In Our Time - The KT Boundary

Episode Date: June 23, 2005

Melvyn Bragg and guests discuss the KT Boundary. Across the entire planet, where it hasn't been eroded or destroyed in land movements, there is a thin grey line. In Italy it is 1 cm thick, in America it stretches to three centimetres, but it is all the same thin grey line laid into the rock some 65 million years ago and it bears witness to a cataclysmic event experienced only once in Earth's history. It is called the KT Boundary and geologists believe it is the clue to the death of the dinosaurs and the ultimate reason why mammals and humans inherited the Earth.But exactly what did happen 65 million years ago? How was this extraordinary line created across the Earth and does it really hold the key to the death of the dinosaurs?With Simon Kelley, Head of Department in the Department of Earth Sciences, Open University, Jane Francis, Professor of Palaeoclimatology, University of Leeds; Mike Benton, Professor of Vertebrate Palaeontology in the Department of Earth Sciences, University of Bristol.

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or wherever you get your pods. Thanks for downloading the In Our Time podcast. For more details about In Our Time and for our terms of use, please go to BBC.co.com.uk forward slash radio 4. I hope you enjoy the program. Hello. Across the entire planet, where it hasn't been eroded or destroyed in land movements, there's a thin grey line of clay.

In Italy, it's one centimetre thick, one centimetre thick. In Central America, it stretches to three centimetres. But it's all the same thin grey line, laid into the rock some 65 million years ago and very likely it bears witness to a cataclysmic event experienced very rarely in Earth's history. It's called the KT boundary and geologists believe it's the clue

to death of the dinosaurs and the key reason why mammals and humans inherited the Earth. But exactly what did happen 65 million years ago. How is this extraordinary timeline the KT boundary created across the Earth? With me to discuss the boundary is Simon Kelly, head of Department of Earth Sciences at the Open University, Jane Francis, Professor of Paleoclamatology at the University of Leeds,

and Mike Benton, Professor of Earth Sciences at the University of Bristol. Simon Kelly. Can you just tell us what the KT boundary is? It's an instantaneous boundary formed at the same time across the entire world. Now that sounds like something that you would expect, but it's almost unique in geological history. There isn't another layer of rock that we can find almost everywhere in the world. we find it in America, Denmark, New Zealand, everywhere. And it consists of clay, broken down glass,

which breaks down to clay. The layers in America are about two centimetres thick. But inside the layer, you can actually, in America, break it down. The lower part of the layer is just clays. It's just broken down spherials of glass, which are broken down in water. And in the upper layer, you find all sorts of. of mixtures of other minerals, quartz, some diamonds, strangely, and quite a lot of soot.

So there's this particular layer. And you can find it in the Southern America states. You can find it in Canada. And you can see that it gets slightly thinner as it goes away from the Southern American states into the North American states and into Canada. And you find the same layer at exactly the same point in Italy, and it's slightly thinner. It still contains the same minerals, it still contains soot. You find the same layer in Denmark, still the same constituents. What about in Asia? In Asia as well, in China, you find the layer.

It tends to be thinner and further away from America. Simon Kelly, can you just take us a bit further with why this is so rare and even unique? Because you said that quickly at the beginning, and those people, cloned to myself, I don't know a great deal about geology. What makes it so rare and perhaps even unique? Most geological processes, like say, rivers depositing sediments or even oceans depositing sediments. The sediments are deposited in a particular area,

and you get changes in fossils, and so you can identify a particular area of sediments. But you're never absolutely sure precisely that the ages are the same. So I could have a set of sediments in America and a set of sediments in Europe, and it would be very similar. They have the same fossils in, but they could be different by half a million years. So they're not absolutely the same age.

they're roughly the same age have the same fossils in. In the KT boundary, as far as we can tell anywhere in the world, it's precisely the same age. And that's a unique, that's a unique situation in geology. There isn't another boundary that looks like that. So in Cabin letters, something happened.

And that was first lighted on, as far as we know, by the French scientist Georges Cuvier in the beginning of the 19th century. What did he find? First of all, what did he find then, and how were his findings received? Cuvier was walking in the Paris Basin, and he noticed a very strange change in the fossil record,

which correlated with a change in the rock. So he was looking at thick layers of chalk with a particular suite of fossils in. And then at the very sharp boundary, it moved to being a clay, which is a very different kind of depositional environment. You've got very little life. And then the fossils above that boundary were all very different.

