In Our Time - The Cambrian Period

Episode Date: February 17, 2005

Melvyn Bragg and guests discuss the Cambrian period when there was an explosion of life on Earth. In the Selkirk Mountains of British Columbia in Canada, there is an outcrop of limestone shot through with a seam of fine dark shale. A sudden mudslide into shallow water some 550 million years ago means that a startling array of wonderful organisms has been preserved within it. Wide eyed creatures with tentacles below and spines on their backs, things like flattened rolls of carpet with a set of teeth at one end, squids with big lobster-like arms. There are thousands of them and they seem to testify to a time when evolution took a leap and life on this planet suddenly went from being small, simple and fairly rare to being large, complex, numerous and dizzyingly diverse. It happened in the Cambrian Period and it's known as the Cambrian Explosion.But if this is the great crucible of life on Earth, what could have caused it? How do the strange creatures relate to life as we see it now? And what does the Cambrian Explosion tell us about the nature of evolution?With Simon Conway Morris, Professor of Evolutionary Palaeobiology, Cambridge University; Richard Corfield, Visiting Senior Lecturer at the Centre for Earth, Planetary, Space and Astronomical Research, Open University; Jane Francis, Professor of Palaeoclimatology, 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, in the Selkirk Mountains of British Columbia in Canada, there's an outcrop of limestone shot through with the seam of fine, dark shale. A sudden mudslide into shallow water some 550 million years ago

means that a startling array of wonderful organisms has been preserved within it. Wide-eyed creatures with tentacles below and spines on their backs. Things like flattened rows of carpet with a set of teeth at one end, squids with big lobster-like arms. There are thousands of them. And they seem to testify to a time when evolution took a leap and life on this planet suddenly went from being small, simple and fairly rare

to being large, complex, numerous and dizzyingly diverse. It happened in the Cambrian period, and it's known as the Cambrian explosion. But if this is the great crucible of life on earth, what could have caused it? And how do the strange creatures relate to life as we see it now? And what does the Cambrian explosion tell us about the nature of evolution?

With me to discuss the Cambrian explosion in the context of evolutionary science. He's Simon Conway Morris, Professor of Evolutionary Paleobiology at Cambridge University. Jane Francis, Professor of Paleoclamatology at the University of Leeds, and Richard Corfield, visiting lecturer at the Centre for Earth, Planetary Space and Astronomical Research at the Open University.

Simon Coghawain Morris, you were part of the team that made great discoveries in the Burgess Shale in the 70s and 80s. Can you describe the slice of life you found there and how many and how different were the organisms you find? First of all, perhaps you could tell us all what it's like. Is it a big open quarry? What's it like? No, it's a small quarry and it's set in the Canadian Rockets. So every time you look in any direction, you have the most fabulous views. glaciers and mountains and beautiful sky, sometimes full of snow, even in August, mind you. And on the side of one of the mountains, there's an excavation which has been going on now for many years,

and it's a layer of shale, which was once mud on the seafloor, and it's now been elevated up by mountain-building processes into the Canadian rockies. So it's actually very ancient, it's around about half a billion years old. And within this shale, there are some ordinary fossils, things like trilobites and shellfish, like brachia pods. but they're also fabulous soft-bodied fossils, just amazing things. And it was discovered actually before the First World War by a very distinguished American, a chap called Charles Walcott, who worked in the Smithsonian Institution. And he knew he'd stumbled on something totally remarkable.

But I don't think anybody quite realized how remarkable it was until we started work on it in the late 60s in Cambridge, mostly, under Harry Whittington, a wonderful man still alive. And the fossils are fabulous beyond belief. They're exquisitely preserved. And the way I look at it these days is to imagine almost we have a time machine or if you like one of those trawlers you see in the North Sea

and we're lowering the net and we bring up organism after organism. And some of the things we see are actually relatively familiar, at least to the specialist zoologist, worms, one sort of the other. But other things, as you say, carpet with a teeth at one end,

are absolutely ludicrous. Or so they appear. They're very, very strange. Or so they appear. And really the struggle has been to make sense to them. because after all, evolution is true, just as the sky is blue. They've got to be related to something.

