In Our Time - Mammals

Episode Date: October 13, 2005

Melvyn Bragg and guests discuss the rise of the mammals. The Cenozoic Era of Earth's history began 65 million years ago and runs to this day. It began with the extraordinary 'KT event', a supposed asteroid impact that destroyed the dinosaurs, and incorporates the break up of Pangaea, the enormous landmass that eventually formed the continents we know today. It is known as the 'Age of the Mammals', and it is the period in which warm-blooded, lactating, often furry animals diversified rapidly and spread across the globe on land and in the sea. According to evolutionary theory, what conditions created the opportunity for mammals to thrive? What environmental factors lead to the characteristics they share - and the features they don't? And how did they become the most intelligent class of animals on the planet? With Richard Corfield, Senior Lecturer in Earth Sciences at the Open University; Steve Jones, Professor of Genetics at University College London; Jane Francis, Professor of Palaeoclimatology at the University of Leeds.

This BBC podcast is supported by ads outside the UK. 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 programme. Hello, the Cenozoic era of Earth's history started about 65 million years ago and runs to this day. It began with the extraordinary KT event,

a supposed asteroid impact that's said to have destroyed the dinosaurs and incorporates the breakup of pangea, the enormous landmass that eventually formed the continents we know today. This is known as the age of mammals, and it's a period in which warm-blooded, lactating, often furry animals diversified rapidly and spread across the globe on land and in the sea. According to evolutionary theory,

what conditions created the opportunity for mammals to thrive? What environmental factors led to the characteristics they share and the features they don't? And how did they become the most intelligent class of animals on the planet. With me to discuss the rise of the mammals is Richard Corfield, senior lecturer in Earth Sciences

at the Open University, Steve Jones, Professor of Genetics in the Galton Laboratory at University College London, and Jane Francis, Professor of Paleoclamatology at the University of Leeds. Richard Corfield,

can you tell us when the first mammals started to emerge in Earth's history and what they were like? Yes, I think the most extraordinary thing about the evolution of the mammals is how very, very ancient they are. The KT boundary took place 65 million years ago when the asteroid hit the earth,

but the mammals actually date back to 200 million years, which is very close to the beginning of the second great era of visible life, the Mesozoic. So 200 million years ago puts us close to the base of the Jurassic. And the interesting thing about the mammals is that they evolved almost contemporaneously with the dinosaurs. and this immediately leads into interesting questions about what were the mammals doing for the 135 million years while the dinosaurs were stomping around

before they got it in the neck at the KT boundary. So the mammals themselves evolved from mammal-like reptiles, the so-called therapsids, who in their turn evolved from the pelicosaurs, which were a type of reptile pre-dinosaur, which had a big sail on its back, which was thought to be involved in thermoregulation, a way of controlling body temperature.

Now the interesting about the pelicosaurs, this group which gave her eyes to the mammal-like reptiles and then the mammals, is that they eventually did away with this evolutionary innovation, this sail on the back. And the speculation is that during this time they were developing one of the most important traits of the mammals, which is the ability to control internal,

body temperature, so-called homeothermy. And that's a major departure from all the other vertebrate groups, with the exception of the birds, who are poikilothermic. They are cold-blooded in popular parlance. They cannot control their own body temperature. So when we talk about mammals, what we're actually talking about are often furry vertebrates whose females lactate. They produce milk from modified sweat glands and use this to nurture the young.

and they are homeothermic. They can control their internal body temperature. Briefly, what were they doing for 125, 135 million years? Well, I think you got it right, actually. I think they were hiding. I mean, the earliest mammal, for example, is mega-zostradon, and there's a similar variety called aeosodon. And these are very small shrew-like creatures,

about 10 centimetres long, probably nocturnal, for the very good reason that in the daytime, they stood a very high chance being eaten by something big and scaly. What kind of evidence does a paleontologist have to rely on to reach the conclusions you've reached? Well, the interesting thing about the evolution of the mammals is it's basically the evolution of teeth. There's a very practical reason why teeth are important when you look at the evolution of mammals is because mammals are predominantly terrestrial animals, predominantly not always, as we know,

and they tend not to fossilize very easily. And the things which tend to get left behind are the teeth because they are extremely preservable in the fossil record. It's quite hard, at least it was quite hard for me as an undergraduate to get excited about mammal teeth, but now I see the point of it because not only are mammal teeth preserved,

they also are very varied. Dinosaurs had basically one type of tooth, whereas mammals had four. very early on in their evolution, and these diversified into many different types. Can you, Steve Jones, you're a geneticist, a molecular biologist, do you use a different method to draw your conclusions about the mammals different from, well, obviously you do, can you tell us what it is?