And he remarked on this, And in fact, it developed into a sort of catastrophist theory. If you were able to ask Kuvia, he would have described the way that life evolved as periods of creation, then equilibrium and then catastrophe. And this KT boundary is the boundary between the Cretaceous and the tertiary periods. And although it's K, we call it K because K is for chalk and C was pinched by Carboniferous.

That's right, right. How was it received, though, when he found out it? People say, oh good, you found something very important. We're going to take this up. No. In fact, at the time the British geologists were sort of in the ascendancy, William Smith at the same time that Cuvier was working. Remember this is before Darwin. Cuvier was working the same time as William Smith,

who was developing this way of understanding rocks and processes in rocks, a gradual build-up of rocks. And that was taken up by people like Lyle a little later and developed into a theory, which is the way that we still understand. geological processes. The present is the key to the past. It's uniformitarianism, which means that if you're looking at a sandstone,

which looks like it's deposited in a river millions of years ago, then it probably was. And that's the best way to understand ancient rocks. So they played down. His great discovery, as it were, which turns out to have been right, was in his lifetime played down. Can we just take that on a bit, Jean Francis.

Let's go back to where in the Paris Basin, he discovered this great divide, divided by the thin grey line of clay between the Cretaceous and the tertiary. What was striking, Simon Kelly's hinted at it, but what was striking in the differences in fossils? Well, the KT boundary has been studied quite intensely since that time, and now geologists, paleontologists, particularly the scientists that study fossils, have got a good feel for what the change was all about

in terms of the sort of animal and plant record. And we know now that there were sort of mass extinctions. This is the time of mass extinction, a big mass extinction boundary. And we know that there are animals like the Ammonites, the big sort of curled fossils that everybody sees, the big sort of sea spirals, the kind of big squid-like animals in the sea, they disappeared at the boundary.

Some of the big sort of marine reptiles, a big gnashing mosasaurs and pleasiosaurs that lived in the oceans. We see their bones and their skeletons in the fossil record before the boundary in the Cretaceous rocks, but there's certainly none in the... the tertiary rocks. The plant record kind of survived okay through the boundary, except in North America where some plants went extinct. And we also see changes in animals that lived in the surface of the oceans. So some of the forums went extinct. Or they certainly were knocked out for a while.

So there is a big change in the fossil record in the rocks that we see, the fossils that we see in the Cretaceous and the things we see in the tertiary. But nevertheless, at the time, to take up the story from Simon Kelly with Cuvier. It was played down that there was this mass exchange. I understand it. Please put me right. Gradualism, the idea that things gradually evolved moved in.

And the Cuvier dramatic theory was gently pushed, or maybe not so gently, I don't know, pushed to one side. Yes, I think everybody thought that evolution was fairly gradual and the thought that there was some massive catastrophe on Earth

is really something that's come up much more recently. You're a paleo-clamatologist. Can you tell us about the climate of that period about 65 million years ago and how that might have affected what we're going to talk about, why there was this mass extinction, what caused it, what factors were? Well, it's interesting to sort of consider the climate effect in this boundary. And we're dealing with a really different world 65 million years ago. We're in what we call a greenhouse world.

Today is an ice house world because we've got glaciers at the poles. but 65 million years ago there's no evidence for major ice caps in the polar regions. Very different world. So there are forests at the northern. Yeah, there are forests at the South Pole and the North Pole. They're dinosaurs walking around on the South Pole. So the Cretaceous territory happened at this time when we were in much warmer climates, generally. We do know there are some smaller climate changes near the boundary.

So climate was in the Cretaceous, the climate was very warm with almost tropical temperatures. at the South Pole. And then the climate was cooling a little bit towards the end of the Cretaceous. There's some warming cooling periods, but it carries on being warm, even after the Cretaceous boundary. How might this have played a fact

in the KT boundary and the mass extinction? How might the climate have borne in on it? Well, I think this is sort of really current research at the moment, but I think we were going through a change from these really warm climates in the Cretaceous and the climate was cooling a little bit towards the Cretaceous tertiary boundary. So there were some,

there were some organism and some animals were responding to this cooling of the climate. Because what you're saying is really perhaps some signs of extinction were evident before an event, if it occurred, occurred. Yes, I mean there is a whole troupe of that this is still a very contentious issue, you know, the Cretaceous territory boundary, and I'm sure you'll find that three of us here have different views on what was going on. But certainly I think some parts of fossil record do show a gradual change, and a lot of the animals had already gone extinct before the Cretaceous territory boundary. So we're dealing with sort of the last remnants in some of the lines. Well, let's keep laying out at the map of this.