They've got to be, in some sense, the ancestor of something. They've got to be the descendant of something. But when you have these very strange-looking fossils, some of which did look very odd indeed. Then we had a bit of a struggle. Basically, the last 30 years has been making loads of mistakes and getting a few things right. But much more interestingly, we now have some serious problems,

not with evolution, but what are the implications? because, after all, all these amazing animals, where did they all come from? But what we have, then, is the sea bottom has been heaved up to the top of the rockies, and because it slid when it did, it is wonderfully well preserved, as it were, at the bottom of an Irish bog or something like that, bottom of a peat bog. Yes, in essence, we don't actually know precisely why these fossils are so fabulously well preserved. What we do know is that the environment into which they were transported, They were living on the seafloor, and suddenly they went, whoa, hang on a minute,

and everyone's going rumbling down slope, and it's very turbulent. And then the sediment rapidly settles out. They're buried very quickly. They're buried in all sorts of orientations. And there's no oxygen there. But that doesn't really explain their special preservation. There must be something else, and people think maybe bacteria have some role in this. Why is it called an explosion?

Can you set it in the context of geology? I mean, we're talking about a massive example of a diversity of life here, But life has been going on for billions of years before then, almost since the planet began, say almost up to four billion years ago, and only half a billion years ago we have this immense explosion. So in the context of four billion years of geology, is it going, budo, budu, boom, but boom, but then wham,

it goes off the Richter's sky, and then it goes down again. Well, it doesn't go so much bum-de-bum-bum-de-bum, it goes, bang, like that. And really what is the question is, what is that bang all about? after that there's a question, does it then gently go into sort of, you know, a senile decrepitude? Like us you mean? Well, welcome to the 20th century, or 21st century,

or as it actually then it starts a whole set of new motors, which then start to go to places like, let's go on to land, what's that like, let's go into the air, what fun, let's develop intelligence. But the seeds of all that, I think, are in this so-called Cambrian explosion. It's a very interesting event. Richard Corfield, just because the sudden plethora of evidence doesn't necessarily mean there's a sudden emergence

of rich and diverse forms of life. So could you put those two together? There is this massive evidence turns up. What do you see as the first implications of that? Well, it seems to me that the central question of the Burgess Shale, one which I think has now been answered, is whether or not it's an artifact of preservation

or whether or not it's something real and to do with evolution. In other words, whether or not fossils appeared because they could become fossils because they had hard parts or whatever or whether in fact evolution did do something spectacular 543 million years ago and did it incredibly quickly

and the key to understanding that I think is to do with the sub-discipline of geology known as stratigraphy stratigraphy is effectively understanding the layering of rocks and the way things are superimposed on top of each other in other words younger time is on top to a first approximation rocks, of course, can be folded. And also sedimentary rocks can often be missing so that you have hiatuses between what appear to be continuous strata.

And so if this was an isolated instance, you might decide that the Cambrian explosion is simply there because of a hiatus and sedimentation suddenly started again, and so what we're seeing really is an artifact. but the stratigraphy is effectively about integration. It's about integration in time and also in space so that you can go around the world and find stratigraphic sections in different rock types

and that's a key feature, sandstones, mudstones, limestones. Which can, when put together, when integrated, will give you a continuous picture of what was happening in the world at that time. and when you do that, eventually you can become very sure that you're not missing any time and that you do have a clear picture of the passage of geological time. And that's where we are now. Sorry. No, you please finish.

I think it's very clear that the Cambrian explosion is a real feature of the fossil record. It's not an artifact. Evolution truly did something spectacular at that time. The question is, of course, why it did it so quickly? So we're not talking about just a sudden appearance of evidence. We're talking about a sudden, if we can use sudden in the terms of it took about 10 million years or maybe 15 million years and it happened half a billion years ago. We're talking about a sudden expression of mass diversity of life.

That's what you seem to say, which we are agreed to be talking about now. There was a time at the top of the pre-Cambrian when you did not have this diverse fauna. It had hard, part of the fauna was Shelley, and that came in quite. quickly. But as Simon's just said, what you see when you look at the Burgess Show, which actually is in the middle Cambrian, is that you have a very diverse, soft-bodied fauna as well. Can you give us some idea of what happened? Simon described it as a small hum, and then he gave a large bang, which took our sound engineer very much by surprise. But without doing that, can you just tell us what the small home was before,

that three and a half billion years before we have the Cambrian explosion? Well, since the end of the Hadean bombardment, say, at 3.9 billion years before present, the earth was relatively young at that time. Life had been all about bacteria. In fact, it's famously known as the age of bacteria, from about 3.9 billion years before present until effectively the end of the neoprotozoic, which is 600 million years before present. So to put that on a 24-hour clock to help orientate ourselves in time, life got going at about three minutes past 12 in the morning and then things suddenly started to happen at about the time that you put the kids to bed in the evening at about quarter to nine.

