New Fossil May Trim Branches of Human Evolution

Writing in Science , researchers say a 1.8 million-year-old skull found in Dmanisi, Georgia indicates that early humans may have evolved from a single lineage rather than from multiple species. Anthropologist Adam Van Arsdale tells us what this could mean for the way we view human evolution.

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JOHN DANKOSKY, HOST:

This is SCIENCE FRIDAY. I'm John Dankosky. Ira Flatow is away. Like any family tree, the homo sapiens line has multiple branches. Our earliest ancestors, homo habilis, appeared in East and Southern African about 2.3 million years ago. Homo erectus showed up about 1.8 million years ago and finally, modern humans, Homo sapiens, around 200,000 years ago.

But like any family, it can get kind of complicated. A skull found in Dmanisi, Georgia, could shake up this tree and trim some of the branches. Reporting in Science, researchers say the skull could be evidence that the genus Homo is actually one slow evolving line rather many multiple species. Could this change how we look at human evolution?

If you have a question, give us a call. Our number's 1-800-989-8255. That's 1-800-989-TALK. If you're on Twitter, you can always tweet us your questions, write the @ sign followed by SCIFRI and if you want more information about what we'll be talking about this hour, go to our site, www.sciencefriday.com. You'll find some links to the topic.

Now, my next guest is not part of the study reported in Science but has studied the fossils at the Dmanisi site. Adam Van Arsdale is assistant professor in anthropology at Wellesley College in Wellesley, Massachusetts. Welcome to SCIENCE FRIDAY.

ADAM VAN ARSDALE: Thank you. Thank you for having me.

DANKOSKY: So first of all, describe the site where the fossil was found and why it's so interesting.

VAN ARSDALE: Dmanisi is a really remarkable site for a lot of reasons. One of the first is that unlike a sort of big open area that you might expect in East Africa, the site itself is actually in the midst of medieval archeological ruins so its initial discovery in the early 1980s was actually because there were archeologists excavating the basements of these medieval structures.

So when you're working at Dmanisi, you're working surrounded by these medieval structures that date back more than 1,000 years, but you're digging through remains that go back 1.8 million years, even a little bit older. So the setting itself is quite remarkable. But the...

DANKOSKY: Go ahead.

VAN ARSDALE: The fossils are really what make the site spectacular, though. One of the really unique things about Dmanisi is that it's a single site at a single place in time. All of the fossils at Dmanisi, all the hominid fossils are coming from a very shot narrow window of time and they're all very complete, where we have lots of preserved elements from multiple individuals, which makes it a unique comparison to look at other fossil localities in terms of the variation presented by the Dmanisi fossils.

DANKOSKY: So tell us about this skull five, the one that everyone is talking about here.

VAN ARSDALE: It's probably the best preserved fossil human skull anywhere in the fossil record older than 100,000 years of age. And the fact that it's nearly 1.8 million years of age is just remarkable. Now, unlike a lot of human evolution stories that make headlines, it's not really something new. It's not a new species. It's not from a new location. It's the same old species we've had for a long time and at a site that we already have four relatively complete skulls.

So what makes it remarkable is it really rounds out that picture variation that we see from Dmanisi and together that picture variation has been quite illuminating.

DANKOSKY: Well, first of all, describe a little bit what you think this individual would have looked like, how big, how tall, describe the skull itself.

VAN ARSDALE: It wouldn't have been a very large individual, kind of moderate in body size, probably towards the small end of the human range. We have some postcranial elements so parts of actually the lower limb that are probably associated with this new skull that would suggest it's probably something in the neighborhood of four and a half to five feet tall as an adult.

So at the small end of the human range and not very large, but also not tiny, not like earlier australopithecines, like you might know Lucy. The skull itself, however, is remarkably primitive and in many of its features it's very huge. It has this massive jaw. Imagine sort of the Jay Leno chin on a four and a half foot tall early fossil human, and a big projecting brow ridge, a very prognathic or projecting face so it would have been definitely a remarkable individual to look at, a combination of sort of primitive large facial structures with an overall relatively small body size.

DANKOSKY: And about 40 percent the size of our brain.

VAN ARSDALE: Yes. Very small brain size. This new specimen they estimate at about 550 cubic centimeters, which, yeah, is about 40 percent of our brain size today.

DANKOSKY: So aside from the skull five, talk a little bit more briefly about what else is found at the site.

VAN ARSDALE: So the variation preserved in these fossils, as I said, is remarkable because it gives us a relatively complete picture about what normal variation looks like. If you go outside and walk around and you look at the people around you, the features, that structure, the anatomical differences between them are largely aspects of how old those individuals are and whether they're male or female.

And at Dmanisi, across these five individuals, we think we have younger individuals, older individuals, male individuals and female individuals. So within one site, just coming from a narrow strip of time, we think we have a relatively complete picture about what variation looks like in a normal population 1.8 million years ago.

DANKOSKY: So now the researchers said that the differences between these Dmanisi fossils are no more pronounced than those between five modern humans or five chimpanzees. What made them come to this conclusion?

VAN ARSDALE: So they did a fairly technical analysis where they actually looked at the variation in the shape of the skull itself. Actually, what kind of shape differences exist between these specimens that some of them are male, some of them are female, some of them are different age, and they basically said that it's a similar kind of range of variation as you'd find under a human sample or a chimpanzee sample that occupied a similar kind of characteristics, you know, also males and females, also young and old.

