In Our Time - Ediacara Biota

Episode Date: July 9, 2009

Melvyn Bragg and guests Martin Brasier, Richard Corfield and Rachel Wood discuss the Ediacara Biota, the Precambrian life forms which vanished 542 million years ago, and whose discovery proved Darwin right in a way he never imagined. Darwin was convinced that there must have been life before the Cambrian era, but he didn't think it was possible for fossils like the Ediacara to have been preserved. These sea-bed organisms were first unearthed in the 19th century, but were only recognised as Precambrian in the mid-20th century. This was an astonishing discovery. Ever since, scientists have been working to determine its significance. Were the Ediacara the earliest forms of animal life? Or were they a Darwinian dead end? Either way, it is argued, they reveal some of the secrets of the workings of evolution. Richard Corfield is Senior Lecturer in Earth Sciences at the Open University; Martin Brasier is Professor of Palaeobiology at the University of Oxford; Rachel Wood is Lecturer in Carbonate Geoscience at the University of Edinburgh.

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 four. I hope you enjoy the programme. Hello, in the 1940s, a prospector called Reginald Spryg was working in southern Australia, searching for uranium for Britain's atomic bomb project.

When he came across impressions in the rock, unlike anything he'd ever seen before, Spreek thought there might be fossils from before the Cambrian era. This was an astonishing discovery. It was widely thought that, bacteria aside, the dawn of life on earth came with the Cambrian era about 542 million years ago. Charles Darwin thought there must have been pre-Cambrian life forms,

but he never imagined anything like this. These mysterious fossils became known as the Ediacara biota. But were they the beginnings of animal life, or were they a failed experiment? and what hitherto unknown aspects of evolution have they revealed? With me to discuss the Edia Carabota are Richard Corfield, visiting senior research fellow at the University of Oxford, Martin Brazier, Professor of Paleobiology at the University of Oxford,

and Rachel Ward Lecture in Carbon Age Geoscience at the University of Edinburgh. Richard Corfield, why did people think there was no life other than bacteria until about 545 million years ago? Well, that goes back to the nature of the fossil record and the nature, the way it was perceived by 19th century geologists. It's conventional, or it has been more or less since the dawn of the science of paleontology. For paleontologists to slice up time using fossils. And the phanaozoic, the dawn, the age of visible life, which is five,

142 million years old to the present day, is conventionally subdivided on the changing succession of fossil faunas. What happens at the base of the Cambrian, which is defined after a tribe in Wales, which reflects the fact that so many 19th century, primarily British geologists were there laying the groundwork for it, is the fact that the fossils appear to be underlain by nothing. And that is in itself a reflection of the fact

that our understanding of the fossil record is highly coloured by the fact that they are shelly. They leave behind hard parts in the fossil record, which enables us to visualise them and divide up the fossil record. If you have layers of rock, and in fact 80% of Earth history is like this, with no visible fossils in it, unless you have very modern techniques,

which we now have, you're in trouble and it's almost impossible to divide geological time on the baking. of this biostrategography. And this was Darwin's big dilemma. He knew that fossils appeared suddenly in the fossil record, the shelly fossils that he could see 542 million years ago.

But for his theory to work, which was gradual evolution by natural selection, it required something to have happened before it. And he was rightly very concerned that there was no fossil record that he could see to explain the sudden appearance of fossils 542 million years ago. 42 million years ago. So you're talking about technology advancing knowledge in this sense? Right, because

