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IRA FLATOW, host:

Later in the hour, we're going to be talking about decoding the genome or at least some of the genes of the woolly mammoth. But first, talking - speaking about mammoths. A team of scientists say it thinks it knows what caused the extinction of the woolly mammoth, along with other fantastic creatures from the Pleistocene era.

And if you weren't around then to know what they are, they include the giant sloths, the mastodons, the saber-toothed tigers, 13,000 years ago. The scientists said - did this - they studied the layers of soil, a kind of, you know, a timeline of geological history, and they found evidence of an extraterrestrial impact - maybe, you know, a comet or an asteroid, something like that, something happened at about that same time that the mammoth and the other creatures died out. They say such an impact could have set off a chain of environmental effects that just killed the large mammals.

Their paper is out this week, in this week's issue of the Proceedings of the National Academy of Sciences. One of its authors is here to talk about the study, and why not everyone is convinced that they've got it right.

Peter Schultz is a professor of geological sciences at Brown University in Providence, Rhode Island. He joins us by phone. Welcome back to the program, Dr. Schultz.

Dr. PETER SCHULTZ (Geological Sciences, Brown University): Oh, thank you very much and thanks for having me on the show.

FLATOW: Let's talk about this. You haven't found a big smoking crater, have you?

Dr. SCHULTZ: No, no smoking craters - smoking gun, I think we've got some, you know, leftover pieces of the bullets, at least the cartridges. We're still trying to get this sorted out.

FLATOW: You say you have a suggestion of an extraterrestrial impact. Tell us what you found.

Dr. SCHULTZ: Well, the primary thing is that there's a ribbon of dirt that occurs at one certain layer within a whole group of sediments. And this ribbon of dirt contains a great deal of carbon, as well as other traces of things like iridium, which is a cosmic signature and helium under the cosmic signature as well as some detritus, dust that apparently had been blown around.

And this occurs in a lot of different sites. And so it's very suggestive, but this is certainly not the complete answer. There has to be a lot more work done yet.

FLATOW: And where did you find this?

Dr. SCHULTZ: Various sites all across the U.S., up into Canada, into Arizona, on the East Coast down in the Carolinas and very often - at least in places that have been studied - was with the - some of the sites have been occupied by humans at one time.

FLATOW: These are found only in the Americas. In…

Dr. SCHULTZ: No, there's a report that there are some in Belgium and - but I think this is where we sort of spread out from here, to see if this was a regional event or global event. But, you know, I think the real point is that something happened and we're trying to figure out if it could have been an impact.

FLATOW: Mm-hmm. Would it be something that would have to actually impact the surface or could it have exploded above and rained down the layer that you're finding?

Dr. SCHULTZ: Well, that's a really good question, and we're trying to figure this out. I mean, the first step in this is to confirm what we have - other people need to confirm what we're reporting - and at the same time, we've got to get that geology down really tight. This is kind of like CSI. We have to get this nailed and then we can start speculating on what happened.

But I think we can think of scenarios that would cause devastation, but it really challenges our understanding of various things, including the rate in which these things come in from outer space.

FLATOW: Mm-hmm. This is TALK OF THE NATION: SCIENCE FRIDAY from NPR News. I'm Ira Flatow talking with Peter Schultz, professor of geological sciences at Brown University.

So you're basically in a fact-gathering mode at this point?

Dr. SCHULTZ: I think that's right. This - and you have to remember, this is actually correlated with a sudden change in the climate. You know, the climate had been warming for a while, and then there was a sudden down turn. I mean, within about a hundred years and lasting about a thousand years, it got cold again, very cold.

FLATOW: Huh.

Dr. SCHULTZ: And the mystery is why we're finding this stuff at about that same time level that that's sort of a mystery.

FLATOW: And you've got that time level pinned down?

Dr. SCHULTZ: Yes. That is pinned down, but of course, we just need to see more sites to find out how good this pinning is.

FLATOW: Mm-hmm. Is there any way to say whether it's a comet or an asteroid, a meteorite or any of those, or just something different?

Dr. SCHULTZ: Well, with the data in hand, it's more suggestive of either a comet or an asteroid made up of a lot of carbonaceous material. But that's not a slam dunk. We're still trying to figure this one out.

FLATOW: What's been the generally accepted explanation of the death of all these animals?

Dr. SCHULTZ: Well, usually, people impose humans that they simply hunted these beasts to death and that's a very popular notion, theory actually, that has been suggested both for North America, as well as South America. And it's appealing because this was a time when there were a lot of aboriginals who were through this area of North America and South America, so that's really sort of the standard explanation.

FLATOW: The Clovis people who were around at that point.

Dr. SCHULTZ: Yeah, that's right.

FLATOW: The land bridge - they could go over across the Bering Sea and that kind of thing, and came down and basically hunted these animals.

Dr. SCHULTZ: Yeah, yeah. And - but there have been criticisms on that hypothesis as well.

FLATOW: Yeah.

Dr. SCHULTZ: And so, on the one hand, this kind of makes sense because it's happened in the past. On the other hand, to have something so big, so widespread, you know, it gets kind of difficult to swallow.

FLATOW: Can you give us some idea of a trigger for the mass extinction, a sequence of events that might have gone on here?

Dr. SCHULTZ: Sure. You know, you can think of these, you know, several different scenarios. One of them could be that there was a whole cluster of these things coming in, coupling most of the energy to the atmosphere, you know, just others basically frying the atmosphere and frying everything beneath it. That's one scenario. That requires, though, something very specific in terms of a whole group of the bodies coming in.

Another possibility is that there was a real big strike in the ice sheet that was still existing in Canada, and that somehow resulted in both the atmosphere heating or perhaps even disturbed the ice sheet that induced a lot of freshwater going into the Atlantic Ocean and then changing the - what's called the thermal-haline circulation pattern. But again, these are all scenarios and really speculations, and we just got to have something to hang our hat on yet.

FLATOW: Yeah. So you can either - in any of these scenarios, you could either have a giant crater or no crater.

Dr. SCHULTZ: That's right - if it happened to hit in ice, then when the ice disappeared, that would pretty well wipe out the evidence. So now we have to look in lake sediments. We have to go into all these different localities and really confirm that what we have is real.

FLATOW: And that's a large area you're covering, right?

Dr. SCHULTZ: Yeah, but - it's a large area. On the other hand, there are a lot of people who study this interval of time. This is just a mystery of why it suddenly got cold, so…

FLATOW: Yeah.

Dr. SCHULTZ: The only question is whether or not they've used the right tools to see whether or not there was an impact.

FLATOW: Could this stuff be already in records of these areas that lay hidden, waiting for you to just dig through the data?

Dr. SCHULTZ: Oh sure. Yeah, that's exactly right. And I'm hoping that somebody will get out there and prove us right or wrong. This is - I think it's a really sobering thought that we could have had something this recent that could have been this devastating.

FLATOW: Wow. That's interesting. Thank you very much, Dr. Schultz, for taking time to be with us.

Dr. SCHULTZ: You bet.

FLATOW: Peter Schultz is a professor of geological sciences at Brown University in Providence, and he joins me by phone - joined me by phone from Williamsburg, Virginia.

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