Mike Benton, in 1980, a geologist called Walter Alvarez and his father, who was a prize-winning Nobel Prize winning science, called Lurivirus, examined the chemical components of the K2 boundary, and they found iridium. Now, why was that significant and what came from that? It's only 25 years ago, but it did radically change view of that time. Yes, the Alvarez's father and son, they thought iridium would be an important thing to look at because it's a very rare element on the earth's surface. They believed, and they were broadly correct, that it doesn't not naturally occur on the surface of the earth,

that it comes largely from meteorites and small meteorites, tectites and so on that hit the earth, so that if you find iridium, you've got evidence of some sort of impact, although at that stage they were thinking only in terms of the normal kinds of low-level, impacts that happen all the time. Eridium chemically is close to platinum and gold. Louis Alvar is the father, the physicist, he'd invented a machine that would measure minute quantities of iridium because hitherto people had not been able to do that, parts per billion. And I think initially they went to look at this to try and find out if they could use it as a geochronometer. They thought that if we know there's a fairly predictable, regular rate of arrival of

meteorites on the earth. These are small ones, nothing to be alarmed about, of course. You could use that, you could estimate the quantity of iridium that would be scattered per square meter or whatever. You could look at the volumes in the rock and you could use that to actually try to link from rock thickness to time. This had been a long time debate. How did that tentative almost theoretical exercise

turn into the arrival of the mega meteor which blasted everything and changed the planet if it did indeed? This is where they were very... They made the junk didn't that's right they were very clever here because what they found was going through the sequence of late cretaceous limestones they found low levels a few parts per billion and that was a regular ticking clock there and suddenly then they found a peak or a spike of iridium it multiplied by 70 or 100 times now they could have said this is a concentrated bed this is a condensed unit of clay which took a thousand years to be deposited instead of one year and hence you get

a thousand times or a hundred times the amount they turned it on their head on its head and said, no, the thickness is regularly accumulating. This spike means massive impact. And then they set out to prove from, I'm coaching from notes that you people have supplied, but it seems from merely two small samples, they made massive predictions, which were born up by the other side. Yes, it's one of the most astonishing stories. Can you tell us of the most astonishing story?

That's right. They did the first study in a section in Gubeo in Italy, and the idea then was to try and establish a standard of time. having turned the whole idea on its head, interpreted the spike as evidence for one of the most astonishingly huge impacts ever, they found one other section in Denmark, and they found the same phenomenon there. It was a marine section. Also, with two sections, they went to print. And I think most people would not be brave enough to go that far. But they did, and with a long paper in the journal Science, which attracted a huge amount of attention, and based on what many geologists at the time said was very limited evidence, they really stuck their necks out.

in a serious way. Based on these two sections, they said this is a worldwide phenomenon, not just in Europe, not just in these marine beds. You'll find it on land and in the sea, and that this clay layer that we've heard about, of course, has this enrichment in iridium, which means massive impact, so they said, and they back calculated if the thing covered the earth,

they could work out the size of the event. And they worked out the size of the event, which was, and the interesting thing is this calculation was made. It is astonishing. It's wonderful to read about it. They sat down these two samples and said, well, what must have happened, or happened is something about 10, 12 kilometres across, travelling at 15 kilometres a second, hit the Earth, some of rather,

and that caused all this. It's a magnificent sort of projection of imagination based on fact, isn't it? And Simon Kelly, were they right? Indeed, they were. It's now been measured in about 80 different places around the world, and the eridium anomaly is found in every section. which is an incredible achievement. At the time, it was extremely controversial.

And I think if I'd have been in the conferences at the time, I would have enjoyed laughing at them because at the time the geologists were not looking for something like this. And anything which comes from outside the field, remember these guys are physicists. We were simply not believed. And this referred to, they found the case, crater, didn't they, in Yucatan?

Yes. And that was where they said that Mitya had impacted. Yes. How did they find that? And what grew from that? What does that tell us that crater? In fact, there was a large debate for about 10 years before the crater was discovered.

Anyway, they discovered the crater in Yucatam. Yeah. Once you've got the crater, once you know where the impact happens, other things start to fall into place, like the thickness of the layers. like the extreme events that we can see around the coast of Mexico and around the coast of America, where the deposits are not just thick,

but there are metre-thick tsunami deposits. So we've got, we've established the meteor, we've established the impact, and there doesn't seem to be any argument about that, and then the fallout caused this thin layer of clay. Now, moving on to, was that that which caused the massive extinctions? What do you think about that, Jane Francis? Oh, well this is a controversial issue.