Yeah, it is. It is, it's superficially different, but perhaps fundamentally the same. And what genetics has done is turn us all into living fossils, because we all contain, all mammals contain, all humans contain, we all contain a complete history of evolution from the very beginning in our own genes. And of course that's changed, and you can reconstitute the past from the present, which is what Darwin did when he looked at different species of mammals gathered across South America, let's say,

but we can do it much more accurately now with genes. The process is a bit like reconstructing ancient languages. I think it was Dumas who said that English is just French badly pronounced, and you can see that English and French German. No, that's another argument. But you can see the similarity between the English and French rather easily. You can see it's more difficult to see the similarity between English and Turkish,

although it's there. But if you look hard, you can actually reconstitute ancient languages which are the ancestors of them all. And we can do that with genes, but we have to make an awful lot more assumptions. We make some tremendous assumptions about the rate of change, about the population size, about why the natural selection was acting.

on these genes, we tend to assume that it wasn't. And the thing which is really quite surprising is how well this molecular clock argument fits now with the palatological argument. For example, the molecular clock suggested that mammals were much older than we had thought, and lo and behold, the fossils now agree with that. And so you're very much together on this. This molecular clock, can you just tell people a bit more

about the molecular clocks, Steve? Well, the molecular clock, as you can guess from its name, It turns on the assumption that the rate of change of DNA ticks like a clock and that the more different two lengths of DNA are, the longer they've been separated. It's rather a daring assumption. If you compare rats and mice with rats and mice versus humans, it turns out that the clock has gone faster in the rat-mouse line. So, you know, we're not all...

I don't all have the same drummer, as it were. But the crude assumption that change and time are related to each other It's held up rather well. And we can't get back to the Therapsids, I don't think. Incidentally, I remember once seeing that misprinted as the therapists. Humans have descended from therapists, if I said. I was greatly cheered some of my psychiatric friends. We can't get all the way back, but we can get a long way back.

There's no question. How did mammals evolve from? And what did they evolve from? Well, we've heard about that. We can move on to there. Can you talk about the early diversifications? The very early diversifications. Well, again, the molecules are not that good

of telling you about the very old diversifications. The odd thing is, as we've heard, for about two-thirds of mammalian history, we were a... Can you just sling in some dates every now and then? Because, I mean, when you're saying not very early, you chaps tend to mean 120 million years. So if we just keep saying 120, 90, 60,

give us a bit of a steer on the way through. I think the big bang in at least the mammalian fossil record was this notable event, which even the BBC noticed, the KT extinction, 65 million years ago. There is this huge span of time from about 200 million years ago

to 65 million years ago where the mammals were really rather conservative. They stayed as small, furry, shrew-like beasts, although they did diversify. And if you look at a tree of the fossils, it turns out that there were dozens and dozens or many lines of mammals before 65 million years ago

who simply disappeared and didn't go anywhere. So, you know, we are the survivors of a very small subset of all those very, ancient creatures. And it's very easy to sneer at them because they're extinct. We don't really know what they were doing. Jane Francis, what sort of climate was there on Earth during the Cretaceous period before the death of the time? Before 65 million years ago. Well, the climate, the whole time Richard mentioned, as the mammals were evolving, we know is what we call a greenhouse climate. We know that it was pretty much warm all across the globe,

right up into the polar regions. What is pretty much warm mean? Well, tropical forests. Tropical forests in Antarctica, which was over the South Pole, and up into this high Arctic regions. There's no ice on earth at all, because presumably the CO2, the carbon dioxide levels in the atmosphere were higher at that time. So we had a very warm climate. Perhaps in the middle of the large continents,

you mentioned Pangea, this very large landmass. When Pangea was altogether, the centre of the continents would have been quite arid. But generally, throughout the Cretaceous, 100 million years ago, Pangaea was breaking up and there were seaways between the continents. So we had warm and moist and moist. climates over much of the globe. And this was important because it meant the vegetation was fairly lush

all across the globe. I don't think we had very strong seasonal climates. So they were fairly uniform, fairly warm and probably a pretty nice place for the early mammals to live in. And of course that helped tropical forests to go or subtropical forests, lush forests.