And the Cambrian explosion is more or less at the time of the 9 o'clock news. And on that kind of time scale, that 24-hour clock, what's really interesting about the Cameron explosion is that it took only five minutes. It took the age of bacteria, whatever it was, several hours to get that far. and then in five minutes you have this extraordinary explosion of complicated animals.

Which takes me to Jane Francis, and why do you think was this, as Richard has described it, comparatively, very, very sudden emergence at that time of this huge diversity of life? Well, the current idea is that there was a big climate event that really changed the earth. And we actually have to go back a bit further in time in the pre-Cambrian to about 700 million years. so we're dealing with some time before the bird of shell in the explosion of life. And the big theory at the moment is that the Earth was completely covered in ice, the snowball Earth hypothesis. And if you look on...

Several kilometres thick. I mean, we're talking about ice that is... Well, this is still debatable. There's a lot of controversy and a lot of debate about this. But if you go to nearly every continent on the planet today, it has a core of these pre-Cambrian rocks. And if you look at the rocks, a lot of the rocks in that sequence are glacial. or tillites, they're rocks that are formed under the presence of glaciers.

So intuitively you think, okay, well, that continent in the pre-Cambrian was probably over the polar regions, maybe the South Pole or the North Pole, and that's where you'd expect glaciers and the tropical regions would be warm. But when geologists have looked at these rocks and they've used a technique which looks at the fossil

magnetism in the rock to work out the latitude at which these continents were sitting, they've discovered that they were actually situated over the equator. So it's completely counterintuitive to present day. So this pose a really big problem for geologists and got their minds going

completely. And if you start thinking about a continent over the equator, which would have been warm in the pre-Cambrian, or warmer than the poles, and then you start putting glaciers on there, then intuitively you have to freeze the earth. If you've got ice on the equator, you must have ice

at the poles. And the idea is that the whole earth was, at some time, maybe several times in the pre-Cambrian, covered by this white ball of shiny white ice. So we have the snowballed earth. Now then that's the earth. What about the seas? Were they frozen too? Well, we don't have much evidence of the actual seas themselves, but people do think that there was a covering of sea ice on the sea as well. So if you go to the extremes of the snowball earth theory, we have the continents covered by ice and the sea as well. So completely encased in ice. So that's then, let's say, just for ease, I mean, that these numbers are kind of.

to most normal people who are not geologists like you through. They're ridiculous anyway, but never mind. 700 million years ago, we got snowballed earth. That would have been a problem, Bukkiota, wouldn't it? Anyway, snowballed earth. And as a result of that changing, the theory is because of the big climate change from that to a much, much warmer Earth, this happened.

Can you take us through that stage, which is the important stage, isn't it? Yes. We've got snowballed, then we've got 150, 200 million years there. The Cambrian explosion. Right. So what happened? How did it melt? What was in the melting that made the difference? Well, if we assume that the earth was covered in ice,

and that's still debatable in them, there is an ice... Let's assume it was a white ball of ice. Somehow you've got to melt the ice. And the idea is that plate tectonics were still operating, and there were volcanoes, and they were still producing carbon dioxide. And that built up really quickly in the atmosphere, because the interesting thing is that on top of the glacial rocks, you see limestones.

and limestones are typically formed in very warm water, like the Bahamas or the Great Barrier Reef. And so we went from this dramatic change from an ice house into an ultra-greenhouse, if you like, with tropical seas at the equator. So there was a really big change in environments very quickly, relatively quickly in geological terms. And the idea is that that caused a really big stress on Earth

and that stress called the evolution of life. Right. I'd like to ask Anna Richard briefly, Do you think that the reason Jane's given is a sufficient explanation? I have a few problems. Partly, some people say this is slush ball, not snowball. It might have been soft around the edges. And more importantly, it's a heck of a long time before the Cambrian explosion gets going.

We know in between there's another set of fossils which are just about as strange as anything you find in the Burgess Shale. And if my mind goes sideways with some of the animals I worked on from Burgess Shale and China, they are completely rotating when it comes looking at these other fossils. So actually we have a stage of problems, but the difficulty also is if you look at the sediments deposited when snowballed earth was active,

the organisms there, they're not terribly interesting. They're microbes with apology to microbiologists, but they're okay. They don't mind too much. It's a bit tough, but then life is used to going through times, which are a bit tough. So it's a beautiful idea. There's an element of truth in it,

but I don't think it quite adds up. There's too long a gap. Richard? I certainly agree that there is a big gap between the end of phase two of the snowball earth, which is 580 million years before present. I mean, that's one estimate. Of course, the estimates are always based on radiometric dates,

and there's an error bar associated with that. And as Simon correctly says, there is a fauna which falls in between the end of the so-called snowball earth and the Cambrian explosion, and that's the Ediacaran fauna of Australia. And that was discovered by a uranium germ. strangely enough called Reg Sprigg, a good Australian name, in 1947. And he was prospecting for uranium and he found these extraordinary impressions in the rock.