Now we have to play some mind games. So if you can imagine this meteor meter hitting the earth, the idea was, if you played some mind games, the meteor act was slammed into the earth, created this big crater, and that it would have thrown up massive amount of dust into the atmosphere. And that this would have called what we call a nuclear winter scenario. That was how it was described in the early days.

And the idea was that all this dust in the atmosphere really blocked off the sun. There was no photosynthesis for the plants, the food chain was affected and then that's what killed the dinosaurs and a lot of other animals. There just wasn't the food around. So we had this sort of winter darkness for many years.

And also with the big meteorite impact, there was supposed to be a massive amount of soot deposited in this layer, which was interpreted as evidence of wildfires. So imagine a world, you know, the whole world was alive with fires burning everywhere, great masses of dust in the air, horrible place.

So is this sufficient explanation then, Mike Benton? Is this why dinosaurs died out? We keep using dinosaurs because I don't know about them, but land vertebrates, 90% of land vertebrates. Is it that reason alone, do you think the evidence is sufficient for that? I used to be very skeptical in the way that Simon referred.

I think most Earth scientists thought this was bizarre and just too easy. I think though the coincidence of this impact and I think the necessary physics of the scale of what would find follow on from it. I think as Jane described, the dust cloud, the freezing, the cutting out of the light, perhaps the heat shock, I don't know. Those things likely happen because those are physical consequences. The fact is that many, many major groups of organisms did die out at that time

rather catastrophically. And I've observed this over the last 10 or so years as people have looked at the fossil records ever more closely. Certain records, for example, of the ammonites, this major group of marine organisms, used to look rather gradualistic, meaning species disappeared step by step one at a time for many, many meters below the boundary. But when people have gone back to the sections and collected more intensely, they're filling the gaps. So a lot of these records that used to look gradual, stepwise, dropping off, become more and more catastrophic with time, and more species go up to the boundary. And that's true of the dinosaurs as well.

But there seems Simon Kellogg to be another candidate for the building towards, extinction, the so-called Deccan traps, these enormous volcanoes in India. Can you tell us a bit about those and why they play a part and at this time too? Deccan traps were an enormous area of India, in fact, and they probably looked a lot like Icelanders today, huge basalt eruptions which covered vast quantities of the area of India. And at the time, they would have been just like huge lava lakes. But one of the important things about that kind of volcano is, it releases a large amount of sulfur dioxide and carbon dioxide.

So it sort of poisons the air as it's erupting. We know that the decan trap started to erupt slowly at about 68 million years. Now remember that the boundary is at 65. So a few million years before the boundary, these volcanoes start to go off. And we also know that they exploded, and there's a huge amount of eruptions, unlike anything that we see today, thousands and thousands of times more volcanoes going off at the same time, at exactly.

65 million years. So there's this huge pulse of eruptions at about 65 million years ago. Does it seem a very odd coincidence these two massive things happening, Jane Francis? You've said during this programme that this is contested areas, but I haven't quite got what's contested because it seems to me going around the table. The meteor was a fairly clean sweep until we came to the deck and traps. Now, what's the contest going on here? I don't mean in brutal controversial. But what's going on? Well, there are several mechanisms of extinction probably at this time. we can discuss it at the end

but I think probably it's a mixture of everything going on at the same time but certainly we've got we know that at the same time there was a meteorite impact 65 million years ago and at the same time there was the eruption of all these lavas in India and there are two camps of people

some believe that the meteorite was the main killer of all these animals main killer of the dinosaurs some people don't believe that at all although these days I think people believe there was a meteorite that the effect it have is fairly controversial and then the lavas are erupting at the same time and they're changing the climate with this massive amount of CO2

and we do know that there are other times in the past where there were massive eruptions of lavas that had a fundamental change on life, on evolution. So these mechanisms have been known before but what's happening now with the Cretaceous territory is they're all happening together so it must have been a really dramatic world to live in. Are we sure enough are you, not me, but are you,

Are we sure, scholars, sure enough of the dating to say something as specifically, as you've just said, Joan Francis, that these are happening together. Because from the dating that I've read in the notes, some of it is, well, six and a half million years either way or something like the end. So when you're saying this is happening together, people think together means simultaneously, they think it means together. How sure can it be of that, Mike Benton? I think the dating is as good as it can be for geological samples. I think for the impact you can date melted rocks in the bottom of the crater essentially. So that is coinciding precisely with the impact and they can be dated. Sorry, I don't a bit pedantic.

