there was a gradual increase in that brain size through some of the fossils that had been attributed - largely younger than about 1.8 million years, to things like Homo habilis or Homo rudolfensis as you move up into half our brain size and then three quarters of our brain size and things like Homo erectus to us around 1,400 cubic centimeters to 1,600 cubic centimeters.

At two million years, sediba sits with a 420 cubic-centimeter brain, and that's right at the bottom of where we see australopithecines, only slightly larger, even in the recent Flores hominids and in fact in an animal that's not that small; it's about 1.3 meters in size. But it's not the brain size that's important about sediba. What it appears from the synchrotron brain scan that we did, and Dr. Kristian Carlson led this part of our study, is that there appear to be more - there's a lot features in parts of the brain and maybe even some incipient - without - being too basic - bumps and twirls that might indicate areas like Broca's area potentially enlarging.

We pointed to these and said that that in fact if you look at the overall - particularly the shape of the anterior aspect of the brain, behind the eyes, that these areas mirrored more the Homo-like condition. And I think it's important and I think that probably Bernard would agree with me on this. It's important to understand when we say the Homo-like condition to possibly sort of exclude from the discussion for a moment things like Homo habilis and rudolfensis but talk about later Homo, things that we would more generally agree are definitively in the genus, things like Homo erectus and Homo sapiens and Neanderthals.

FLATOW: Well, that's your argument, is that - this - you believe it isn't possibly a direct lineage to humans, Homo sapiens.

BERGER: Well, what the basic argument is that when you go from the head down, you look at things like the hands, which we published, which is the most derived hand that's been discovered. It's more derived in many ways towards that later Homo condition and things like the hand in fact forms part of the type description of Homo habilis. And the pelvis is more like what we see the pelvis of later derived Homo - Homo erectus, Homo sapiens, Neanderthals - than any other australopithecine pelvis.

We don't have any pelvises definitively of any of those early, quote and unquote, "early Homo in the middle." Then when you add those things up and the very small teeth, which were in fact in the range of human variation and other features, we say that well, at least sediba must be considered as a potential ancestor in that lineage. We're not saying it is. We say it deserves consideration. And it does so in a surprisingly different way than the way we were hypothesizing with those early Homo candidates - habilis and rudolfensis.

FLATOW: This is SCIENCE FRIDAY from NPR. I'm Ira Flatow talking with Lee Berger and with Bernard Wood. But, Dr. Wood, you're saying let's wait a little bit more, that we need to find new fossils that match this one?

WOOD: The - well, it would be nice. And I'm sure and I'm - but the jungle drums, they suggest that Lee already has, you know, he already has more material from...

BERGER: Good jungle drums.

WOOD: ...Malapa.

(SOUNDBITE OF LAUGHTER)

WOOD: And that will be, you know, that will be great. I mean, that, you know, we can only congratulate these people for recovering such excellent material. It's just that the combination of morphology that you see in sediba, although...

FLATOW: Give me specifics.

WOOD: Well, you know, here and there, you see sort of Homo-like aspects, so, you know, it's got a longish thumb rather like ours. But that's matched with some rather ape-like things in its wrist. It's got some Homo-like aspects in the ankle region, but for every Homo-like feature, there is an ape-like feature to go with it. And so it's also clear that if you look elsewhere in the tree of life that there's a lot of morphology does not just occur once in evolutionary history. It occurs several times.

And if it occurs several times and the technical word for that is homoplatic(ph). If it occurs several times, it means that that piece of morphology cannot be used as a proxy for saying something it belongs to a particular lineage or not because it occurs in more than one lineage. And the difficulty it seems to me, is that a lot of the features that have been highlighted maybe connecting this with Homo are likely to be homoplatic. And that means that it - sure, you know, it might be Homo, but it also might be something else. Now just in terms of that time...

BERGER: And I think that's where we would diverge as a team with that opinion larger or at least we'd like to make a bigger point. We feel we're talking about areas that are very complex and are relatively unique. The hand is a complex area made up of a large number of bones that we would argue at two million years. We're seeing what is something at least in the distal part of the hand, around the fingers and the (unintelligible) and thumb that is effectively undemonstrated in any other fossils, the same with the brain and in fact with the pelvis.

And one of the important things to remember is there almost is no other fossil record that can be definitively assigned to anything between 2.4 million and 1.7 million. There are some isolated bits and pieces. People will find an isolated pelvis, for example, and it will look like a Homo pelvis, and they will say, oh, this is a pelvis of Homo. But sediba shows you can't take a pelvis like that and say, oh, it's Homo because sediba has that same pelvis. You can take the isolated - if you took the mandible or the isolated teeth of sediba and found those individual teeth, you'd say, gosh, these are Homo teeth.