And you asked Richard, where they live? Well, they probably hid around in these forests. There would be plenty of little niches and bushes for them to hide under. plenty of food for them to eat. And presumably a very good place for the dinosaurs to live. Yes, well, I imagine the dinosaurs were thrashing their way through the forest. It was warm and there were lots of vegetation.

And the small animals were there living in the undergrowth or coming out at night. Can you just tell us briefly about this famous K2 event and the effect it's supposed to have had on the mammal environment? The KT, the Cretaceous territory boundary, that's when the asteroid hit the earth and the dinosaurs went extinct. In terms of climate, there wasn't much change. actually. And before the KT and the Cretaceous, I think it was the dinosaurs that stopped the mammals really expanding. And it wasn't really a climatic factor. And after the KT, yes, there was probably a

little bit of environmental, you know, catastrophe for a short time. We know there's probably some cooling events just as we come into the K2. 65 million years ago. But afterwards, for another 10 million years, we still have these warm tropical climates. And in fact, up to 55 million years and the earth got very, very warm. So it was still a pleasant place to live in and still we find in the fossil record we still find evidence of lush forests

in the high latitudes. So there was no major climatic factor at that point, I think, that really affected the mammals. It was just after that then everything started to happen. Richard Gough, do you think it's necessarily the K2 event that did away with the dinosaurs

and led to the rise of the mammals? I think it's indisputable that the death of the dinosaurs at the KT boundary led to the diversification of the mammals. Can we just again nail that KT? There might be a few people who are just saying to myself, I can't quite remember, why did it

nail the dinosaurs? Because an asteroid 10 miles across, hit the earth travelling at transonic speed, created a crater 100 miles across and threw dust into the atmosphere which blanked out sunlight for

several months and dinosaurs, of course, by being cold-blooded, were not able to regulate their body temperature in a world without sunlight, and so became extinct. Sorry, interrupted you. That's okay. No, that's in a nutshell. I mean, Jane has alluded to this transient cooling

at the Cretaceous tertiary boundary, and it is there in the record of the oxygen isotope, record of isotopes of oxygen, you can see a cooling at the KT boundary. But beyond the KT boundary event, which, as it were, cleaned the board, for the evolution of mammals. Remember, as we've said,

that the mammals had been around since 200 million years ago. That's the base of the Mesozoic. The KT boundary defines the end of the Mesozoic. So that's why the Mesozoic is the era of the dinosaurs. Another important factor, which has actually just recently been published in science, is, as one of my colleagues at the Open University

pointed out to me the other day, is the increase in oxygen levels from about 200 million years ago. That's the base of the Mesozoic, to the present day. And the geochemical evidence is very good that oxygen levels have increased, more or less gradually,

for the last 200 million years. But there were times when oxygen was higher than at other times, oxygen in the atmosphere. And one of these was in the late Cretaceous, the heyday of the dinosaurs, and it appears, this was about 90 million years ago, 18, 90 million years ago, and it appears that there was a major diversification

of placental mammals at about that time. I'd like to come back to oxygen, but thanks for bringing in placental mammals, because that brings us back to you, Steve Jones. Can you talk about placental mammals? They were the most successful mammals. We're told there around 3,845 species. I'm reading this, obviously, where there are only 2, 6 species of marsupials

and 3 species of egg-laying mammals. So what does this say about the placental, placental mammals? Well, it's obviously a rather good trick to have a placenta. And it's interesting, actually, the placenta, which is the sort of coffee bar between the fetus and the adult has actually evolved in other groups. There are fish with placenters, for example, line-bearing fish.

And what this has done is to allow the young mammal, before birth, to get to a much more advanced stage before it enters the world by virtue of the fact that its mother can feed it indirectly. And this has clearly been a rather successful strategy which may indeed have something to do with this oxygen story because one of the things a placental mammal must do and a fetus must do is get oxygen from its mother.

And if you look at the molecular end of it, it turns out that when you look at the first step in the oxygen chain, which is the hemoglobin, which is the stuff which is a protein that picks it up from the air, it's a well-known gene. In fact, it's a well-known family of genes in mammals. We have several versions of these globin genes, some of which are only found in the fetus, and the fetus pulls oxygen across the placenta from its mother.