What is precisely in your terms? I know geologists think precisely is within 4 or 5 million years. It's a bit better than that. But still not good enough, it has to be said. It's probably plus or minus 1 or 2%, I suppose. 1 or 2% of 65%. Which is still less than a million years error. But maybe Simon could give us.

The date on the KT boundary is probably to the nearest 100,000 years. But it's still quite a lot. It's not the actual isotope dating. It's not the dating by measuring the rocks that ties the two together. In fact, you can find the iridium layer within the basalts in India. So we know that the bassobs were going off, not because we can correlate things, but because you can actually see the origium layer in the basalts.

So you know the two are going on at the same time. Do you think those basalts, Mike Burton, those basalt flows, would themselves have been enough to create what generalisation referred to as a nuclear winter? I think people used to think that that was the sole cause, and they tried to explain the clay layer as a consequence as well, because that could be some kind of volcanic ash, for example. And they also try to explain the other physical features you find in this crisis layer, the various, say, glass spherules and stuff like that.

I think now people have abandoned. that and they accept that they have to explain the clay layer by the impact and I think nobody denies that the impact happened. People have tried to make a link of course from impact to volcanism and said oh the impact happened in Mexico and that weakened the crust on the other side of the earth and caused the eruptions. The eruptions started long before the impact of course and I think that type of model of the earth being beaten about like some some great big cake or balloon or something is probably not very valid. Whether the eruptions on their own could cause extinction, that's interesting.

There were other events, as Jane said, where that's almost certainly the case, that they could produce enough gas, they could produce enough global warming and other bad effects. Yes, there are times in the past when there have been, there's another extinction event called the Pomo Triassic extinction, 250 million years ago,

which is actually called the mother of all extinctions. The KT boundary spurred this whole new sort of look at extinction events in the past. And the Pomo Triassic event, we're pretty sure there's no evidence for a meteorite. impact. So that's pretty sure of that boundary. But there is in the Permian-Triassic boundary a massive amount of lava as outpouring

which is now considered to be part of the cause of the Pomo-Triassic extinctions. And that's a very similar sort of larva to the one that was coming out in India at the KT boundary. But at the KT boundary it's a little more difficult to be completely clear because although there was a huge set of volcanic

eruptions in India, just around the edge of the volcanoes in India, there are lakes. And in the lakes, there are dinosaurs. So at ground zero, when this huge volcano was erupting, ground zero right next to the volcano, the dinosaurs are happily puddling about in the lakes, laying eggs,

and the eggs are preserved in some cases because the next set of eruptions rolled over and preserved it. So that will militate against the idea of volcanoes wiping out dinosaurs if they're living happily in the foothills. It shows you that it's not. Like persons are on the slopes of Pompey at the moment. Yeah. But the critical issue is timing of these things in the interrelation of things.

I think you can't just view them as sort of one day on earth. There was a meteorite impact and volcanic eruption. Well, it must have happened one day if they do that. If they had days then. Three seconds, we're told. The meteorite impact happened, obviously, at an instant. But the critical thing is that these lavas were erupting a long time before the KT boundary. And if the real key to the lava eruption is the carbon dioxide and the gas is coming out,

with the volcanoes. They would have taken a much longer time span to erupt. And they're affecting the climate on a longer time span. So you've already kind of changed the climate system, change the environmental system, probably weakening it or warming it. And then suddenly, wham, in comes a meteorite. So it's sort of the addition of all these factors

that probably really is a clue here. Yeah. I think you have eruptions going on nowadays. And we had a big eruption a few hundred years ago, 1783 in Iceland, which caused huge problems in Europe. In fact, there's something like 10,000, 20,000 extra deaths in England in a particular year because of one eruption. Now, if you think of that as a punch,

and if the punch happens every few thousand years, things recover. If the punches come faster and faster, that things don't have quite their time to recover, and it becomes a cumulative effect. The ecosystems don't recover quite before the next punch. And you have a sort of punch-drunk earth, if you like. Can we move on to the fossil record and evolution?

I know you're like from the notice, some disagreement between you here, but it'll be helpful to clarify it. Can we start maybe with Simon? Is there any chance at all that the KT boundary and the distinction of the dinosaurs are not linked? No. I think that the evidence, well, okay, you're picking them the dinosaurs. The dinosaurs are very difficult. They're just useful. I don't mean enough to pick on, I can say land vertebrates, but it just gives us a clue.

