Dinosaur Extinction and the Rise of Mammals The dinosaurs' demise has been linked to the rise of mammals, but a new study says the connection isn't so direct. Scientists have assembled a complete family tree for all existing mammals, and this "supertree" tells them how different groups were created and when they diverged.
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Dinosaur Extinction and the Rise of Mammals

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Dinosaur Extinction and the Rise of Mammals

Dinosaur Extinction and the Rise of Mammals

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You're listening to TALK OF THE NATION: SCIENCE FRIDAY. I'm Ira Flatow.

Still to come a little bit later, should you have an MRI to screen for breast cancer? But first up now, the mass extinction of dinosaurs 65 million years ago. You know, you read about it in the textbooks. There's a linear line. The dinosaurs were wiped out. That allowed the mammals to flourish and come into their own here on Earth. And that is the common belief. So what about it?

Well, a new study based on a new family tree for all the mammals - over 4,000 of them - tells a very different story. The story is out in this week's issue of the journal Nature, and one of its authors is here to talk about the rise of mammals on Earth. Ross MacPhee is a curator in the Division of Vertebrate Zoology at the American Museum of Natural History in New York. He is here with us in our SCIENCE FRIDAY studios. And if you'd like to talk about the mammals and the dinosaurs, our number: 1-800-989-8255.

Welcome back to the program.

Mr. ROSS MACPHEE (Curator, Division of Vertebrate Zoology, American Museum of Natural History): Thank you very much, Ira. Good to be here.

FLATOW: I mentioned a little broad brush about this. Tell us what the common knowledge is and what this new thinking is.

Mr. MACPHEE: Okay, I'd be delighted to, but I've got to make, first of all, a slight change to your intro. When people think about mammals nowadays, they think of usually just one type, the placentals, of which we're an example. The other big group, although not big in North America, is the marsupials, really restricted now to Australia and to South America. But way back in mammal history there are at least a half dozen other groups that were of significant size and actually were the first mammals in a real sense. These were the ones who were really trotting around under the feet of the dinosaurs and…

FLATOW: Came before the dinosaurs.

Mr. MACPHEE: In some cases they did. Some of the lineages are actually that old. The difference is that they've all disappeared, leaving us behind. So that's why we think the modern world is essentially made up of marsupials and placentals. There are a few monotremes around, but they're so small in number that they really don't affect any of the calculations.

Okay. So with that slight modification in place, what I'm now going to go on to say is what our study showed is that our ancestors - that is to say the ancestors of the lineages that have resulted in all the modern mammals that are around us today - didn't do well. They didn't do much of anything frankly immediately after the demise of the dinosaurs.

FLATOW: Theory was that dinosaurs were holding them back from (unintelligible), right?

Mr. MACPHEE: This is a piece of popular wisdom. It's never made a lot of sense to me because, you know, most dinos were in the very, very large size, down to minimum of big chicken.

FLATOW: Right.

Mr. MACPHEE: And most of the mammals that they competed with or at least were in the same area with were very much smaller. These were guys with very long noses that ferreted around in the forest floor, or in some cases were arboreal. But the notion that the dinos had much to do with mammals and actually kept them from going forward is really probably just a tale. It has nothing to do with the actuality.

FLATOW: How did you discover that? What made you overturn this idea?

Mr. MACPHEE: Okay, so in terms of the actual study that we did, we did three things. The first was to create this massive family tree that you mentioned, which we call a super tree, which in a way is like meta-analyses and other lines of work, although the procedures we used are different. In other words, we went to a lot of studies - actually thousands of them - took the best, pasted them together and came out with what we think is a fairly good representation of how everything's related.

The next thing we did was put a timescale on it using molecular evidence. And this gave us a notion of when all the splits took place between different groups. The third thing we did, and this is the surprise, is that when we decided to look back the very earliest stages of the differentiation of the ancestors of modern-day mammals, we found not only did their first beginnings extend all the way back to about 85 to 100 million years ago but that they really didn't do much.

Having diversified, having a few lineages going, they really didn't do much until about 50 to 55 million years ago. In other words, 10 to 15 million years after the bolide impact, which changed everything on planet Earth.

FLATOW: Do we know what - there's a growth spurt right then. Do we know what the cause of that might have been?

Mr. MACPHEE: Well, you know, I can tell you all the stories you want. The trouble is that they're very, very hard to test. But let me give you one example since your previous interviewee was talking about climate change. In fact, the very warmest temperatures of any time in the last 65 million years occurred within this range of 50 to 55 million. We call this the thermal maximum for the Cenozoic for the last 65 million years. And the way to think about this just in metaphorical terms, it's as if you took the climate of a place like Texas and moved it 20 degrees north to where Minnesota is.


FLATOW: Wow, that's some shift.

Mr. MACPHEE: That's some kind of shift. So the idea here is that not only was it very dramatic change, but it was also a very rapid one within the space of a few thousand years, or a few tens of thousands of years. And that, doubtless, has had enormous ramifications for every living thing on the planet at the time. But did that actually spark us? Did it actually get us going? That we don't know. And in fact one of the things that we're going to have to deal with going forward is for not just us, but a lot of other people interested in the question, is to find out whether there are any connections between our rise, climatic events of this sort, and anything else.