I think we'll probably get to this a little bit further down the programme, but there are now genetic paleontology as opposed to classical geological paleontology, whereby we can map divergence states between major groups of animals on the

basis of the similarity or dissimilarity of their genetic composition. And of course this is a technology which would have been completely alien to Darwin. Remember that DNA on which all of this technology is based was not discovered until Watson and Cricks work in 1952 in Cambridge. So can you give us some idea of the frustration that Darwin must have felt that his grand plan, his grand idea, the great idea, didn't have, in that

sense, a root, a good enough route? Well, he was very concerned about it. I mean, he said that it's a perfectly legitimate criticism that my theory of evolution succeeds all fails on the basis of this. And he was very concerned about it. I mean, for the last 542 million years, you can see the change in succession of fossil faunas. You can see things evolve into other things, particularly in the deep sea record, for example. But for things to appear suddenly, relatively recently, 542 million years ago, was a big problem for him. And, of course, now what's happened is that we use better techniques, microcontractors,

and other techniques like that. And we've searched more of the fossil record from other areas of the world, including, as we will discuss, South Australia, Newfoundland, and found unusual areas called Legastatin, where soft-bodied fossils are found. So these Martin Brescia would seem to down to have been miraculously preserved, because, as Richard was saying,

we had the shells, which gave us the false. which gave us the evidence, which gave us the history, which gave us the means to have a theory of evolution to develop that. How were the Ediacara biota first discovered and how did the person who discovered them think, well, these are pre-Cambrian. They're fossils, but they're not shell fossils. Well, that's quite a large part of the story. In fact, we didn't really define the base of the Cambrian until 1990. So it was a bit of an open question. As we've heard, there was Victorians looking for fossils below the Cambrian back in the 1850s.

And in fact, Darwin, in the origin of species, mentions the longmind and some fossils that Salter found as possible evidence for early life. Did Salter think he'd found something pre-Cambrian? Because he took his own life, and I wondered if it was frustration and not being able to convince everybody that this revolutionary thing that he'd found was real. Well, that's certainly a fascinating thought. but it looks as if he had manic depression of some sort and so maybe he can be excused some medical grounds but at that time he thought he was looking for Cambrian materials

of various sorts. He just found something lower than the trilobites and most of the things found up until you mentioned Red Sprigg were sort of discs and blobs and things people could easily dismiss. But what Sprigg came up with in 1946 was something really really, quite dramatically different. I've got to brought a specimen in. This is our radio

perhaps not reach the heights of its communicability, but there we go. And he settled down in a little... We've got to give listeners a bit more than that. You can't pick it up and put it down and say, you're going to... If we're going to do it, you've got to do it. Yes, okay. Right.

It's... Almost this very piece Regisbury picked up while he was having his lunch in a little River Valley in the Eide Kara Sheep Station, and the valley was filled with slabs of red sandstone, just like this. And when he was sitting on the riverbed, all he could see was just ordinary sort of mottled from lichen growth and everything, but he happened to turn it over, and it was on the underside that he suddenly discovered these remarkable impressions.

This is a piece of thin sandstone if you stretch your hand about the size of a hand. Yes, that's right. And the impression on it here is probably a bit. about two inches across or so. And it's an ovate impression that looks like a very large fingerprint. It's got lots of internal detail in it, not something you could easily dismiss. He found that nearly every slab he got

had structures like this. Some were circular, some were great elongate things, and some were like these fingerprints. Some were a metre long, as I understand. Some were a metre long. And they clearly grew from very small things to the size of a pea. And it looks as if these creatures

could grow to almost an unfilfitting. amount of length. They didn't seem to have a fixed growth program. Now, why did it happen that when Sprigg got hold of this, he made it stick and brought in other people and he began to grow the examination of this, and when Salter had got hold of it in almost 100 years previously, it had come to a dead end. That's a very good question. Sprig had perhaps the Nouse of an Australian field geologist. He wasn't going to let people knock him around, and so he stood up for himself. And he showed his fast. around various meetings in Australia and he even reported them in London. He even tried to get them published by nature. But he got very short shrift for all his hard work

and they said that his fossils were of very little scientific interest. Let's go back to that extraordinary thing in your hand, which is older than 542 million years. We'd sum between 5 and 42 and 600 million years. Can you just say why, again, sorry, just to be really clear, why he thought that was particularly significant. You've described it, but people would say, well, aren't a lot of fossils quite like that? Well, I think that's what people were saying at the time.