The fetus actually is a statement of the battle of the generations. The placent is a statement of the battle of the generations because the interests of the mother and of the fetus are, of course, different. And there are really quite a lot of but to and fro where the fetus trying to get more and the mother trying to give less, which is a statement again of the importance of conflict in evolution. and the mammals, without doubt, are radiated enormously, partly because, for example,

of sexual conflict between males and females, perhaps of conflict between parents and offspring. And that conflict began in the mammals long, long ago. So how did that conflict help? I mean, why did the conflict between the mother and the child in the womb and male and the female? I mean, what exactly do you mean by conflict in this context? Well, if you take it in terms of the fetus, for example,

you can see a very neat example in the placenta and that there are particular genes which express them, themselves, which do their job, in the placenta. And the version which comes from the father does it differently from the version which came originally from the mother. They're kind of genes that increase the rate of feeding of the fetus. If you've got the father's version makes the fetus work harder, feed harder, the mother's version tries to slow down the passage of food.

Yeah, this is really a debate about the immune system, isn't it? The placenta is a device which basically gets around the mother's desire to reject the fetus for a relatively long amount of time. In our case, it's nine months. Now, in marsupials, the female marsupial expels the fetus, the so-called neonate, after a very, very short period of gestation. And one reason for that is because, for example, in a kangaroo, a marsupial, the immune system separation, because they don't have a true placenta, is not as advanced as in the placental mammals. And then there's one step before that, which is actually what

is the very elegant linkage between the reptiles and the mammals, is that the most primitive mammals alive today, the so-called duck-bill platypus and the echidna, lay eggs. And so what you see in the broadest possible terms, and this doesn't necessarily mean that I'm trying to erect a phylogenetic

tree from one to the other to the other, is that the prototheos, the monotrems, and the most primitive egg-laying mammals, and then you have the metatheria, the marsupials with this kind of halfway house immune metabolism, and then you have the uetheria, the true placental mammals. And what you actually see is a progression towards being able to hang on to your fetus

for longer and longer, which means that you can do more and more for it in the womb, and therefore it's not as exposed when it gets into the environment. But these three are not evolving sort of, as it were, A, B, C. They're coming on at the same time, aren't they, Steve Jones? Yes, and you can see, actually, there's a great natural experiment happened, and it's happening again, which is when the placental mammals...

What do you mean it's happening, you know? Well, if you look, for example, as what's happening to the native mammals of Australia since humans got there, it's clear that the placental mammals are, cats and dogs, all the others. When it comes to a battle with the other more primitive forms of mammals, tend to win.

And the non-ewetheria have been wiped out again and again. as continents come together, for example. So clearly we have a better plan. But evolution doesn't aim towards perfection. It only goes as far as it needs to, as it were. As long as there were no placental mammals fighting against these kangaroos and the like, they were perfectly happy.

When we arrive, then there's trouble. Right. Can you just tell it? Can we develop the oxygen point, Jane Francis, a little bit, and talk a little bit more about the effect of climate on the mammals after the K2 from, say, 65 to about, for the first few years, with the oxygen. increase. Then I want to talk about it getting cooler. I mean, Richard introduced the idea of more oxygen.

I think that's a beguiling notion, but maybe we could develop it a bit more. Why was this sudden almost doubling of oxygen in you guys' terms of comparatively short amount of time? Well, I think, as I mentioned before, just after the KT, things were warm. We know from evidence in the geological record in the fossils the climate was warm.

And then there's another peak in oxygen, isn't there, in about the Eocene, which is about 55 million years ago. Which corresponds to another radiation of mammals, yeah. Yeah, so all these are steps. There's an interesting, also an interesting event, which is sort of a hot topic in geology at the moment, which is looking at the climate 55 million years ago, when there's this real peak of warmth,

and we see in the geochemical record, a sudden burst of carbon, which we think must have had an origin in methane. And a lot of people are working on this in the moment, because it looks like at a certain point, the climate warmed just enough that it released a large amount of methane that was stored, if you like, as frozen methane,

on the sea floor. And if you change the sea level, if you warm the seas up a little bit, then the idea is that this methane was released very, very quickly into the atmosphere and caused a very, very massive warming, just in a few hundred thousand years, which logically is a very short time. The climate after that was carried on to be warm for a few million years. But then there was a big change. Then there was a big change. And this is the fundamental change in our climate system, which affected just more than the mammals, it affected vegetation, ocean circulation, practically everything on Earth.

