The Ancients - The Cambrian Explosion: When Life Began?

Episode Date: June 14, 2026

538 million years ago, life on Earth changed forever. In an evolutionary burst known as the Cambrian Explosion, complex animals rapidly appeared in the oceans, laying the foundations for almost every major animal group alive today.Tristan Hughes is joined by the mighty Henry Gee to explore biology’s 'Big Bang'. What triggered this extraordinary leap in evolutionary complexity? Why did creatures with eyes, shells and limbs emerge so suddenly in the fossil record? And what can the Cambrian Explosion reveal about the origins of animal life and the history of our planet?MORERise of HumansListen on AppleListen on Spotify The Age of DinosaursListen on AppleListen on Spotify We're going on *TOUR* to Australia and New Zealand! - grab your tickets here.The Ancients is now on YouTube! Watch here: @TheAncientsPodcastPresented by Tristan Hughes. Audio editor is Aidan Lonergan. The producer is Joseph Knight. The senior producer is Anne-Marie Luff.All music courtesy of Epidemic SoundsThe Ancients is a History Hit podcast.Sign up to History Hit for hundreds of hours of original documentaries, with a new release every week, PLUS early access, ad-free podcasts. Sign up at https://www.historyhit.com/subscribe.  Hosted on Acast. See acast.com/privacy for more information.

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Just visit historyhit.com slash subscribe. 538 million years ago, something mysterious happened in the murky depths of our seas. Our planet came to life. It's known today as biology's Big Bang, the Cambrian explosion. Simple organisms gave way to complex, mobile creatures with eyes, limbs, shells and teeth. Predators and prey emerged, and, for the first time, life on Earth really began to take shape. A sudden burst of evolution saw practically all major animal species begin to appear in the fossil record. But why? What sparked this burst of complexity?

Why did evolution move so slowly for so long, only to erupt like this? Welcome to the ancients. I'm Tristan Hughes, your host, and this is the story of the Cambrian explosion, Biology's Big Bang. Our guest is a fan-favorite returning expert, the paleontologist, evolutionary biologist, author and editor, the one, the only, Dr. Henry G. Henry G, what a pleasure it is to see you in person for the very first time. Well, it is Tristan. I mean, here we are actually together simultaneously, both at once and at the same time. Who thought?

Who'd have thought it? I've got to ask, first of all, are the chickens doing around? They're chickens. There are two of them left. Two of them left. They're bluebell and poppet. And they're still squawking and laying eggs, so that's nice. But today we're going far beyond the origins of chickens, which in itself is a fascinating story, isn't it?

It takes millions years. But the Cambrian explosion, and this has been described, the Cambrian explosion. as life's big bang, as the biological big bang. Yes, today we're going to explode the Cambrian, and it's always been a big mystery. If you cast your mind back to the Victorian times, to the days of Darwin, and pioneering geologists like Hutton and Lyle, who got an idea of which rock came before what, but of course they had no idea how long the lengths of time that these rocks had taken to sediment, and the lowest rocks in the sequence with fossils in was called the Silurian.

I mean, it's all since been divided up into flats with a kind of retail outlet underneath. Now it's called the Cambrian. But before the Cambrian, there was nothing. After the beginning of the Cambrian, there were lots of fossils, clams, and lots of trilobites, which are these animals that look a bit like woodlice or pill bugs, full of feelers and antennae and legs and eyes and armor and lots of other animals like that. So people wondered, how is it that at the beginning of this period there's lots of life, lots and lots, but immediately before absolutely nothing.