They looked like jellyfish, but people sort of expected to see jellyfish like things in the Cambrian, and it was dismissed. And in fact, it was about another 10 years before people began to realize these could actually be really rather old. It wasn't too easy to date them in Australia. But something turned up in England in Charnwood Forest in 1957, with school boys. With schoolboys, which, and school girls, which completely changed the story.

And a couple of schoolboys were climbing at a quarry on the edge of a golf course. And they saw something like a great big fern frond impressed in volcanic ash. They called in a friend called Roger Mason, who was a keen fossil hunter. And he ran into Leicester University to report it to Trevor Ford. So the story goes, said, go away, silly schoolboy, or whatever, dismissed it. And so Roger brought in his father, and the father got Trevor to go out into the field, and there were these fossils in rocks that were thought to be much older than the Cambrian. In fact, they were thought to be a lot older than the Cambrian than we now know them to be.

They're about 30, 40 million years older. And eventually, within a few days, the director of the geological survey was out, and the whole story was out that there are actually big fossils in the pre-Camrian. and so it began to roll. Rachel Wood, can you tell us more about them? Now, when I say them, will you tell the distance what them are? Yes, the Ediacarobiota is in fact remarkably diverse. However, these body fossils, they're remarkable for being the very oldest, large, multicellular life forms that we know.

As Martin says, they range from a few millimeters or centimetres up to a meter. I think the largest form known is up to two metres long. What's remarkable about them is that they don't really resemble anything that we're familiar with living on the seafloor today or not immediately familiar with. They had no immediate means of locomotion, no limbs, no swimming appendages. They appear to have no sensory organs, no eyes and no antenna.

And as far as we can tell, they had no guts or internal organs. So these are really remarkably similar. A simple bioa. Did you say swimming on the seafloor on the seafloor? On the seafloor. They didn't move much or at all, isn't it? Some did. But they were talking about seafloor creatures.

Seafloor creatures, these are exclusively marine. If you were to snorkel over the Ediacaran sea floor, in fact you'd probably have to take a submersible down because some of these biotas were growing at considerable depths, hundreds of metres, you know, down in very, very dark waters, maybe down to a kilometre in some cases. So they were about sunlight as well? Without sunlight.

The whole ecosystem. and probably was operating without sunlight in many cases. But if you were to swim over this ancient sea floor, you'd see, as Martin said, a diversity of disks, several centimetres in diameter up to maybe 20 centimetres, wrinkled discs, flattened discs, some with radiating patterns. But the fauna is often dominated by these fern-shaped forms, like the iconic charnia, which was found in Charnwood Forest.

These appear to have a sort of hold fast and then a long stem and then a fern-like frond at the end, which could be a spindle-shaped or a comb. And these appear to be attached to the seafloor and would waft in the currents, these deep-water currents. Can you tell us what the earth was like in the period when they merged? There's a dispute whether they merged 30 or 40 years before or up to a million years before

or up to 90 million years before this Camryan explosion in 542 million years ago. But can you tell us what the Earth was like? Admittedly, vast periods, but you people deal in vast periods as if I'm catching a train in half an hour. So there you go. Can you just tell us what it was like?

Yes, the pre-Cambrian world was radically different to our world today. About 100,000 to 1,200 million years ago, we had just a single landmass called Redinia. And this straddled the equator, so there was no tropical equatorial seaway. So the whole arrangement of the oceanographic currents was very, very different to what we see in the modern Earth

where we have distributed continents in the poles and also around the equator. So the... And tectonics, in other words, plate tectonics, the way continents move around the earth, is thought to be a major driver of geological change. The way continents move around can be a driver of changes in the oceans

and indeed can be a trigger for biologically, evolution too. And at about 750 million years ago, this huge supercontinent, Radinia, started to split apart. And this seems to have triggered a number of really quite dramatic glaciation events.