DANKOSKY: If you want to join us, 1-800-989-8255 or 1-800-989-TALK. Some criticisms of this study say the measurement method wasn't very discriminating. Do you see it that way?

VAN ARSDALE: I actually don't see it that way. I mean, there's no perfect way to analyze fossil specimens. Every fossil has a different degree of preservation so the analytical approaches you bring to bear in your questions are always going to vary a little bit depending on exactly what you're looking at and exactly what questions you ask.

In the case of these researchers, you know, the methods that they took are very much in line with what many other researchers are doing today and even more interestingly, actually, although I wasn't on this paper, I published a paper earlier this year where we came to the same conclusions using similar materials, but using a very different method.

So the fact that two groups of researchers have come to the same basic conclusion about the variations we see in early homo, using very different analytical approaches is, I think, as strong statement.

DANKOSKY: Okay. So let's get to the big thing here. Some people are going to look at this and say, this skull is evidence that the homo genus comes from a single line that essentially, as we put earlier, it's going to shave off some of the branches of the human family tree. Maybe you can explain this analysis and whether or not you think that this is essentially what we're finding here.

VAN ARSDALE: Well, so to take it a step back a little bit, so for the last 40, 50 years there's been a huge amount of work done in East Africa getting a wonderful fossil record. However, the fossils we've recovered from East Africa are primarily coming from open air sites that are scattered across wide geographic areas and that also encompass more than 400,000 years of time. So we have individual isolated specimens coming across big geographic and temporal ranges.

Dmanisi, as I said, has a lot of individuals with a lot of variation at one point in time. So over the last 40 years as we've discovered more and more fossils, the differences between them have been fairly striking, in part because we're making comparisons across such distances. And so those variations have, more often than not, been classified on the basis of being different species.

So some people would argue that we have as many as four or five different species of early homo at the same time. But here at Dmanisi, all in the same time and place, we have this huge range of variation and the fact that the site is so constrained in terms of the area it encompasses, means that it gives us this tremendous window to reinterpret this variation and to, instead of seeing of this variation as being representative of different species, see it as natural variation, the kind of variation that evolution operates on, shapes and patterns, across time and space of a single evolving lineage.

DANKOSKY: Could it be as simple as what we see when we just walk down the street in any American city? We see a lot of different types of people who clearly come from different places. Are we just essentially looking at ourselves, some sort of variation within a species that we just haven't really seen before?

VAN ARSDALE: In a way, but there are two ways in which just looking at people on the street is a little bit different. The first is that when you're looking at people on the street, we're all from exactly the same period in time. Evolution again is changing variation across time. And when we're looking at these fossil samples, particularly the early homo records spanning from, say, 1.5 million years of age to 2 million years of age, we're looking across a huge amount of evolutionary time, especially given the comparison to living humans today.

The second reason that comparison might not exactly be correct is that humans today in the pattern of variation we have is largely a product of what's gone on in the last 15,000 years, what's gone on since populations developed, agriculture - and we've had this massive demographic expansion. So today populations are likely much more similar to each other than they actually were in the past when we were very small localized populations scattered across fairly large geographic ranges with a very minimal amount of cultural technology to use to adjust to different environments.

DANKOSKY: I can imagine that this paper has caused a bit of controversy within your field. Is that fair to say?

VAN ARSDALE: I think it's fair to say, but my hope is that it will actually provoke renewed conversation and dialogue and discussion about these topics. I think there's been a lot of interest in the evolution of early homo for a long time, but there's been, I think, a disconnect between the kinds of questions we're asking and the kind of data we're looking at. And I'm hoping that these new finds, this new publication, really brings a lot of people together, even in disagreement, but to discuss scientifically, actually, what are the important evolutionary processes that play in the evolution of early homo.

DANKOSKY: Well, and that's interesting. I mean, what are the new questions that you think you're going to start asking now that maybe you haven't been asking over the course of your career?

VAN ARSDALE: Well, I think we could begin looking more at what are these factors that are affecting intraspecific variation, the variation we sample within a species. I think the question of are there one species or more than one species at Dmanisi has been pretty definitively answered. And if that's the case, we can use Dmanisi and the variation we see there as a lens to reinterpret the evolution processes that we see all throughout East Africa and everywhere else basically that we start finding fossil hominids shortly after 2 million years of age.

And if we look at that question, and if we look at where our genus comes from, sort of what are the key evolutionary changes that gave rise to the genus homo from some sort of ancestor, presumably some kind of australopithecine ancestor, I think the changes that give rise to homo are all changes that involve an increasing size of the evolutionary niche occupied by humans. We get bigger, our brains get bigger. We begin to use tools to culturally adapt to problems in the environment. And we begin to disperse into new environments.

All of those are associated with an expanding ecological niche. And all of those are also associated with it being more difficult basically to develop the long-term sustained boundaries necessary to create new species. So when this team has argued that this is evidence for a single evolving lineage, I think that's consistent with the fundamental changes that gave rise to the genus homo.

DANKOSKY: We've run out of time, but I'd like to thank my guest. Adam Van Arsdale is assistant professor in anthropology at Wellesley College. Thank you so much for joining us.

VAN ARSDALE: Thank you.

DANKOSKY: Coming up next we're going to be talking more about the human brain. We'll look at the social networking inside, right after this short break.

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