But they're not. They're in these patches that Bernard did describe quite well that a long-armed creature with a more human-like hand or with its remarkably ape-like heel, and the point we're making is that given the lack of a record that now we do have something and maybe that record was just misleading us. It doesn't mean it is. And we are, you know, we've been portrayed in the media - or particularly I have been portrayed in the media as pushing this and saying this is the ancestor of Homo erectus.

And I promise you that there's more of a - the press grabbing that statement. What I'm saying is that we now have a fossil that - a well-represented series of fossils, not just little pieces that says you can't use just little pieces to interpret the big picture anymore, and it has to be consistent.

FLATOW: All right. We'll get back. We have a few more minutes to talk about it. 1-800-989-8255 is our number. You can tweet us, @scifri, if you'd like to. We'll be back with Lee Berger and Bernard Wood after this break. Stay with us.

(SOUNDBITE OF MUSIC)

FLATOW: You're listening to SCIENCE FRIDAY. I'm Ira Flatow. We're talking this hour about history of the human - well, Homo sapiens with this finding of Australopithecus sediba in South Africa, talking with Lee Berger, from Johannesburg at the University of Witwatersrand, and with Bernard Wood, who is at the National Museum of Natural History at the Smithsonian in Washington. Our number, 1-800-989-8255.

Let me ask you this question, Lee. You talked about narrowing down. You can pin down the date of this to a very narrow region, a few thousand years in the middle of over a million-year-old fossil.

BERGER: Yes.

FLATOW: How were you able to do that?

BERGER: It was really a lot of luck and a lot of hard of work. We've had some major advances in dating fossils in Southern Africa. It was our big bugbear for decades that we couldn't accurately date these fossils. And we had to use formal dates, and so they had big errors. What happened a few years ago was the development of uranium-lead dating to date the flowstones. Flowstones are limestone layers that form within these caves. And you look at the decay of uranium within them as it was reset as it formed.

And at Malapa, we have a remarkable situation. We have a basal flowstone which underlies this deposit. The deposit formed very, very quickly, literally in weeks, months. And then, we found about eight or 10 months ago, a capping flowstone. Those flowstones were the same age, and they had relatively narrow error bars. And what happened as the Malapa site formed, there was a paleomagnetic reversal. The Earth's poles reverse periodically in history. And it went from a normal polarity to a reverse polarity as that deposition happened.

Well, there is only one event that fell within the overlapping error bars of those lower and upper dates, and that was a little event at 1.977 to 1.98 million years, and thus allowed us to tie this to a moment in time effectively.

FLATOW: Let me try to wrap this up into some sort of lesson to be learned about this fossil find. And if I hear you saying correctly, Lee, you're saying that what this proves beyond whether it's Australopithecus or Homo or whatever, is that you no longer - if you find a little piece of a hominid or a little piece of a fossil, you can no longer type it as easily as you might have thought you could because your fossil shows there are pieces from all different times in that body?

BERGER: I think that's a fair question. I don't want to say and I'm not saying you can't use isolated remains to interpret function and form. I think we're going to have to be very careful, though, from the lesson of sediba in asking certain types of questions from isolated finds. No, I do not believe that you can take certain parts of the anatomy, like the pelvis, like the distal parts of the hand, like the mandible or isolated teeth and - or things like the talus, even parts of the foot and say, oh, this is definitively a member of the genus Homo and vice versa that you can't take bones and say, oh, this is necessarily an australopithecine.

Those type of questions probably sediba says that they're not appropriate to those type of fossils. It may be true in other species, but this species says that's probably not why they're doing it. If it's true in one species, it's probably true in others. As Bernard mentioned with homoplating(ph).

FLATOW: Bernard, what would it take to convince you that this is a transitional species?

WOOD: It would take a lot more wisdom than I possess.

(SOUNDBITE OF LAUGHTER)

WOOD: It would take, you know, the problem is that, you know, just because it's the right age, that doesn't make it an ancestor. And I'm, you know, for example, I'm probably old enough to be Lee's father, but I think he would agree that I'm not. So, you know, just being around at the right time is not enough. I think what, you know, what really - what sediba has done is that it's reminded us that, you know, we have only seen just a few of the cards, and we are extremely tempted to come up with a comprehensive reconstruction of human evolutionary history on what is probably just a very small part of the evidence.

And I think sediba has reminded us that, you know, that we shouldn't do this. You know, we should just, you know, lay back, you know, relax, make sure we analyze these fossils carefully and then, you know, the truth will gradually emerge, and the truth being the reconstruction of the evolutionary history that at least the minority of us in the United States that thinks happens.