And they were unlike anything which had been seen before. They're kind of discs and quilts is the easiest way to summarize them. And they're found in a rock which you would not normally expect to have impressions of soft-bodied fossils like that, which are sandstones. and then only 10 years later, an example of this Ediacaran fauna, as it came to be known, was found in a very unlikely and unprepresessing place, which is Leicestershire. And so at the other end of the earth, you have the same type of fossil. And then other examples have been found, for example, in Newfoundland. And so there appears to be this Ediacaran fauna sandwiched somewhere between the end of the snowball earth

and the Cambrian explosion proper. But Simon, let's come to the Cameron explosion now. we have this snowball earth, which Jane said it is not, you're not absolutely certain that it is, but I'm not going to be a total devotee to snowball earth. And you've introduced a bit of slush into the argument, Simon, so that's okay. And you've taken us to Australia, so have it intermediate between whether it was snow or slush and the Camberon explosion.

We get to the Cameroon explosion, and you've talked about the hum and the bang. Now, what other theories are for the cause of it? Because you don't seem the three of you are certain as to, as to, you're not going to plump for acorn. Somebody has suggested, let me have a look here. Yeah. Andrew Parker, he said that the development of the eye was, in your court now, son. Thank you very much, and I won't plop it back immediately. Andrew Parker's ideas are very ingenious in as much as they argue that actually inventing vision,

suddenly, you know, in the way that those of us who are lucky enough to have good vision, and imagine what it might be like to be blind, suddenly think, good heavens, the world's around us. There's a problem with it, really, in as much as, first of all, animals are very good at detecting their environment in lots of ways through electrical fields, through echolocation, through chemistry, and those sorts of things. And the second thing with Andrew's argument is he regards it as being extremely abrupt.

He more or less writes in his book, in the blink of an eye, I think it's called, you know, one day a trilobite suddenly saw. Well, trilobites are a little late in the camera explosion, and we know through good Darwinian principles that the evolution of the eye, It's a gradual process. And as Richard Dawkins, I think, has rightly emphasized, a 5% vision is a heck a lot better than no percent vision. So it's a nice idea.

We don't know, but I don't mean that's because we're sort of silly and ignorant. It's really intriguing. You can look at all evolution. Is it the outside world which drives you? So some people have argued that actually the levels of oxygen in the atmosphere which we know do fluctuate may have gone suddenly up. And a lot of oxygen generally up to a certain level is rather good news. You can be more active.

I don't buy into that. The other possibilities, actually it's an internal system, but it might be if you just invent something trivial, like eating more effectively, if you go towards predation, or if you have particular genetic mechanisms which allow you to reorganise your body, in a way perhaps you want to divide it into a series of what are called segments. You repeat them, then you can begin to specialize.

So there's really a dilemma between extrinsic forcing factors, Jane Snowball Earth or Simon Slushball Earth, for example, or intrinsic factors to do with ecology, in part, and especially developmental genetics. Jane, what about the emergence here in the Persia shale, and we also see it in China and in Greenland, which we're discovered later, of hard body parts of shells and skeletons?

This is coming in, as I understand it, for the first time, and this is a big factor. We've just got to keep remembering that this is massive diversity, this is a huge, huge leap, and there seem to be things, living things there, which have not continued through now, they may or may have not.

We're talking about something very big, but we are talking about hard shells, skeletons, things you can pick up, people pick up on beaches and buying shops still all over the place now, now then. Well, this was a fundamental change. After the greenhouse earth or slush-ball earth,

we know that the continents moved apart, so they were lumped together as a big continent at one point, and then they started to move apart. And as the continents moved apart, the sea levels rose, around the edge of all these continents and they made shallow shelves, nice areas for animals to live.

So you increase the area for animals to live and you can increase the diversity of animals. But also at that time, or just a bit later, actually, we do see the sudden appearance of shelly fossils and shells and hard parts. So the Ediacaran fauna that Richard mentioned is a soft-bodied fauna. And it's really amazing that we do find fossils of these.