There was one at about, dated at about 700 to 720 million years ago, and another one at about 600 to 625 million years ago. And these were the infamous snowball events, huge glaciation. There's undoubtedly massive

glaciation down to the tropical waters, which is of course unknown today. There was a further glaciation at about 580 million years ago, perhaps not quite so extensive. And the Edacarabioda appeared within 5 million years at the end of this glaciation. So there's some suggestion that these glaciation, so this world obviously was dramatically different, but there's a suggestion that these glaciers were either innovators of, evolutionary innovation, or they were some sort of bottleneck. It allowed the fauna to get through, but then the fauna radiated after these glaciations had gone.

Can I go back to you, Richard Corfield, to take up the story. Before the Ediacara, there were bacteria, which had been going on for nearly three, at least two and a half billion years before. Do we know any connection between the bacteria and the Ediacara before we come to the Cambrian explosion? Yes, I think so. One of the things I wanted to say is just to sort of orientate people in time on this, that the Ediacaran Biota existed for a relatively narrow interval of time, between 635 and 543 million years before present.

If you think of the history of life on Earth, 4.5 billion years as the age of the earth, to the present day, divide it into 24 hours, the idiacaran's only existed for about half an hour on that clock from 837 to 907 in the evening. And this was a very strange alien world populated by these weird organisms that we've heard about, these hubcaps, these pitter breads, and something which looked like the logo on the flag of the Isle of Man. But the connection between them and the bacteria is that bacteria,

as the late great Stephen Jay Gould used to say, are the real lords of creation. They occupied the Earth from about 3.8 billion years to when the Ediacarans took over. So a massive slice of the history of life on Earth. But then, as Rachel says, there were these very strange, very severe probably glaciation, which were glaciers extending to sea level in the tropics. So you have to imagine planet Earth swathed in white. And through all of this, the bacteria,

have been going. And at the same time, they've developed photosynthesis, probably as we've discussed on this program before, by one bacteria sidling up to another bacteria and saying, hey, would you like me to be your chloroplast? And so it gets involved with this other bacteria and then it's suddenly creating oxygen. Because for most of the history of the earth, oxygen levels were very low, say 1% or less. At about the time which immediately preceded the Ediacar and fauna, these two glaciers, these two very severe glaciation that Rachel was talking about, it appears that oxygen levels increased by this bacterial innovation from about 1% to about 21%, which is the concentration we have at the moment.

And this may also have been a trigger which resulted in this explosion of the Ediacaran fauna. And then that, in turn, was succeeded by something else, which we would recognise as animals which are clearly related to ourselves. This is the Cambrian, the Great Cambrian Explosion, which we'll have come to in a moment to do. But still, Martin, can you take that on? Yes, Richard has mentioned some of these ideas that physical external conditions drove the evolution of the Edia-Caribiota and then the Cambin explosion. But I think we need to turn the coin the other way around, upside down, if you like,

and consider whether to some extent internal causes within biological evolution were driving some of these physical changes too. One thing that you see before the snowball earth is the complex cell that Richards referred to called the eukaryote cell, which is the sort of thing you find in algae and seaweed and protozoans today.

And we have fossil evidence that that was building up in the period up to the Eidiaran explosion of its own. Certainly from about a thousand million years ago to round about 600 million years ago we get more or more evidence of large body masses of things we might loosely call algae. And that interests me because it seems to me

that the biosphere was gradually becoming more and more multicellular and multicellularity carries with it impacts on the climate and the way the earth works because it means the material will not degrade so quickly. It will therefore get buried more quickly, and therefore it could lead to more carbon dioxide getting lost into the rocks. And it seems to me an interesting possibility that biological evolution itself could have been one of the factors

that led to the snowball glaciers. And the other side of that is, as Rachel has referred to, as soon as you get the Ediacarabioda, there are never any more snowball glaciers again in the whole of Earth history, where they're quite well known in the Precambrian. Snowball glaciation being so extremely, massively, almost totally occupying planet Earth.