And that was the beginning of ice on Antarctica. 38 million years ago, we see the build-up of ice and Antarctica to make big ice sheets. And that fundamentally changed our climate. It changed the ocean circulation system. And from that point, we can see evidence that the Earth was cooling. There was no ice in the northern hemisphere for a long time. But what happened is when ice was building up in Antarctica,

the climate also became dry globally. And that drying out of the climate, the global climate, got rid of most of those tropical forests that I mentioned were common in the Cretaceous. And instead, tropical forests were restricted to the, actually honoronically restricted to Antarctica until it got too cold. But a large part of the world was dominated then by, first of all, shrubby woodland drying out the forests,

and then grasslands. and that really prompted a big change in the mammals because a lot of the early mammals were used to, Richard mentioned the kind of teeth that they had, they were used to eating nuts and berries and sort of rainforest-type food and suddenly all that was in short supply. And so the animals that could eat grass really took over at that point.

So were dominated by herbivores? Yep, and they were particularly dominated by animals that had very long teeth or large teeth, high-crown teeth. because the thing about grasses is that they contain little things called phytoliths, which are little grains of silica. So it's like, if you like, eating a mouthful of gravel, I guess. And so for the early mammals that didn't have teeth that were adapted to eating this really abrasive food, their teeth wore down very quickly.

And so a lot of strains, I think, that didn't have these big teeth that could grow fast or high-crowned teeth, they became extinct. So the animals that had high-crowned teeth were then the survivors in the grasslands. We're talking horses and sheep. Yeah, what we call the angulates, yeah. The animals that we know that live in sort of grasslands today. Steve Jones, can we turn to this great landmass, pangir? That began to break up.

Can you tell us when? There was a 30% smaller landmass than there is. Now, can you tell us when it broke up and what the consequences were for the mammals? Well, I'm not a great dating man. Well, maybe the other two. Just throw it up front. Jane's got a finger raised to put you anytime. time you want to date.

Just... Sorry about that. I was just... I'll great you on that one. I'll be called Blind Date. Yeah. I once saw a T-Shirt in Australia that said reunite Gondwana land.

And actually the breakup of the continents, some of which actually were immeasurably long before, the ones we all know about. The earth is shattered and rejoined many times. That really shows the importance of geography in evolution.

because you can actually see quite clearly in the molecules and indeed in the fossils great groups of creatures which don't have anything much in common apart from the fact they seem to come from the same place. There's a great group of mammals called the Afrotheria, for example, elephants and the like who were found in Africa. And that fits with the fact that Africa broke off this landmass and began to float away. Madagascar is a subset of the same thing. So again we've got this rather strange coincidence

between the genetics, living creatures, telling us that they're more or less different from each other, and geology agreeing that indeed they lived on those different ancient continents. Jane? Yes, as Steve mentioned, when Pangaea broke up, which was about 150 million years ago, sort of in the Jurassic, and before that, all the continents were amassed together. In particular, Gondwana, as Steve mentioned, was joined together.

That's all the southern hemisphere continents. And for a long time, animals could walk all the way from South America. they walked all the way down through Patagonia, all the way across Antarctica, all the way into Australia. And in fact, I think the marsupials have an interesting story because it appears that when those continents were joined together as Gondwana, marsupials seemed to have appeared in South America. And they sort of walked, if you like, or spread through Antarctica into Australia. But what happened after that is that because the continental plates were moving around and Gondwana broke up and Antarctica moved south, Australia moved north, and South America was isolated,

and the placental animals that were developed in South America no longer could escape into Australia. So Australia actually does not have a good placental record of that age. It was fairly much isolated. It became isolated in the Cretaceous and started to drift south. But I'm going to have to stick up a bit for marsupials here and take up a point that Steve mentioned earlier about them somehow being inferior to the placental mammals.

which actually I don't believe is the case. It is true that marsupials are, female marsupials are permanently pregnant. The female kangaroo, for example, has an egg in the uterus, it has a neonate on the teeth, and it has a Joey hopping around. So marsupials may seem like the trollops of the mammalian world, but in fact they're not. What we're talking about here is a distinction between R selection and K selection. Now, this is an old idea in ecology.

