'Genetic Fossils' Change Extinction Picture

IRA FLATOW, host:

This is SCIENCE FRIDAY from NPR News. I'm Ira Flatow.

Back in the old days, a couple of years ago, when you wanted to study extinct animals, maybe figure out when they went extinct, you had to find some animal bones and, of course, by dating the bones you could figure out when the animals were roaming the Earth. And using that technique, scientists estimated that the woolly mammoths - woolly mammoth went extinct about 12,000 years ago. But now using a new technique, looking at what they call genetic fossils, scientists have revised that number, woolly mammoths and ancient American horses may have been roaming around North America a couple of thousand years longer than we thought.

Joining me now to talk more about it is my guest Eske Willerslev. He is professor of evolutionary biology and a director of the Center for Geogenetics at University of Copenhagen in Denmark. Welcome back to the program, Dr. Willerslev.

Professor ESKE WILLERSLEV (Director, Center for Geogenetics, University of Copenhagen): Yeah, thank you.

FLATOW: You're welcome. You were able to see evidence that these animals there were older or were around longer using genetic fossils. Tell us where the DNA, the genetic part came from.

Prof. WILLERSLEV: Well, it came from permafrost in Alaska - permafrost areas there. There was a specific site called Stevens Birch in mainland Central Alaska where there's very well dated and undisturbed permafrost surfaces. So, we could basically go in through these permafrost layers and then look for what animals had been present there because they are leaving traces of DNA behind through the urine and feces through time. And that's what we find in evidence of woolly mammoth and horse in days that are dated between 10,500 and 7,500 years ago. So, considerably younger than what people had found through macrofossils.

FLATOW: Why this a more reliable method than the old method?

Prof. WILLERSLEV: Well, I mean, you could say the chances of finding DNA traces rather than the remains of the fossils of these really late surviving members of the species is much higher simply because that one dead animal leaves behind one corpus, right? But during its lifetime it leaves many DNA traces behind through urine and feces. For example, a modern African elephant is basically urinating about 50 liters everyday and that is basically containing numbers of DNA molecules and the same goes for the feces. So I can say the chance of basically discovering these traces are much higher through the DNA than through the macrofossils.

FLATOW: Mm-hmm. And so this tells us something about their extinction then?

Prof. WILLERSLEV: Yeah, sure. I mean, because there has been these, you can say, fault dominant processes of this extinction of big mammals that took place in the Americas, and Europe for that matter, and Asia, presumably around the end of the last Ice Age. And some of the theories said well it was humans or it was feces (unintelligible) to the Americas with human (unintelligible). But those processes - basically the month that the extinction took place very rapidly after first human contact - within thousand years of the first contact. And because the oldest secured evidence of human presence in Alaska is about 14,000 years - well, that's, of course, fits very well with the bone record but it fits pretty poorly with the new results because they suggest that humans and mammoth and horse co-existed for several thousand years before mammoth went extinct. And the same goes for another theory that suggested was an extraterrestrial impact that hit, you know, the North America around 12,900 years ago and wiped out most of these animals. But again, we see actually survival of these guys much longer.

FLATOW: So, okay, what's the�

Prof. WILLERSLEV: So, I would say this is the only theory that persists really of the prominent ones is the some degree is the climate theory, you know, that it was climatic changes that somehow took out these animals through changes of vegetation. I mean, our results doesn't prove that but at least it's the only theory that still stands.

FLATOW: So, there was a gradual extinction, not the�

Prof. WILLERSLEV: Yeah, exactly.

FLATOW: �extinction those dramatic events would have caused.

Prof. WILLERSLEV: Yeah, exactly.

FLATOW: How - what other kind - how far back could you go in, let's say, the fossil record? Could you go back to millions of years ago to look at the soil around? You know I'm just thinking off the top of my head - something like Lucy or something like that?

Prof. WILLERSLEV: Yeah. I mean, well - I mean, the DNA molecule is a fairly, you can say, unstable molecule over time, right? So, it will be key over time, of course, in permafrost regions, cold regions it persists probably the longest. And there we have evidence that DNA can survive, you know, maybe between half a million and a million years, but not much further back than that. So, you can say we wouldn't be able, for example, to deal with the dinosaur extinction or something like that.

FLATOW: And how complete is the DNA that you find from these mammoths?

Prof. WILLERSLEV: Well, it's fragments. It's fragments of DNA, but the database you can see of the DNA, even from extinct mammals is actually - is very good today, you know. There exists DNA, you know, obtained from the�

FLATOW: Right.

Prof. WILLERSLEV: �mitochondrial DNA obtained from most of these extinct megafauna and therefore it's actually possible to go and compare it to this huge, huge database containing all DNA sequences known today, and you can also the statistically show, for example, you know, this is the mammoth or this is a horse or this is a bison or moose or whatever.

FLATOW: Mm-hmm. Does this�

Prof. WILLERSLEV: Even from short sequences, if you catch the right sequences, you know, that is very variable across species.

FLATOW: Yeah. Does this dating technique make the old ones now obsolete?

Prof. WILLERSLEV: No. I think - I mean, you should look at it as they are going hand in hand, so to speak, because there is also, you can say, the advantages of this new approach is that it's very cheap, it's very fast and it can also - it increases the chances of detection. On the other hand, you can say the drawback or you can say the uncertainty (unintelligible) of course that you cannot date DNA directly. You have to make an argument that the DNA has the same age as the sediments in which you find it. And in our case, you know, I think we can make a very good argument.

But in some cases, that argument can be more difficult and therefore I think you can also see the method as a way to basically say, okay, you basically have to go and dig more bones in this area, you know. You are very unlikely to have, you know, got the last members right and - of a certain species. But dating bones, collecting bones is a very, very time consuming process and also extremely expensive. So, in that regard, I think that the two approaches, you know - I think the new approach is, you know, is really interesting and can be very useful, but I think they should go hand in hand.

FLATOW: Well, Dr. Willerslev, I want to thank you for taking time to be with us today.

Prof. WILLERSLEV: Thanks. Bye, bye.

FLATOW: And good luck to you.

Prof. WILLERSLEV: Yeah.

FLATOW: Dr. Willerslev is a professor of evolutionary biology and the director of the Center for Geogenetics at the University of Copenhagen in Denmark. And he was talking to us there from Copenhagen. Transcript provided by NPR, Copyright National Public Radio.