Completely totally, completely occupying. So what lived had to go deep down? Yes. So the Earth may have been locked in layers of sea ice that may have been kilometres, several kilometres thick. So it's very extreme, and it seems to be, to some extent, a reality. Well, the Ediacarabiota, once they appear, we never get those glacations again. We only get what we might call high latitude glaciation,

the sort of thing we see today. And it seems to me rather an odd coincidence that the Ediacarabioda evolved without knowing, if you like, there were going to be no more snowballs. And one way of looking at it is that the Edicarabayota was sort of the biosphere's response to controlling the carbon cycle in a new way. It was a revolution in the way that carbon was being processed.

Rachel. Yes, this idea of the earth oxygenating, and was it chicken or was it egg, there's some very interesting new data that suggests that this oxygenation that happened after the 580 million year glaciation, didn't happen gradually but happened in a series of steps, maybe two separate steps.

One is about 560 million years ago and another quite close to the pre-Cambrian-Cambrian boundary at about 550 or so million years ago. There's evidence that's from, this is based on geochemical evidence. And what's very curious is that these stepwise oxygenation seems to correlate with innovations within the Ediacarabioda itself. So the oldest part of Ediacara biota seemed to be dominated by these organisms that were rooted in some way to the sea floor, probably very intimately associated with a microbial mat, which I'm sure we'll come to later. The later forms possibly gained some ability to move, some motility.

And then at the very, very end of the Ediacara, just before, about five million years before the pre-Cambrian-Cambrian boundary, coincident with this sort of final oxygenation event, we have for the first time the appearance of skeletal fossils, little tiny tubes and cups, and then actually remarkably large fossils that resemble modern corals and sponges. So there may be a sense of, it seemed that there's a connection between these oxygenation events

and the evolution of the Edacar and biota. Yeah, come quickly, then I come to, Martin, and then I want to come to... Just to follow up on that. We know from some work that was done in Australia, the evolution of zooplankton, that is to say little creatures swimming in the water column, will itself have increased the oxygenation

because they gather up all the phytoplankton and all the sort of green crud that's floating around in the water, package it in faecal pellets, and it descends rapidly to the sea floor. So the water column would predictably have become much more oxygenated with innovations like the frou gut, which is one of the big things that happens in the Kemin explosion, and also with the evolution of burrowing,

because it ventilates the seafloor. So we still can't really be sure that these oxygenations are due to some external cause, or whether they're due to the actual innovations within the biosphere. My hunch is actually the latter. We're sliding around a bit here. Can I ask you just to summarise what evidence there is for the emergence of the Ediacara biota, How did they come about? What is the general view, or is there a general view?

I mean, the observation is that very, very soon after the end of this last glaciation, this last snowball earth event, you find these impressions in the rock record, which were reliably dated, as Martin says, which, by the way, is a testament to the British Geological Survey, because in fact it was because they'd mapped these rocks so well in Leicestershire that there was no longer any doubt that they were pre-Cambrian. That's right. So the observation is that they're reliably dated

as occurring immediately after this last snowball event. And for that observation to be watertight, we have to know that that snowball event finished, in a clean and completely discreet time horizon around the world. And we do know that by one of the most extraordinary pieces of good fortune in proto-ozoic geology, which is that this event is capped by a carbonate rock like a seal on top of a tomb, the so-called cap carbonates. And this is a global phenomenon which has a very distinctive chemical signature.

and so what you're seeing is that, and that's in fact why the base of the Ediacaran is defined in Australia because this cap carbonate is very well exposed close to where Reg Sprigg found the fossil like the one which Martin has brought in. And you know the Ediacaran fauna is above it but not below it. There's an ambiguity about their nature. Can we move to that? I mean, some people have thought them close to jellyfish, others to fungi. Can you develop that? Yeah, one of the strange things about the Ediacaran fauna is that there seem to be one or two representatives,

species, in very high-level taxa. And a quick scoot through our taxonomy reminds us that we have kingdoms, phyla, classes, orders, down to genuson species, Genus Homo sapiens species, us in other words. And if you think about the arthropods, there are millions probably of species within that class or order.