Our selection is where you produce a lot of young and do not invest a lot of time or energy in bringing them up. It's like shooting a crow with a 12 bore. Placentals, on the other hand, invest a lot of time in bringing up just one at a time. It's like shooting a crow with a sniper's rifle. So these are two alternative strategies. Now, the R selection strategy, which the marsupials employed,

worked incredibly well. for tens of millions of years, all throughout the Cretaceous and into the Cinesia. For example, if you go to Riversley Station in Australia, you'll find that there is an incredibly diverse marsupial fauna fossilised there. You had Thalaka Leo, the marsupial lion. You had Thalakas, the marsupial cat. Every niche which we would associate with placental mammals in the northern hemisphere was occupied by a marsupial equivalent in the southern hemisphere.

What seems to have happened, I mean, certainly it's true, for example, that when we exported cats, I'm a huge cat fan, Arnold Schwarzenegger of the placental world, but when we exported cats to Australia, it had a very negative effect on the marsupial fauna there. But by and large, marsupials are extremely successful in their own right. and I think that a more important reason for their being out-competed by placentals,

for example, when the Isthmus of Panama joined, was their large brain size. Right. Can we talk about the joining of Isthmus of Panama first Jane, and I'll go over to the brain size with Steve? This happened about three million years ago, so we're coming much closer to our time. Can you take a Richard's story, though? I mean, why it was important.

Anyway, tell us about the joining around three million years ago. We're getting very close to today, really. Yes, yes. And what happened? The Istmuth of Panama didn't exist before that. and it affected two things. It affected the ocean currents and the climate. So North and South America joined up.

Yes. So there was a land bridge. Landridge, but it separated, it made great oceans instead of one swirling sea. That's right. So the first thing that could happen is that the animals that were isolated in Northern America could walk across to South America and vice versa. So there was an exchange of animals for the first time. And there are different ideas about which one was dominant.

I don't know if that's solved. But secondly, what happened was to put this bridge of land there did actually separate different ocean circulation. So before that, the ocean, there was an ocean current between the Atlantic and the Pacific, and it kept the ocean's sort of constant salinity. When this land bridge came there, it stopped the Atlantic currents flowing into the South Atlantic. And so instead, the currents in the Atlantic became much saltier. And as they became saltier, they became denser. And they flowed north along the North American margin to as far as ice.

And then it became so salty, so dense that they actually sunk down into the lower levels of the ocean. And that meant the warm water, the warmth that they were taking with them, did not get all the way up into the Arctic. So for the first time the Arctic actually had no heat source and it started to get cool. So that was again another big change in our climate that affected all life on Earth, in that the ice sheets started building up in the Arctic regions. And we were plunged into what we know in the northern hemisphere as our last ice age. But that again, the mammals could survive. being warm-blooded, being able to regulate their body temperature.

Yes. So how did this join in together? How did the Istmits of Panama affect the mammal evolution? Well, I think certainly it caused this great climatic change, and I think we see that the conductor of the evolutionary orchestra is climate. There's no question. But then the individual sections respond in different ways. And when the geography changed, in some ways exactly as when geography changed

when we invented the sailing boat, got to Australia, then the northern placental mammals, many of them got into the south and basically did much better than many of the native placentals and indeed the kangaroo-like mammals which are in South America and extinguished them. There's still plenty of them there. Why was that, though? Because you're the great defender of the marsupials or a marsupian man to your fingertips. I have a great fondness for the marsupias. Well, I think Stephen Jay Gould had the best theory, actually, for what happened to the marsupials in South America. South America had been isolated for 67 million years, from about 70 million years to just

three million years before present. And during that time, a very diverse marsupial fauna had evolved in South America, including a group of cats, exactly parallel to our cats, called the boar hyenas. But how did they go? When the Landridge joined North and South America, placental mammals went across into South America and out-competed them. But It may have been, this is Steve's theory, Stephen Jay Gould's theory, because placental mammals in North America had larger brains, and that went along with increased locomotorability, i.e., they could run faster, and more efficient slicing and biting apparatus.