If you imagine a load of lilos or air mattresses suddenly being buried on the beach, you really wouldn't expect to see much sign of them millions of years later. But that's essentially what they are, all soft-bodied parts. We can't find any hard shelly parts to them. But then just over the edge of the Cambrian is sort of about 550-45 million years ago, there is this zone where we find these tiny, tiny little shelly, shelly creatures. And it's almost like some small, soft-bodied animals and they've decided they're going to form shells. maybe the chemistry of the seawater has changed but they suddenly start forming armour around themselves

and that takes us into whole new real. I mean you've said they decided it. You don't really mean that. Well, there's a big question. What happened that the shells came about it? You're handing over to Richard. I thought he's handed, he was very, very frightened.

So what happened? Well, the first thing is that I mean, not much need have happened except that there is evidence in the latest pre-Cambrian that things were experimenting with a creating rock particles around themselves or sponge particles. Just a second, hold on. I'm probably maybe interrupting wrong here,

but you sound as if these sort of Lilo creatures are sitting around like the three of you in a laboratory designed to experiment with things. The Lilo creatures have gone. What's happened? You've now have nothing. You're in a dead zone. This is still half a billion years ago.

Right. But the gap of time we're talking about between the Lilo creatures and the small Shelley fauna is just 10 million years. if that. I know it sounds as though we're speaking in a laboratory, but what we're actually saying is that evolution occurs by throwing up variation randomly.

Okay, I mean, to put it technically, what's happening is that nucleotide sequences are being rearranged on chromosomes, and that happens all the time. That is a source of variation in the fossil record. And so if one of these were to throw up, an organism which secreted hard parts. The implications of that are profound and far-reaching. First of all, it gives you some protection from predation.

The other thing is it means that you're suddenly able to keep your delicate reproductive organs somewhere where they can't get hurt. The other thing is that you're suddenly able to filter feed in such a way that you don't clog up your feeding apparatus. So what I'm saying here is that a relatively small genetic change, suddenly is amplified by its value in terms of survival. And that particular STEM group would suddenly diversify into lots and lots of other STEM groups,

all sharing or having suddenly noticed the benefit of having hard parts. There's a spin on that as well in as much as often in evolution. You see what we call co-option. There's actually a seminar yesterday in Cambridge. And the person was pointing out that if you want to make a hard part in a sponge, which we generally regard as the most primitive animals, you use a particular protein. That protein is also found in us,

and it does something completely different. So a really interesting aspect of this, is we suspect a good part of what you need is there ready and waiting. And the crucial thing is, how does the jigsawls suddenly come together? In other words, most of the building blocks actually evolved not millions of years ago,

but billions of years ago. So you have a sort of inherent complexity, and something has to push it over the tilt, and it's interesting. But when you... Sorry, Jane, you and... say something, I want to go back to Simon. I'm just going to say once you've evolved the ability to make hard

parts, not only are you protected against predators but you can be a predator yourself and if you can go out with this armour around you then anything that is soft like a lilo, you can just a big chomp and that's it they're gone. So... Sound of hissing air. Yes. But when you're looking when you're working for those years over in the

Berger's cell sign and you're there, you're seeing that there are records not just of hard skeletal, hard shell and skeleton fossils, but of the soft imprints as well. What What do you draw from that? Are there fewer soft ones? I'm just trying to get it. We're still trying to get it. Why did it happen now? We'll move on in a minute or two, because you're giving us everything from snowballs to climate to this, right.

Well, the one thing is it's clear from the Birges Shale, which we think is otherwise representative, because we find very similar faunas around the world, is actually the number of animals with hard parts was pretty small. Most guys are soft body, but that's as true then as it is today. The real question is, how do you get around, how do you effectively walk more efficiently, how do you hunt more efficiently, how do you see more efficiently? And that's all part and parcel of the Cameron explosion. So one's still driven back to the fundamental question. Do the Lilo crowd, our Ediacrans, have anything really to tell us about the Cameron explosion?

And second of all, if they don't, and that's my suspicion, in fact, then we've got to have something which is coming together in a very sort of specific way. But it's also worth remembering that evolution, of course, does this sort of experimentation, if that's the right word, and I'm a bit cautious about that again and again and again. I mean, for example, Jane is an expert on fossil plants, and when you see all those flowering plants we're familiar with, actually they had a rather similar explosion in the time of the dinosaurs. Richard, you want to go.