That's what's going to continue to make it an interesting story.

FLATOW: 1-800-989-8255. Talking with Ross MacPhee of the American Museum of Natural History in New York. So, you did this totally with DNA analysis or was it when you went back and looked over all the records - there had to be fossil records there also, right?

Mr. MACPHEE: Absolutely. Another sort of wrinkle I want to put in the story is that there were mammals that did very well after the demise of the dinosaurs. But you and I have never seen them because they all disappeared within a very short time, really, after this thermal maximum. At least by 30 million years ago they were almost all gone. And there were a whole slew of them. Very different from the groups around today. You would still probably recognize them as mammals if they were here with us, but they're not closely linked to us.

And so they have made no contribution to our evolution or to our phylogeny. And with the methods that we used, the super tree analysis, this is all based on living mammals. So in looking back in time, we're actually looking back the track way that we've created with this family tree based on living mammals. And it doesn't intersect with any fossils because fossils can't contribute DNA obviously, at least ones of that age, and we have to treat them differently.

But here's the interesting thing, a lot of paleontologists - in fact, evolutionary biologists in general - would really have no problem anymore with mammals of the sort that we are having arisen between 85 and 100 million years ago. There's a lot of favorable molecular evidence in particular that supports that idea. The fossils, however, from this time period - and now I'm going to go from, say, 75 million up to about 55 million - the fossil mammal record at this point is dominated by these weird groups that have very little to do with us, although if they're (unintelligible) by formal definition mammals, they didn't have anything to do with us.

And some of those did very well. They did a lot of diversification in the few millions of years after the demise of the dinosaurs. They were diversified before that point. They diversified later. So what the - this sort of other paradox that we now have to come to grips with is that as we did nothing - we were just sort of sitting on our bottoms 65 million up to about 50 million - these non-thereon mammals, as we call them, did comparatively better. And then at 50 to 55 million the graphs sort of switch.

These other groups go into a swift decline. Our guys take off explosively. So in addition to everything else, we have to wonder whether there was some interaction phenomenon between mammals from sort of different sides of the tracks.

FLATOW: Somebody ate somebody.

Mr. MACPHEE: Wish it were that easy.

FLATOW: Could it be a climate, like you said? Could the temperature change have affected these other?

Mr. MACPHEE: You know, it could be.

FLATOW: (Unintelligible) so successful, you know.

Mr. MACPHEE: It could be, but the trouble is you've got to look at sort of both sides of the question. What kept us down?

FLATOW: Right.

Mr. MACPHEE: By us, I mean the ancestors of modern groups of 65 of the non-thereons took off. Then conversely, what changed at 50 to 55 million where we finally threw off the gloves and decided to diversify and the other guys left town. There's puzzles within puzzles here that are going to take a while to sort out.

FLATOW: Is this the first time you found that spot where they crossed over? I mean…

Mr. MACPHEE: No, this is something - I'm now going to stand up for traditional paleontology. This is something paleontologists have known for at least a hundred years. We knew about these non-thereons. We knew about the rise of modern groups. The way it was traditionally interpreted, however, was that our ancestors really didn't get the spark until sometime after the disappearance of the dinosaurs. There was an explosion - a very short fuse, as we call it. And what's changed now is all of this molecular evidence that suggests the fuse was very much longer, at least, as I say, 85 to 100 million years ago for the initial diversification of mammals like us. And that's put a lot of pressure on traditional paleontology to explain.

If it is the case that we go back that far, then why haven't paleontologists been able to identify ancestors? And I think what our study may suggest in an indirect way is that not only weren't there very many of them for a long period of time but they were persistently primitive. And as I'm sure you know, the story of the evolution of mammals and really of any vertebrate group is really teeth succeeding one another through time. We very rarely have other kinds of evidence for different species and what they were like.

FLATOW: 1-800-989-8255. As you know, our listeners always put their thinking caps on. I know they'll come up with the ideas for you to investigate like Scott(ph) in Arlington, Nebraska. Right, Scott?

SCOTT (Caller): Yeah. Yeah.

FLATOW: Go ahead.

SCOTT: See, I've got a question, but it may not have a whole lot to do with what you're talking about. But has anybody done any studies having to do with when the magnetic poles shift on the Earth, if that has anything to do with the evolution of mammals and dinosaurs and the changes that occurred that there. I'll take my answer off the air.

FLATOW: Thank you. You got Dr. MacPhee to at least raise an eyebrow on that one.

Mr. MACPHEE: Scott, that's an interesting question. Lot's of people have asked that. In fact, going well back into the 1960s, when there was the first assembly of a decent record for pole switching. The thing is - and just in a nutshell - the switches don't most of the time, and especially in the time that we're talking about, seem to be correlated with anything particularly important in terms of rapid evolution and diversification.