Well, no, let's stop here for a second because it's useful. Are they a useful example, or do they kind of get in the way from now? They're getting the way. All right. Well, pick a better example. It's me, fine. Take the big marine geysosaurs and motorsaws. He's really big, you know, double-decker-sized gnashing teeth and jawed big animals that lived in the sea. I mean, they certainly disappeared, didn't they, at the KT boundary. You don't find the fossils in tertiary rocks. They ruled the Cretaceous seas and then they disappeared. The difficulty with the dinosaurs, there weren't many of them. Of the big dinosaurs, there weren't that many. So it's difficult to, when you get very close to the boundary,

if you don't find a dinosaur, does that mean it went and lived somewhere else? Does that mean it disappeared? And there were so few of them, it's very difficult. If you go down to the micro fossils, it's easier. We find a few, a number of fossils. You only find them in particular areas. I mean, there's an area of Montana, I think it is. Yeah, to some extent that's.

true. People argued that there was a gap of maybe three meters at the top, which was a divide of dinosaur bones. In fact, when you go to the Hell Creek formation in Montana, that's where people have sampled intensively. And they found, in fact, they go all the way up to the boundary. But they are rare, as you say, because big animals tend to be rare. I suppose the reason that dinosaurs and some of the others are not so useful in trying to understand the event is you'd expect them to be the kind of creatures that would go extinct under any kind of pressure because they're big and big animals

then live in small populations they rely often on specialised food and the rest. And probably the most striking things are the boring things like plankton and the pherominae, the small calcareous shell, things that's floated about in the sea. They suffered a dramatic extinction absolutely

at the boundary and it's much harder to imagine how they could have been driven to extinction. What kind of crisis would have caused that? Can we talk in a bit more detail about the fossil evidence after that. If there was a big crunch, then perhaps you'd expect to see a massive fossil, I mean, for an outside, a mass of fossils in this clay line, of everything that had been crunched or coming soon afterwards.

What does the deep fossil evidence tell you? If these vertebrates were knocked out, then in that specific time, a specific issue, as far as you geologists can get, and palakia can get to be specific, wouldn't it would want to expect that there to be fossils going into this clay line? No, what we actually find is that people have to do very careful work looking through the Cretaceous sediments

and they gradually document where these fossils are. So they find, imagine you are looking for, let's say a pleaser bone, you find them all the way up to the boundary in the Cretaceous rocks and then they disappear. You don't actually find them in this clay layer because it represents sort of a different kind of environment, but they sort of disappear.

However, I should say that there are quite a lot of fossils that carry on through the Cretaceous tertiary boundary. So let's not draw a picture of a sort of an earth that was completely devoid of life after this. There are selected types of animals and plants that went extinct at the boundary, but there are lots of others that carry on.

And there are also, geographically, we can look at the difference. Before we move to geographically, can you tell us what which carried on and why you think they carried? Well, for example, the classic case of the mammals, you know, the mammals were there in the background all the way through the Cretaceous and the classic idea is that the mammals

were little sort of rat-like things scurrying around the feet of the dinosaurs, and they really didn't have a major place on Earth, but they were sort of evolving slowly. And as soon as the dinosaurs disappeared at the KT boundary, this gave a chance for the mammals to really take over the earth. And then suddenly you do see an explosion of different kinds of mammals after the boundary. But let's just say we're going to, as I understand it, Zamb, Kelly,

the freshwater vertebrates like the crocodiles survive. Now what's going on there? One of the things that makes the KT boundary, that the impact hypothesis, so strong, as it predicts specific kinds of extinction. So if you were able to borrow and get out of the way for a few days or a few weeks, and then you could live on insects.

Let's say you're a small mammal. You probably survive. If you're a big dinosaur, you've no way to go. If you were living in the surface layers of the ocean, dependent on photosynthesis, it's bad. If you're scrabbling around on the bottom and you're eating the bits of other animals that have died and things like that, it's not as serious.

And what you find is there's a pattern of extinctions. An interesting one is the crocodiles, because although land vertebrates are devastated at the KT boundary, things like crocodiles, freshwater vertebrates, seem to survive. And the idea is that the crocodiles can hibernate by going to the bottom of lakes and survive quite a long time like that. I don't know. I mean, it's a nice way to put it, but of course they would suffocate.

They need to breathe air. And the instant killing model that people talk about is a flash of heat that may have followed very early on after the impact and affected all around the world. And certainly it's true that if you can borrow, once you're 10 centimetres or so below the top level of the soil, any heat shock would not necessarily reach you. Ducking underwater, I'm not so sure,

because you would pop up and breathe from time to time, and that would apply to turtles and crocodiles and some marine birds and other groups that survived. So that's where the thing gets very difficult. Well, I'm interested in it being difficult. So why did they survive if they had to pop up and take this? We need to do experiments. I mean, the physicists have calculated the nature of the heat shock.