Just pursuing this idea of the possible causes for the Cambrian explosion, one of the exciting thing that's happened recently is that paleontology has got into bed with developmental genetics. And one of the things that the geneticists have found is that there is a complex of genes, called hox genes. And these are common from everything

from a fruit fly to a mouse to a cat to a human. And these seem to be like the coarse control on an amplifier. If you tweak it a little bit you can suddenly increase the volume very much. If you fiddle around with hox genes,

you can put legs where you would have antennae, you can have an extra pair of wings, you can end up with something out of a David Cronenberg movie very easily. And the fact that hox genes are common to this entire group called the bilateria and can be traced to an origination date of around about the time of the Cambrian explosion

suggests to me that all of a sudden evolution had access to the course control of genetic variability. Before you go along with that, can you talk about... Will I go along with it? Hold on. You will not have to say. I will reply, yes. But your question first. No, no, reply first and I'll ask my next question.

That's absolutely correct. I mean, we do have a fundamental genetic architecture, which astonishingly is the same in the fly on the window as it is in ourselves. But are you suggesting that came in then? Yes, it must have, yes, because it's something we see in all these different groups of animals. But on the other hand, what is not quite clear is the extent to which the genetics is really a fast-track process to make an embryo, and the extent to which actually the morphology changes, and then the genes look over their metaphorical shoulder and say,

hang on a moment, we've got to do this in good order because actually evolution is certainly genetic, but it's much more than that. And there's a huge amount of the sheer complexity about the way the embryo develops is more than genetic. So I'm not trying to dodge the question, but we are really on the edge of things

we hardly begin to understand. Jane, do you think that a lot of what we can see in the Burgess Sale are dead ends, that could be called dead end, sort of evolutionary wastage, is that peculiar to that time, because we've been talked, one hears about the weird and wonderful creatures, which Simon can perhaps tell us a little bit more about.

But does it still go on now? Is that possible? I'm sure if you trace some of the lineages of fossils, we will come to lots of dead ends, and there are dead ends all over the fossil record. I mean, in the plant record that I'm familiar with, you know, we know there are plants evolved, and then they died off. Often it's an environmental cause that's killed them rather than something that just waste away.

So, yes, I think this is common in the first. fossil record, but it's certainly in those older times. Although I'm a bit, although Richard is fairly sure we know all the record as a field geologist, I think that if we keep looking in future years and we really have go on the hunt

and we really have a feeling for what we're looking for, then I think if we came back, say even in five, ten years you might have a different story here. A different story about what? About the kind of fossils that we found in that time and what they tell us about this pathway of evolution. So a dead end

that we think is a dead end now might not be a dead end when we've done more, or you, not. People like you have done more work. Yeah, well, and dead ends and certainly in terms of origins as well, you know, this sudden explosion. You know, they've been remarkable finds over the last few years and we know that...

Well, we found more of the Ediacra fauna and more of the Burgess Shell fauna. If you read books that written probably 10 years ago, you'll see that they just write about one fauna from one continent. Now we know that Ediacra is found, as Richard said, on several continents

and they span about 15 million years. It's a terribly difficult time period to work in because we're going so far back when we're talking nearly a billion years. It's really difficult to date those rocks and there aren't that many of them compared to what's around now. So in a small piece of rock

we're looking at a huge time slice. But the fact of the matter is that to the first approximation and Jane is absolutely correct about the incompleteness of the sedimentary record but I go to China, I go to Australia, I go to Greenland, I see the same things. So there's definitely a signal there. It's not just a distortion, it's not just an artifact.

Well, can we try to put it in some sort of evolutionary context for the last third of this program and start with the great man, Darwin, who was aware of the Cambrian work? And he said, to the question why we do not find rich fossiliferous deposits belonging to these assumed earlier periods prior to the Camberian system, I can give no satisfactory answer. The difficulty of assigning any good reasons for the absence of vast powers of strata rich in fossils beneath the camera system is very great. Now, what did that bring to bear, just an opening to say,

what did that bring to bear on the evolutionary argument? Well, the evolutionary problem for Darwin was that he thought that, excuse me, that evolution should be effectively a gradual process and therefore there shouldn't be this sort of sudden eruption of forms. But what we now know about evolution, in fact, is basically two things. First of all, again and again,

we see that when groups of organisms decide to do something, a lot of different groups, at about the same time. And there is a clear sense, though this is unfashionable in some areas, that actually through Earth history, there is a degree of progress. And this is something Darwin is actually very comfortable with, unlike some modern biologists. And in link to that, of course, it goes back to the things which Darwin couldn't possibly know. He was a genius. He's, I think, still in many ways, the best biologists who's ever lived, even from today. But he didn't know about genetics. He didn't know about genetic inheritance. He didn't know about the depths of

Earth history. So it's not to dodge these questions, but the fact of the matter is, that, indeed Jane is absolutely correct, when we meet again in five years' time, then we will have ten times more to say. But to the first approximation, what we must always remember, that we're scientists, which means two things, we're usually wrong,

but second of all, we have hypotheses. And in the case of the Burgessale, what we thought, what I thought was completely weird, is now actually falling into a coherent phylogenetic story. And the problem is, I can get it down to the base of the Cambrian. But after that, I use Letterset, I'm very old-fashioned, full of question marks and it's almost run out.