And so it appears that the placental mammals in North America had been sort of refined by a hotter evolutionary fire than the mammals of South America, carousal mammals. Why are there so, can we look at some mammal features and why is the most common form of mammal, the rodent, there are about 1,700 species? Well, that's a very, the number of species per group is not something I think evolution has sorted out. You know, why is there only one species of homil, which is us? It's difficult to know.

I think if you look in general terms, small creatures tend to be more specious as we say and large ones, there are more different kinds. Creatures with a lot of sexual selection, where there's lots of fighting over mates, tend to evolve very quickly in the different species. Creatures which get isolated in small groups, as perhaps rodents might in particular mountain valleys, they tend to evolve very quickly at their chromosome level, let's say.

So I don't think there's anything particularly significant about there being lots of species of mice and not many species of elephants. I don't think there's any really general rules you can come out with there. You don't think it's because there's more ecological niches. I have this theory that since the early history of mammals is all about

small mice-like creatures that they just stitched up all of the ecological niches and I suspect there's more ecological niches for mice-sized things than there is an elephant-sized things. Yes, that's true. I mean it's easier to be a mouse than an elephant. There's no question of that. There's a certain circularity in saying there's more ecological niches.

But clearly there is a general rule that the smaller creatures have evolved much quicker. Most mammals are land species but they used to be, we used to think that we all came out of the sea, it seems the other way around, we all came out of the land and went into the sea. There are mammals in the sea. That is the way. Can we just going to skip that? Are we going to make a reference of that before we move on? Well, that's the whales, isn't it? Yes. I mean, we did, of course, historically, in deep time, we did come out of the sea.

And our blood, mammalian blood is rather like seawater. So certainly there's... What's deep time? Devonian. Yes, about 400 million. Yes. But then, of course, we then dabbled our toes back in it in the form of whale seals and the like. And ironically enough, perhaps because they had in the sea

and perhaps, of course, because they were so large, the whale fossil record in the last few years has become spectacularly good. And it's almost impossible to believe, with our perspective, and what time's all about. But some of the best whale fossils are found in the Himalayas. But of course, the Himalayas are Arivese. Like all mountain ranges, they just turn up, then they go away.

They're not really important. And at that time, there was a sea where Everest now is, the Teethysts sea, and there are plenty of really wonderful whale fossils in there. And really quite marvellously, the molecules and the fossils are beginning to agree about this going back into the sea. Not perfectly, that's for sure. But there are some surprising parallels coming out. Jane Francis, finally, not finally, we've got a bit more time, thank goodness.

But the ice age, three million years ago we started the ice age. How is it, what effect is this having on the mammals vis-a-vis the rest of life on Earth? Well, the one thing that happened certainly to the climate, as soon as we started going into a really deep ice age, is that the climate became much more seasonal. And I mentioned before in a sort of, in a warm world, you don't tend to have that kind of seasonal change. As soon as we get into a cold world,

I mean, apart from battling with ice sheets that were building up in the northern continent and the south, the climate was much more seasonal. And I think that takes some adaption, because food supplies become much more erratic. You have to remember where all the good food supplies are in the spring. You have to be prepared to sort of store food and be ready for that. You can't just sort of live the kind of life that mammals would have lived earlier on

where there was a constant food supply. So I think you had to be pretty much clever to try and cope with a very seasonal ice age world. Well, come to Brines in a minute, because it's obviously a very big part of this development. Can we just go back, Richard Corfield, to this placental mammal marsupial business that you and Steve have been rather batting backwards and forwards. the musupial species die out, a great number of them.

And the rest of mammals keep evolving, diversifying. Pescental mammals, you keep being born and diverse. So can you just tell us a bit more about that? Well, it's to do with the breakup of Pangaea and this separation of the continents. The reason you have so many different flavors of mammal, I mean, of the marsupial variety and the placental variety, is because you had lots of island-coms,

continents separated. And each of those island continents effectively had the same variety of ecological niches, and they were colonized by equivalents, marsupial equivalents, and placental equivalents. You have, like, a smilusil, the marsupial saber-tooth cat. You have Smilodon, which is the placental equivalent. They're doing exactly the same things and exactly the same niches. And the marsupials were actually doing very, very well indeed. Except, for example, in the oligocene, which is about 30 million years ago,

the rainforest in Australia started to contract. And that contraction wiped out most of the ecological niches that the marsupials were occupying, which is why you don't see the marsupial lion anymore. Things like this, they became extinct. Through no fault of their own, they weren't badly adapted, they weren't second-class citizens, it's just the luck of the evolutionary draw.