And Darwin was worried about this. He wrote in the origin of species that this particular gap was the chief difficulty of his theory of evolution. So this is the period when, I mean, the first thing. fossil record, I guess basically becomes a thing when we start actually seeing skeletons in the surviving record, in the soil and so. That's right. It was the first time we actually saw animals big enough to see with the naked eye with hard parts, armor, jaws, teeth, skeletons, shells. And of course, it's the hard parts that tend to be fossilized. It's only very rarely that you get fossilization of the soft parts of an animal. And that really is.

quite remarkable. So if the world, if the earth is some 4.5 billion years old, how far back do we need to cast our minds for the story of the Cambrian? The Cambrian, they keep changing their minds about the beginning of the Cambrian. There are two ways to decide this. One is when particular fossils occur. Traditionally, the beginning of the Cambrian is seen as the occurrence of a kind of burrow called treptickness. So when animals started to be able to burrow into the sediment, that caused a big change in the global

ecology rather than skating over the surface. Once they started mixing up the sediment, that caused a great deal of change to the Earth system. But the other one is directly dating it using, not carbon dating, that runs out, that's useless before about 45,000.

It's recent history. It's quite recent, recent history. So there are various radioisotope methods that you can use today at the beginning of the Cambrian. But they keep changing their minds. I mean, I looked up this morning, and the latest agreed date for the beginning of the Cambrian was 538.8 million years ago. In fact, the reason we're having this today, listeners, is because it's the 538.8 millionth

anniversary next Tuesday. Right. So that's why we're doing it. So the Cambrian period, as a period or what they call, geologists called a system of rocks started about then, but they keep changing their minds because it's a very fluid. It's a subject of major research, not just in paleontology, but in geology and geophysics, trying to work out what changed in the system. Before we had an idea of the absolute age, people wondered if there were. was a period before the Cambrian, which was completely lost. It had been completely eroded away to nothing in which all this evolution happened, so that that being eroded away, you would then

afterwards get the impression of nothing and then lots of things. But it turns out that it's real. There is a definite gap. Well, we're going to explain that in a second, but first of all, just so we really get it in our minds, I mean, how far back we're going. How much earlier are we talking with the Cambrian? How much later are the dinosaurs? Oh, gosh. Well, the Cambrian was, say, half a billion years ago. The dinosaurs didn't appear until 200 million years ago. So that's just to emphasize just how much we had the earliest. But of course, when the Cambrian had happened, like 8th or 9 tenths of the Earth history had already happened. And still the pre-Cambrian, we know very little about it. I mean, there were a lot more than we did in Darwin's Day, but there's still enormous gaps in our knowledge of what we're. went on before the Cambrian.

Well, can we now explore this briefly? Can we almost kind of cover what is known, all the theory around the story of animals up to the point, up to that 338 million years ago? I'll try and keep this a huge story brief. About two billion years ago was an event called the Great Oxidation Event. Well, it happened between 2.4 and 2 billion years ago, when for reasons nobody is quite sure a lot of oxygen appeared in the atmosphere. Now, before then, there was almost no oxygen. Now, this caused a revolution in the biosphere, and that precipitated the evolution of what we call

the eukaryotic cell, that is the kind of cell from which you and I are made up of, as opposed to bacteria. Now, bacteria had got together to create that. And then, nothing much seemed to happen for a billion years. It's what geologists who don't get up in the morning for anything less than apocalyptic disaster called the boring billion. But things were going on in the background. Now, most eukaryotes that live today are single cell. Amoebas, paramecia, flagellates, dino flagellates that cause these blooms, many horrible diseases, malaria. They're single-celled. But there were signs of multi-cellularity UKAC carriotes about 1.8 billion years ago. Seawedes.

About a billion years ago, there were early seaweed, early fungi, but nothing animal-like until the breakup of a huge supercontinent. Now, we know about the supercontinent of Pangea, and I remember we chatted about that. The Triassic theory is much later. Well, there is a supercontinent science. The Earth breathes on a period of about 500 million years. So the continents tend to glom together into a big supercontinent, and then they break up, and then they glom together again. And a friend of mine, a geologist called Ted Neald, has written about this in a book called

Supercontinent, and I owe it to him to tell everybody that it's not about the importance of pelvic floor exercises. It's about the supercontinent cycle. Well, before Pangyre, the supercontinent before that was called Rodinia. And that started to break up about 800 million years ago, and as a consequence of that, there were two or three snowball earth episodes where they were ice ages so severe that they covered the whole of the earth. Now, just before the great oxidation event, there was another of these snowball earth events, which was even more severe. but that was so long ago we don't need to worry about it.