So already it's been mentioned that the dust cloud and cutting out light, cutting out heat and so on from the sun. But also it's been suggested by the impact experts that when you have an enormous impact, a huge amount of stuff is thrown up into the upper atmosphere and some of it goes out into space essentially, but then returns back in, and it's that re-entry that's supposed to.

supposedly then cause as a sort of heat shockwave that goes all around the earth. And that heat shockwave, they say, is a necessary consequence. I can't judge whether or not. And that would happen very quickly. And it's that they're claiming, for example, would. But, you know, there's a problem here because the heat shockwave would have caused, well, some people believe this heat shock wave would have called the fires that have been predicted at the KT boundary,

because people initially reported lots of soot, or little beads of carbon in these layers. If you had fires, lots of fires, sweeping through the vegetation at this time, you'd expect to see lots of charcoal in the record. And charcoal is quite common in the fossil record. Things burn, they burn to charcoal. You get small cubes of charcoal. You'd recognise them.

You know, they'd make a black mark on a piece of paper. And once you've charcoalified some plants, they're pretty resistant to decay. So there's pretty good evidence all through the fossil record for fossil fires. So you would expect to see massive charcoal. deposits at the KT boundary. And some recent work by colleagues in the University of London shows that they just don't find charcoal in this boundary layer. I don't think it's found everywhere.

I think the soot is found in any deposit. In fact, if you go to the farthest extent where the University of London work was done, there is still soot in the boundary. So there is something going on. I don't understand about the charcoal. I mean, there's charcoal in some of the US sections. No? You're allowed to shake your head.

Well, I don't think the evidence is very clear for big fires, you know. Maybe there is, there certainly isn't, I don't think there is this idea that the fire swept the world at that time. It's the global wildfire. Yeah, I'm not sure that that stands anymore. I think we need to have a look for charcoal in different places in the boundary. But I think that's been that idea. So if that doesn't stand in able, working back, what does that mean about this impact, Mike? Well, I think we're still looking for a killing model.

I don't think anybody is entirely sure. I think that the physicist can give you very good stories about what must have happened based on the scale of the thing. The difficulty is in referring to the idea of uniformitarianism that Simon mentioned earlier that we should look at modern phenomena to understand the past. There is no modern phenomenon on this scale clearly. We haven't witnessed human beings

has not witnessed an impact of this size and therefore these suppositions about either cutting out the light of the sun and stopping photosynthesis or blanketing the heat from the sun and causing a kind of nuclear winter or the sort of heat shock. These are the three kinds of horrible things

that we're told would follow a major impact. Any of which could potentially have killed all of life. That's the point paleontologists always come back with and say, well, hold on, much of life survived. So that you can't have a killing model that's too brutal. And that's the subtlety that's difficult to get right. What, moving on, Simon, go ahead, What role do such cataclysmic events?

That's very rare, as you've pointed out, and then Jane said who was won 250 million years ago, but there doesn't seem to be a handful of cards on this one. It's just two or so, very few. What role do they play in evolution? That's a really controversial question. If you look at the way that the fossil record evolves, you can either look at it as a series of sudden changes,

and it's a huge controversy in the Pellier record, which I'm sure Jane will go into. You can look at it in two different ways, and in fact the whole fossil record can be described as a gradual change, and it can also be looked at as a series of sudden events. If you're trying to understand what the mechanism is that causes this sort of event, if the environment changes, if the ecosystems change, things shut down, and life will die out in certain areas. I mean, and we're causing mankind, you could argue,

is causing a huge mass extinction at the moment. And imagine that going on, although more natural causes. The animals become extinct, and now that means that the environment is driving evolution. The causal mechanism is the changing climate. But more specifically, Joan, what does it mean? Does it, is it stop start? In many areas, as Simon has indicated, it sops and starts again.

I mean, the mammals are there, and they don't start again, but they're allowed to start because they now have space. Is that right? Yes, I mean, I think these, certainly an event like the Cretaceous tertiary boundary. What these, whatever event it was or combination of events, it cleared some spaces for other animals and plants to then flourish. And so it sort of like cleans the system out every now and then if you like and allows things to really take over.