It sort of makes sense, doesn't it, that a change on that scale should have had some kind of effect. But when you begin to parse it, it probably didn't because even if there was no polarity on the Earth for a while, it wouldn't have been for very long, which means that the solar storms and so forth that might provoke increased mutation and perhaps evolution wouldn't have had a very long time scale in which to operate. So it's a very interesting question, but nobody has actually pinned diversification of mammals to a change in the position of poles.

FLATOW: If this longer fuse had been seen earlier, instead of anticipating there was a short fuse you knew years ago that it was a much longer fuse as you do now, would the dinosaur extinction still have played into your thinking, or the previously accepted thing that in that their dying out led to the mammals?

Mr. MACPHEE: Well…

FLATOW: Will you have seen a long enough gap to say, hey, it was just too long for that to happen?

Mr. MACPHEE: It's an interesting point. And what I have to say is that the professional paleontologists really have come to terms with this - did that a long time ago. But in a sort of popular wisdom where you sort of have to have somebody up and somebody down, sure, probably the idea that there was any direct kind of competition or that that very strongly influenced the evolution of mammals would never have arisen.

One of the reasons why that popular wisdom exists, I think, is because when we look at the natural world today, we see animals doing things so they all have a very specific jobs within the local ecology. And in a way we developed the idea that these are very hard-walled, hard-wired structures so that, you know, you've got a particular job opportunity, you're the one occupying it at this time…

FLATOW: Right.

Mr. MACPHEE: …and you leave and there is always a crowd ready to take over as though it's the niches that are real things and the animals are just the placeholders. Really, it's the reverse.

FLATOW: More niches than you think there are.

Mr. MACPHEE: More niches than you think, and in a way it's always an interaction…

FLATOW: Right.

Mr. MACPHEE: …between the environment and the animals. So going back to my original point about what dinosaurs were doing versus what these early mammals of all sorts were doing, it's very unlikely that they really would have been going after the same jobs. Consequently, the idea that they are in any form of effective competition is probably not on.

FLATOW: Talking about mammals and dinosaurs in this section of SCIENCE FRIDAY on TALK OF THE NATION.

I'm Ira Flatow. Here we are back talking with Ross MacPhee. Let's see if we can get a phone call or two in, because there are a lot of interested folks. Anita in Sunnyvale, California. Hi, Anita.

ANITA (Caller): Hi. I'm wondering about that 10-million-year flurry of activity after the asteroid hit. There have been a few other asteroids that have hit the planets and were there any in your meta-study, did you see any corresponding, you know, 10-million-year flurry of activity from other species that have then died out? I mean, is this a consequence of the asteroid impact? And I'll take my answer off the air.


ANITA: Okay.

Mr. MACPHEE: Well, there - first of all, Anita, as I'm sure you know, there's a lot of evidence that the world really did change at roughly 64 million years ago when the bolide hit the planet. You would've had worldwide firestorms. You would have had an immense amount of garbage thrown up into the atmosphere, which would have blocked the sun, photosynthesis shutting down, on and on and on. A whole series of catastrophes which had to have affected animal life, of course.

Now, your question is whether this is a repeating phenomenon. Well, some paleontologists take the position that probably most, if not all, of dramatic changes in flora and fauna were prompted by catastrophes on this scale. Maybe not in every case of bolide impact, but perhaps that's the likeliest.

The other really big extinction in Earth history, which is much earlier than the one that we're talking about right now, was at the Permo-Triassic boundary around 250 millions years ago. And in many ways it was probably the worst extinction of all time or at least in sort of more recent times in Earth history. Maybe somewhere upward of 95 percent of all marine organisms, for example, died out in that extinction and was equally bad on land.

People have long looked for a single kind of cause or a believable catastrophe that might have affected it, and there is some evidence and some actually fairly good evidence that there was an impact at roughly that time which, again, changed the world. Going into Cenozoic times in the last 65 million years, there have certainly been impactors of much smaller size probably by and large, and some of them could have had an affect although none on scale of either the K-T(ph) or Permo-Triassic.

FLATOW: We've got about less than a minute left. What do you need to do, I mean, is this going to take years and years to find that what happened at that crossover point? Why one went up, one went down?

Mr. MACPHEE: Well, I think the first thing I have to say is we have to get our colleagues to agree that mammals like us weren't doing much for a long period of time. But yeah, there's refinements that can certainly be made in the super tree analysis that we undertook. We'll be doing some of that. There's also now vast opportunities for intersection between paleontology and neonatology, in other words the study of living things, to try and make this whole picture of what happened with life on the planet a lot clearer. And that's what we're in the game for. To try and understand it scientifically so that we can actually explain things better than we could before.

FLATOW: Good luck to you.

Mr. MACPHEE: Thank you, Ira.

FLATOW: And I want to thank you, Ross MacPhee, curator in the Division of Vertebrate Zoology at that little museum up the block here in Manhattan, the American Museum of Natural History in New York.

We're going to take a short break. We're going to come back and talk about new studies talking about whether you should have an MRI with your routine mammogram or not. If you're thinking about the possibilities of going to the doctor, what should you be doing? Stay with us. We'll talk right back at you after this break.

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