Now, it didn't happen like that for the placental mammals in the northern hemisphere. And so they continued radiating. And they do have this more efficient system, which it's more efficient in the sense that in a cooling world, it's better to keep your young indoors for as long as possible. And so the placentals, if you like, were pre-adapted for what Jane's just been describing. This slide into the great ice-a-eatres. of the late Cinesac.

Steve Jones, how important were big brains to the evolution of mammals, and how did they get these big brains? Oh, I think the answer is clearly very important. I mean, the story of the mammals is, thank goodness, the story of the triumph of the nerds. You know, it's the smart ones that won. And that actually goes back a long way.

If you look at some of the fossil endocasts, as they're called, the sort of inside of the skull, the very early mammals, 200 million years ago, a bit less, you can actually see already that they've got really quite folded. Their brains aren't like huge, but they're pretty folded. And of course a folded brain has got more surface area, more cells in it than a smooth brain.

You can even go further, and some of these early insectivores from those times, the part of the brain that we know is associated with smell and taste is relatively large. And amazingly enough, it was discovered a couple of years ago, that the largest family of genes in our genome and the mammal genome as a whole, the largest group of genes is associated with taste and smell. So we can link these ancient fossil brains with the most recent molecular biology to suggest that indeed our ability to do something

which might seem simple to pick and choose among different foods began at the beginning and maybe has helped drive our evolution ever since. That fits very nicely actually with what I was saying at the beginning about teeth. I mean teeth may sound boring but if you elaborate them and you make your jaw capable of handling a variety of foodstuffs, then you will be able to exploit opportunities which are not available to say the reptiles.

Yes, if you remember, when we talked about the Cretaceous and the rainforest, one big event in terms of the vegetation in the Cretaceous was the evolution of angiosperms, the flowering plants. And that did change the vegetation radically. With that, you've got lots more fruits, seeds, nuts, a much sort of juicier, richer food for all these small rodents and they adapted to that. They ate more of the insects that lived on all these flowering plants,

all the flowers that were around. So there was a much better diet for them at that point. Let's talk about the sad fate of the marsupials. I mean, what's killed off many of these things were cats, right? And cats are opportunist feeders. I mean, you can have a cat, which is your pet cat, which is your pet cat, he'll only eat one brand of cat food. But if it gets hungry, it'll eat almost anything.

and that's what makes them so dangerous to specialist creatures because once they've eaten the last of one species they can move to another one. And we are ourselves, the ultimate opportunists. We can choose the climate we live in, for example. And we're omnivores as well. And we're omnivores. Your listeners may not know this,

but we're actually all wearing clothes in the studio here. And that's a statement that we're taking the tropical climate with us. And so we really show the importance of climate more than almost any other creature. Can we just have the last minute little so to talk about the brain issue. Is there any notion in any of your studies about it in terms of it? What started it and how it developed?

It's too much to get in this last minute to do, isn't it? But just have a crack. Have you any ideas about why the brain got so big, so comparatively fast? I think that obviously the placental mammals, by exploiting the placenta, were able to complexify the brain in the womb. In fact, one of the problems that huge, humans have at the moment, which is why we have such a long postpartum maturation, is because

if we were left in the womb, the brain would become so large that the skull would not fit out through the birth canal. So I think the increase in the placental oxygen supplies had a big impact on the development of the brain. Steve, do you think the development of the evolution of the mammals was inevitable? No, I don't think anything in evolution is inevitable. In fact, if there's one word in evolution, it's inevitable. Steve Gould, when he used to preach from his Olympus of oddundity,

was always going on about you couldn't predict what was going to happen next. He pointed out, and he's right, that actually we don't live in the age of the mammals, we live in the age of bacteria, as we always did. As we always have to ask you. Well, I should say that's another programme. Right, well, thanks Jane Francis,

Steve Jones and Richard Caulfield. Thank you very much. Bacteria is a good idea, Steve. But we'll have to wait for that. Next week we'll be talking about the surprising philosophy of cynicism. We hope you've enjoyed this Radio 4 podcast. You can find hundreds of other programmes about history, science and philosophy at BBC.com.com. UK forward slash radio 4.