I mean, these are important. events. There are these, what we call it, five big mass extinctions in earth history where there are big changes in sort of life on earth in the fossil record. And they affect different types of animals to different degrees. But there are changes in the fossil record anyway outside these boundaries. You can tell I'm a bit of a skeptic here. But if you take the plants, for example, because I know more about the plants, there are big changes in the plant record. And they're not related to these big catastrophic events. They're probably more related to sort of natural evolutionary process of certain plants and competitiveness, you know, among vegetation.

And big events like this affected them a little bit. They certainly affected them at the time, but plants are pretty resilient and they came back later on. So we see a change in the fossil record over these mass extinction times, the KT boundary. But some things don't go extinct. They then come back and then flourish or they die out naturally or for other causes. Mike, do you think that these cataclysmic event, this catacly, let's say, sick with a KT boundary, that'll do. Do you think that actually disturbs the evenness and gradualism

of Darwinian theories of evolution, or do you think you merely say, well, and now and then we have a cataclysm, and we all go back to the SkyWa? I think they did affect the animal record quite differently from the plant record, and I think there's no question that one or two of the mass extinctions, including the KT, reset the clock completely,

and the world afterwards was very different from the world before, even though lots of things survived. The dominant groups often are wiped out, and hence giving opportunities to others. But in terms of Darwinian evolution, it's neither fore nor against because the interesting thing is, of course, these cataclysms, whatever they're caused by, happens so rarely that there's no opportunity for organisms to adapt.

So I don't think anybody would expect organisms to be able to become good at surviving impacts. I mean, how do you do that when it doesn't happen on a small scale? There is a mechanism in the plants. If you look at the way that the Cretaceous, plants and Cretaceous, plants and Cretaceous worked, there was a sort of a fire economy. Fires, and as Jane said, charcoal is quite common, particularly in the Cretaceous. So you have a series of plants, and you have it in present day plants. You can have a forest fire, and a few years later, all the seeds that were sitting on the ground

have been germinated because they're waiting for this sort of thing. That was good luck. I mean, they're adapted to fires that happen in the normal cause of things rather than impacts and so on. But then that gives them the chance. And then there are certain species, as you say, that will come back, very rapidly, by their nature, their weedy or pioneer species, they can come in after a crisis of some kind. Well, as we hit the last bend before the finishing line,

let's just, for a few moments on mammals, do you think mammals would have developed the way they did, had it not been for this virtual wipeout, it may have been a combination of the meteor, the deck and traps, the changing climate, anyway, do you think that this gave a spur to evolution?

There's a million dollar question, isn't it? You know, if we still had those big dinosaurs around today, would we be here? Probably not. Something else probably would have happened later on. Certainly, you know, the next big issue after the KT boundary was glaciation, the beginning of the ice house world, when we developed glaciers on Earth. So that would have been a big prompt that probably would have killed the dinosaurs.

And then we would have probably had, well, who knows then? Let's imagine that the little mammals with lots of wool and hair to protect them from the cold would have then survived. So certainly the KT boundary was the prompt for the. the mammals to really come on strong and that's why we're here now. Would you agree with that? But yes, and I think it's important to remember we often think we're better because we're around. Mammals are better than dinosaurs and people think of dinosaurs as pretty hopeless as dinosaurs. That's right.

But of course they were on the earth for 160 million years and Jane is right. They were not predestined to go extinct at the point when they did. Indeed, the mammals arose at the same time as the dinosaurs. So they had been around side by side but had been somehow held back ecologically. Then they took off. the ice age would have done for the dinosaurs. People often speculate would dinosaurs themselves have become humanoids, but I think that's a bit daft. That's of science fiction. I think it's easier for small around animals to evolve quickly. Once you're a large animal,

you're very specialized. If it's a small animal, it tends to evolve more quickly, hence the mammals evolving. But you could argue that the smaller dinosaurs were present-day birds, and so that they did survive. So maybe the smaller dinosaurs are what we see, flying around in the sky nowadays. Well, thank you all very much. Thanks to Jane Francis, Mike Benton and Simon Kelly. And next week I'll be talking about Merlin, and you're still voting in your thousands every week for this,

your favourite philosopher, not the greatest, not the best, your favourite philosopher. And Marx is still out there in front, which continues to completely baffle me totally, but Plato's doing very well, all right, I know. I'm looking at the producer who wants me to do this as if this is a racing commentary, Charlie. Plato's doing well, who else?

Wittgenstein's very well, Hume's doing well and that's enough, thanks for listening. Thank you very much for listening. We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programmes about history, science and philosophy at BBC.com.ukh forward slash radio 4.