So Richard, can you tell us what Darwin's recognition of this intimates about evolution? Was he very worried about his evolutionary theory, the theory of gradual evolution? If we go back to the very beginning of the programme, Simon said, you know, for, I'm losing myself in all these numbers,

but from four billion years ago to half a billion years ago, it was a low hum, and then there was this colossal bang. that doesn't sound like gradual evolution, does it? Was that what bothered Darwin, who believed in gradual evolution? Well, Thomas Henry Huxley, who is a great hero of mine, and Darwin, he was Darwin's Bulldogs, famously known as it,

said Natura non-facet, Sultum, you know, you are unnecessarily hobbling yourself by holding to this idea, Charlie. And that's absolutely correct. It was very fashionable as part of the society of the Victorian era and part of their thinking to think that everything would have to happen slowly and gradually.

But in 1972, Steve Gould and Niles Eldridge came up with another version of evolutionary theory. It's an elaboration on it rather than another version, I should say, which showed how evolution could occur very quickly.

And the essence of the idea is that instead of having to wait millennia for infinitesimal changes to percolate through very large genetic population, If you had semi-isolated genetic populations and you had a bit of variability thrown up by a change in a hoax gene, say, then that could very rapidly spread through that population

and natural selection could act on it very quickly. And then when that population was rejoined with the rest of the population and with the rest of the world, it could take over the world relatively easily. And so with the breakup of this supercontinent at around about the end of the Precambrian, you might have had a situation where lots of... of these little isolated genetic packets could operate very quickly, and that again would give rise to the Cambrian explosion. Was this Jane Francis, was this what's the late Stephen Jaygo,

or less the late Stephen Jaygo, called punctuated equilibrium? Yes. And can you just unravel that a bit more? I mean, Richard's gone down that path. Can you take it, can you amplify that, really? Punctuated equilibrium is the idea that things evolved in jumps, and it wasn't a sort of a steady flow, and something dramatic would happen,

and then there will be some stability for a while and then something else dramatic would happen. But I'd just like to go back to something that Simon referred to earlier about the origin of the plants, of the origin of the flowering plants, because I think this is a kind of signal that we see several times in the fossil record. And the flowering plants originated in the mid-Cretations about 100 million years ago. So it's a much younger period of rocks and we have good fossils there, so we can study things in a lot more detail.

And we've been able to work out that these have, we can see the early, early answer. of them and then we have a very good record. But the interesting thing is we don't see what we think is the very early, early flowering plants. You know, there's a big gap and we have the suspicions that there may be 20, 30, 40 million years when the flowering plants evolved and we just have no evidence of them. But that's a bit similar to the problem of the Cambrian explosion itself, is that suddenly it's there and you know it must have been there before.

And so part of it is the evolution of an innovation. which allowed diversification, but that won't have come immediately out of nowhere. And here, with the pre-Cambor and Canberra and Boundary Problem, you have a dichotomy between what the fossil record shows and what you can do by using so-called molecular clocks, which is using first punctuate equilibrium, now molecular clocks. Well, there's nothing wrong with that.

Heady days, Watson, heady days. Well, molecular clocks, effectively an alternative sort of view of the history of life. view, but please continue. The reconciliation of them, I think, is one of the major problems facing Cambrian explosion work today. 1996, some people suggested

that the roots of the Cambrian explosion were twice as old as the fossil record shows. Now, one of the interesting things about work which has happened subsequently using invertebrate genes, i.e. what I think of as bug genes, as opposed to vertebrate genes, which I

think of as proper animal genes, is that there is now a convergence of molecular data, molecular clock data, and fossil evidence, which is converging on this amazing date of 550 million years before present. It's sort of converging, but yeah, I think it's a really interesting question because... Because this is the argument that you had, Simon, with Stephen J. Gould, about punctuated equilibrium. This is a very big argument. So can you tell us how you took exception to why and how you took exception to what he said? Well, I don't buy into punctuated equilibria per se the idea you have rapid times of change over perhaps a few thousand years and in times of stasis.