With the Ediacar and fauna, there might be a phylum or a class with one member in it. And so that's one species. That's one reason for suspecting that they were experimenting into this recently vacated eco-space. In other words, glaciation finished, here's an entire ocean, go and do what you like because there is no competition.

And so if you actually look at the number of niches that they radiated into, the world literally was their oyster. And a bit later on, when you get to the base of the Cambrian, that competition scenario was completely turned on its head with lots of things trying to crowd, to relatively few niches. And so the Ediacaran's had plenty of vacated ecological space. So between 40 and 90 million years, depending on your calculation,

and there's some dispute here, as I've read, these are the dominant forms. Certainly as far as we can see in the fossil record. A lot longer than we've been here. Certainly, yes, that's right. And in fact, remarkably stable for a long period, and much more stability, I think, in the range of forms than we tend to see at later times.

So Richard has sort of alluded to that. So what is that significant enough, please? Well, it sort of suggests that the nature of the surface of the planet was stable and that one possibly had a stage in which there was this sort of, as Richard put it, experimental group of organisms, and that maybe at the end of it an innovation came in which broke that dominance of some sort, I suspect it's something like the through gut or jaws or something. And then this whole ecology dissolves and disappears, in fact.

Can we turn to that? They've just emerged now. They're just about to disappear. Between 40 and after 40 or 90 million years dominating the planet. Rachel, they disappeared just before, at the time of, because of the Cambrian explosion. Can you tell us what the hypotheses are for what? why they vanish. We've had beguiling references to the got and jaws from Martin, which may have been part of the answer. Yes, I mean, there are many competing hypotheses

to explain the demise of the Ediacara. One, what's emerging in, well, has emerged in recent years, is that we almost certainly at that time, the peak-hambrian-cambrian boundary saw the very first mass extinction that the Earth suffered. There's very good evidence that, there was a massive upwelling of very low-oxygen waters, probably globally, that encroached these nice shallow tropical areas and also the deeper tropical areas where these Ediacarum biotas were living. And this possibly caused their extinction. Can you just spell that out a bit more, please?

We know from both the evidence, if you go into the field at the pre-Cambrian-Cambrian boundary, you suddenly see a shift from these nice red sandstones Red is often an indicator that the rocks are well oxygenated and they're overlaying by black shales or black mud rocks. When you come across black rocks in the geological record you realise you're probably dealing with a lot of rocks that are full of organic matter

and that seems to indicate that the organic matter hasn't decomposed because it's been deposited without the presence of oxygen. So we see this layer of black shale or organic rich rocks close to the pre-Cambrian, Camberian Browning, in many, many areas of the world, so it's probably a global phenomenon. So that's one explanation is the oxygen withdrawal. So one explanation is a sort of a,

the ocean becoming toxic in effect to these marine animals or marine biota. But there are also biological explanations. So as Martin has hinted at, there's a huge difference in the ecology of the Cambrian biota compared to the Ediacaran biota, As we've said, a lot of these Ediacarum biotas were fixed firmly to the sea floor.

Some may have been capable of mobility, but they certainly weren't voracious predators. Some talk, I mean, in a lot of you're actually saying they didn't have a mouth. That's right. They were remarkably simple. It is very difficult to understand really how they ate. I mean, a lot of how they gathered nutrients. I mean, one idea is that they imbibed nutrients.

They just absorbed nutrients through their... They have a very, very large surface air to volume ratio. They may have just absorbed nutrients or been some sort of suspension feeders. But that whole ecology really is remarkably simple compared to the Cambrian ecology where we see this explosion of forms with horrific mouth parts and grabbing pincers, claws and so on.