BERGER: And I would add to that. I think the unspoken words that Bernard were - was not saying - and by the way, I agree with everything he said - is we need time more. We need to go out, explore more. What sediba also points out is there's more there to find.

Remember, that this was recovered right in the middle of the most explored area, probably, in the continent of Africa - for these very fossils, lying on the surface so easy a nine-year-old could find it. And there - Africa is a big continent. It is unexplored. The rest of the world's a big place, and we need to get more exploration and find more fossils. And I think that that is a clarion call at sediba rings out, that I think Bernard was alluding to as well.

FLATOW: But the fact that it was found in South Africa also throw something into the monkey wrench of general thought about evolution?

BERGER: No.

FLATOW: No.

BERGER: No. I would hope not, unless you're some, sort of, geographic xenophobia.

(SOUNDBITE OF LAUGHTER)

BERGER: But, you know, I mean, there are biologists who would say, oh, well, certain types of evolution occur more on the periphery of continents than central - but we haven't explored this continent. I don't know. I would be fascinated to see what was living up in the mountain ranges along the edge of the Rift Valley and not just what you're picking up next to a lake. It's only a tiny representative of the life going on up there. And I can't wait until we see fossils out of Congo and out of West Africa and farther on - and we will find that.

FLATOW: Better bring your kid along and the dog.

BERGER: It's a rule of mine now.

(SOUNDBITE OF LAUGHTER)

FLATOW: Is there enough money and the resources to go looking in all these places?

BERGER: Absolutely not. And what we need to literally pour money into is field exploration. There was, I think, a, sort of, ridiculous idea going around a decade or so ago that we - these fossil fields are depleting and that we found pretty much all the great fossil fields. Sediba is a perfect example that that is absolutely not true. And the funding agencies around the world should pour money into making this not the rare sought-after objects on this planet.

FLATOW: Professor Wood, I'm sure you would agree with that.

WOOD: Yes, I would agree with that. But I think there's a need to be a healthy balance between discovering new evidence and making sure that you analyze the evidence that you do discover in an appropriate way, you know. I mean, there's no point in having all analysis in the new fossils. There's not a lot of point in having, you know, a lorry load of new fossils if you don't analyze properly. Well, I think...

BERGER: The nice thing, though, I would say about having a lorry load of new fossils would be that we have all the time in the world to analyze them as long as we have enough of it. And that's the only...

(SOUNDBITE OF LAUGHTER)

WOOD: (Unintelligible) would make of that.

BERGER: I'll take the lorry whenever it can come.

(SOUNDBITE OF LAUGHTER)

FLATOW: One - well, one other...

BERGER: We'll try and deliver.

FLATOW: One other interesting aspect of this, as I mentioned in passing - is that you scanned the fossil, you didn't even take it out of its - out of the rock it was in totally, did you?

BERGER: That's, in fact, right, Neal. In fact, that's a very deliberate thing, not only because technology now allows us to scan it at resolutions that are truly unbelievable. We did it at the European Synchrotron Radiation Facility in Grenoble, France, scanned the skull at greater than 90-micron resolution. And we are, in fact, deliberately leaving parts of the skull as a record for future scientists.

You know, if some of the great fossil finds, particularly from Southern Africa, had been left with attaching matrix, things like the Taung Child and Ms. Ples, we'd be able to date them today. Who knows what technology, at the pace its changing, will allow us in the future. And that kind of activity is, sort of, part of our gift to future scientists a decade, 100 years, 1,000 years in the future.

FLATOW: Well, the fact that it scanned, it's in a computer, it's a 3-D model, doesn't - can people now with, you know, 3-D printers just take that and make their own model of what you found?

BERGER: That's exactly what we've done, and we're making that data available. We are allowing scientists to examine this material, published and unpublished, anything we find. Any bona fide scientist can come to our labs and examine, whether we published it or not. So we're attempting an open access experiment. We've casts available. You, today, could go to the Smithsonian Museum or the American Museum of Natural History and see casts of the material that we published in Science today. They've been in those institutions for months and months and months, available to any scientists who wanted to look at them.

FLATOW: Well, gentlemen, I want to thank you very much for taking time to be with us today.

WOOD: Thank you very much.

BERGER: By the way...

WOOD: You could give your dog a pat on the head for me, Lee.

BERGER: I will, indeed. Thank you very much.

(SOUNDBITE OF LAUGHTER)

FLATOW: All right. Lee Berger is professor and reader in human evolution and the public understanding of science at the Institute for Human Evolution in the University of Witwatersrand, and that's in Johannesburg, South Africa. Bernard Wood, (unintelligible) the university professor of human origins, professor of human evolutionary anatomy at the GWU and also at the Smithsonian National Museum in Washington.

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