Is that saying anything which we didn't know? I don't think it is. Gould was very interested in the stabilisation. He, I think, took almost a Marxist-Lennon this view of this, and as much you have times, well, it's a Hagellian argument at the end, where you have arguments against each other. It leads to an antithesis, and then the synthesis, the thing stabilises. And he thought that the genomic architecture sort of locked into position. and then somehow to be disrupted by a metaphorical revolution. Well, we now know that's not the case.

Genomics are much, much more dynamic and much, much more flexible than that. If you can then extrapolate that up into the Cameron explosion, irrespective whether you buy into Stephen Jay Gould's particular argument, I'm not so sure because that's really talking about the origin of species, which in the end is the origin of everything. But what we have here is a whole world, which has been just radically transformed.

But maybe there's an analogy, maybe. I have a time machine again. I go back 500,000 years, not 500 million years, and I meet a group of bipeds. Very interesting. Some close, a tiny bit of fire, social organisation. What are they going to do next? I never say, no, they're just another group of primates. Hang on a moment.

But in the seeds of that early hominid experimentation, we have basically the fact we can sit in a studio and talk about it. All you need to be intelligent, go to the moon and talk on the radio. Actually, I am sure, around that campfire, 500,000 years. years ago. And I think there's a useful analogy there as to trying to identify, tease out exactly what these roots are. So I mean, with great respect to Gould, the argument has moved on, and with molecular clocks,

the great thing there is actually the organisms are realities. They obey basic rules, and we now know, very recently, and in point of fact, the molecular clock, which is basically how fast you substitute, say, part of a protein structure, is dependent on your

size. And this is true from a bacterium to a sperm whale, or Blue Whale. They obey the same laws. And that's really the crucial thing in my opinion. Searching for laws. Richard, can I just come back to you on this business on another thing as I understand it? Perhaps I'm getting

it wrong. It was Jay Gould. That he believed in his, the book that he used your work, Simon, and praised you to the skies and then you had a go to him. Good good old Truddin. Anyway, he thought that actually

this Camberin explosion was showed that life was profoundly accidental, that it needn't have happened if it were run the film again, there would be a different result, and we wouldn't turn up. Right, okay, well, that's the so-called Wonderful Life scenario, and it's a great book which I've enjoyed very much. But the point is it was written in 1989, and it was based on Simon and Derek Briggs' and Harry Whittington's work.

And their current view, as I understand it, of their view then, that it was quite hard to fit these phyla, these major structural blueprints, into existing filer, the filer that we know today. What's happened, of course, is that since Steve wrote that book, things have moved on, science is eternally dynamic process,

and it's now become clear that many of the animals in the Burgess Shale can be fitted into existing filer. Some of them look a little strange. In groupings, phyla's groupings. Yeah, phyla is major structural grouping, sorry. And the interesting thing about that is that some of them are a bit off the wall like hallucinia and things like that. But hallucinia is basically a lobopod, yeah?

It's a friend of the fly and the scorpion, which we should have realised when we first found it, but, you know, life is slow and graduate students are ambitious, but nevertheless, we're all the way. I think you're being a bit unkind to yourself, actually. No, no, well, I hope, I hope not. You've got it upside down, but you put it the right way up in the end, sign. Thank you. That's true science. Soon or later.

That's how it works. I just want to ask, come to the final question, because there's a lot more to say, but there's a lot more time to say it in future programs. But Jane, do you think to go back to this, because I think this will fascinate it, it fascinates me, that if what had happened, 500 million years hadn't happened,

that human life, because we're all interested in us a lot, human life would not have emerged as it has done. Well, this is a big argument, isn't it? There are two strands of thought. We've got about a minute. Well, there are two strands of thought. Either if you replay the clock again, it will never happen.

We'll just go in a completely. different way and we probably end up as maybe we end up as life but in an completely unrecognisable form and we wouldn't be sitting here discussing and some people like Simon here is a other put forward a strong argument that if we

play the clock again we must go along the same route because that's the best part that we can take similar it's absolutely inevitable in my view and the reason we now know that is if you go and look into the brain of a dolphin or you look into the brain of a crow or you look into the brain of us they do the same

things in the same way, but the brain structure is completely different. They've got to the same solution independently, and therefore these things are as inevitable as walking or flying. I think the important point here is that you need enough time to do that. I mean, I suspect that if the world haven't been hit by an asteroid

which killed off the dinosaurs, can we come back next week? We'd all be sitting here wearing scales. There's another argument. No, no. Scales, I mean, could look rather nice, really. It was good. Ties, anyway. Anyway, thank you all very much. Thanks to Jane Francis, Simon Conner-Morris and Richard Corfield.

And next week we'll be discussing Alchemy. Thanks 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. com.com.com.