So they might just have eaten them up? It's quite possible. Yes, they sort of chewed them to death. That's one of the explanations. A thing you've really noticed, as you travel from the Ediacurran into the Cambrian, is how very two-dimensional the seafloor looked in the edioccurran. There was very little living below the surface. All the information was on the surface itself. And as Rachel suggested, things seemed to just be happy to sit there and slurp up, absorb the nutrients, a bit like fungi or slime moulds.

But as you go into the transition, perhaps three or four million years. before the Cambrian things start to change, there's just a hint of things starting to get a little bit below the surface. But then things happen very, very suddenly indeed. And within a few metres of rock succession or a few hundreds thousands of years, things are starting to burrow very rapidly down in the sediment, either to get at the food that's getting trapped down below

or possibly to start avoiding predators of various sorts and other things that are starting to move across the surface. Richard Corfield. Yeah, we have actually alluded to, but we haven't really spelled it out, one of the most important innovations which go along with us, the bilaterian triple blasts, which is that we have a mouth and anus. And it's, in fact, an incredibly useful thing because you can process food. And the idioccarans probably didn't have either of these things.

And this actually goes along hand in hand with grades of body organization for a long time. In fact, all the original researchers thought that the Ediacarans were like jellyfish. And jellyfish are characterized by having two layers of body tissue when they're just in the gastrilla stage, when the fertilised sex cell is just about to develop. It has an ectoderm, which is your skin and an endoderm, which is the lining of your gut. And the jellyfish have a layer of jelly in between.

What happens at the base of the Cambrian is that the metazoa, who are beginning to borrow, as Martin has said, and expand into these new horizons, because if the jellyfish just lived, if the Ediacaran's just lived on the surface of the sediment, probably in conjunction with bacterial mats, which we actually haven't mentioned, but then these bilaterian triple bars start going up and down.

One of the innovations which enable them to do that is that they said we'll have a third layer of body tissue, which is the mesoderm, which enables you to have a thing called, a sealum, which is the body cavity. And everything between your gut and your skin, where all your organs sit, your liver, your lungs, your kidneys, your pancreas, that's your selam. And so that gives you a nice package, a suitcase, to put all sorts of clever equipment to enable you to process the ability to eat food and adjust it at the other end. And so it might be that the edioccurrence

were just out-competed by superior metabolic machinery. And it was, though, A very striking phrase. Obviously, I'm out in a mass extinction. Do you want to develop what's been said by the two others, Rachel? Yes. I mean, Richard has alluded to this microbial mat. This is really a very, very important part of understanding the ecology of the Ediacara biota. Of course, it's remarkable, in any case, to find soft-bodied fossils.

That huge, most of the fossil record consists of skeletal forms, of bits of shell and skeleton and so forth. And what's so remarkable about these Edacarum biota fossils is that they're found in a type of rock, a coarse-grained sandstone, which really couldn't preserve anything today. And when you scrutinize these specimens, or if you look at these huge expansive bedding planes that you can find in places like Australia and the White Sea in Russia, you see that not only these surfaces covered with these remarkable fossils, but also the whole surface of the rock is crinkly or pastular. has a sort of elephant skin texture. And this has been proposed to be the remnants of some sort of microbial mat, which in effect preserved this biotas as a sort of death mask. So we have to revoke a very special condition on this Ediacar and sea floor,

both to have preserved these remarkable fossils, but also this algal mat may well have been playing an important role in the ecology of these forms that we don't fully understand. And of course, with the advent of... things that were burrowing and churning up the sediment and also grazers with new types of jaw parts, this algal mat would very, very easily have become disrupted. So that may well have been a key reason for the extinction of these forms.

Can I move on to the point that's made several times by what I've read from what you've written? This has been called the DeCarrant failed experiment. They came and went and left nothing behind, or something that was part of the evolutionary chain and was linked to and can be linked to what happened afterwards. Now there's obviously bits in between, there's obviously grey areas. But can you address that, Martin?

So what do you think? I think that the term failed experiment rather misses the point. It seems to me that the edictorans were doing what they were doing very efficiently and very well in a world in which there were no predators and no creatures with a through gut. They were cycling organic materials, getting them back into the biosphere,

helping to stabilise and maintain a habitable surface for 40 million years or thereabouts. And I think it's a bit of a mistake to say that because they didn't carry through they were failed, it's a bit like saying that your great-grandfather because you didn't have an iPod was a failed experiment. because he was doing things to the best of visibility at the time

by reading the Times or whatever it was. So the rules of the game in the Ediacarum were different, and therefore I think they were successful. It's possible that they did not leave descendants, the ones we're talking about mostly, but there are certainly things in the Ediacaran which must have been our ancestors. They may have been small,

or they may have developed relatively, late towards the very end of the story. But I think we have to see it in that light. And that's interesting because it's not a thing a biologist would predict. There's all sorts of things in the fossil record, which no biologist or molecular clock expert would predict from early history. The fossil record shows us that things move in fits and stars. But is it the resistance?

I'm going to do it one moment. Isn't the resistance to the idea that there cannot have been something which lasted so long was so dominant, and it was not part of evolution, because evolution is the idea? What do you think, Richard? Well, I think Martin's is completely right. I mean, they were doing things to the best of their ability,

and then somebody changed the goldposts on them, and what had been good enough of 40 or 60 or 90 million years was suddenly not good enough at all. I mean, that's life, isn't it? But there is, it's not true to say, many of the Ediacaran fossils, which are so strange, didn't come through to the present day. But there are plenty of things in the Ediacaran that did. Red and green algae sponges. Sponges come through to the present day.

Sponges are basically something which, you know, almost decided to develop multicellularity and then settled for a colony of cells. But then the other thing that we have, which we haven't really talked about, is this idea of genetic paleontology, which we touched it at the beginning, which is that it is possible to calculate, assuming that mutation rates are over 4.5 billion years, statistically constant, which they more broadly are, when things diverged. And it becomes very clear, no matter how you slice this,

and there's a considerable amount of debate about rates of mutation in vertebrate genomes, genomes like ours, and rates of mutation in simpler genomes. But it's absolutely clear that the, The modern-day animal assemblages, which we know and love, actually had their roots in the Ediacaran, if not before, in this Snowball Earth episode, the Cryogenian. And so the Ediacarans and the bilaterian triplabasts, us, if you like,

are not so completely different. They were a way of dealing with the world as it existed between 600 and 542 million years ago, and then something which had been in the background, much like the mammals at the Cretaceous tertiary boundary, when the dinosaurs became extinct for whatever reason an asteroid impact on the Yucatan Peninsula, the mammals just decided, well, you know, we are going to inherit the Earth after all.

What surprising things does the way they developed tell us about evolution, right Sean? I would just concur with what Martin and Richard have said. There's an adage in geology, which is the present is the keith of the past, and this clearly doesn't is utterly inapplicable to the precamrian. This precarming world was remarkably different. Not only with the disposition of the continents utterly different, the oceanography, the amount of oxygen, the water column, the water column may even have been full of sulphur, perhaps even a bit of iron.

So this biota really is, as they've said, of its time. And we have to really think of the world as being a holistic evolution of both, the physical, chemical and biological environments. And this is really what we see, through study of the fossil record, we see these new biotas emerging, each of which appears to be adapted to the unique world for any particular time slice. And then these are mown down during the five mass extinctions. And, of course, we're probably living through six mass extinction now.

So yes, evolution is very, very episodic. I can't think of a better ending to the last program of the series. than we're probably living through a mass extinction. Now, thank you very much. That was a terrific program to go out to. Thank you very much to Richard Wood, Richard Caulfield and Martin Brasio. We'll be back in the middle of September with a program about the 13th century philosopher Thomas Aquinas.

Thanks for listening. If you've enjoyed this Radio 4 podcast, why not try others, such as Thinking Aloud, where Laurie Taylor discusses the latest social science research. To find out more, visit bbc.co.com.uk forward slash Radio 4. Thank you.