But partly to do with the Snowball Earth event, animal life appeared. And these were large enough to see with a naked eye. But the first flush of animal life was very strange. And this was before the Cambrian. These were the Ediacaran fauna. Now, a fellow, I think his name was Spriggs in Australia, discovered the Ediacaran fauna in South Australia

in some sandstone. very, very coarse impressions of things that look like jellyfish and other things, squashy sea creatures that were very hard to assign to any particular group of animals. They have been thought to be lichens. They have been thought to be their own thing, some strange creature. Some have been tentatively associated with the modern groups of animals. But since then, Ediacra and faunas have been found in all sorts of exotic locations. from the White Sea coast of Russia to Namibia to Newfoundland to Bradgate Park near Leicester.

Brilliant. If you go to Bradgate Park in Leicester, it's a public park. And in the middle there are these enormous great rocks, which are pre-Cambrian. It's a little splot of pre-Cambrian in the middle of the English Midlands, and it's got Ediacaran fossils on them. But they're very, very hard to see. I mean, some of them are huge. I mean, they're not tiny. There's as big as a pair of trousers, you know, spread along the rocks.

There's what they call Charney a discus is one. And they look like fronds, but you really need to see them at dawn or dusk when the light is slanting. I mean, I remember being there trying to look for them, and I went on a very sunny day at midday. And even though they were right in front of me, and I knew this because I was doing face-timing, my colleague Emily Mitchell in Cambridge, he's an expert. on Ediacaran Fathers. And she knew exactly where I was. So she was in Cambridge and I was in Bradgate Park

which said, no, left a bit, left a bit, right a bit, there it is. So the Eadiacaran fossils, they lived just before the Cambrian and then they disappeared. So I'm still getting over the fact that you can go to Bradgate Park near Leicester today

and look at the remains of bizarre, 600 million years. 600 million year old. And there's people climbing around and walking their dogs and playing football and there they are.

I mean, it is, mind-blowing. That's amazing. That is so amazing. And most people don't know they're there because they're very hard to spot unless you really know what you're looking for. They were actually discovered by a small boy who was, I don't know, playing football or walking his dog. I've no idea. And is it the fact, so back then, you know, these animals, they have mouths, they have an anus as well, we're thinking that kind of thing on? Well, nobody really knows with the Ediacaran what they were like. I mean, some of them might be kind of colonial. They're these

Ediacaran creatures called Ranger Morphs, which look like platted loaves, and some of them seem to have little smaller platted loaves around them. They grew by, like, strawberry plants by shooting out runners and grew baby ones. These are mostly known from Newfoundland. Now, people have split up the Ediacaran into various sub-stages. The earliest one part is found in Newfoundland, and I think probably Leicestershire, but the later part is in Namibia and also on the White Sea, where you see signs of things that look a bit like mollusks, in other words, more modern animals. So there were signs of animal life happening just before the Ediacaran period finished and before

the Cambrian explosion. And I've just got this last thing in my notes is that people describe it as the Garden of Ediacura. Yeah. So this idea that there weren't predators. or prey at that time, they all just coexisted? Yes, it was hard to know what the ecology was like, but there doesn't seem to have been any predation as far as we can see.

Nobody knew, knows how they lived. Maybe they had symbiotic algae like corals do today. So it's a huge amount of unanswered questions about how the Ediacaran lived, and people think, you know, it was a kind of blissful time of the, of things just getting on. But I'm sure that a lot of the animals were probably slurping up even smaller things that don't appear in the fossil record, like larvae or bacteria or tiny eukaryotic cells, because even though multicellular creatures had evolved,

they were still, as there are now, lots of single-celled creatures around. So there was probably a lot of filter feeding and deposit feeding. Nothing chasing each other with nasty, long, pointy teeth. But that comes soon after. It does, yes. So what happens roughly around 550 million years ago, that is the spark, what's an explosion in life? There are lots of different theories, and they're probably all connected. One is that sponges had evolved.

The earliest sponges are about 900 million years old, but sponges do something great. They slurp up a lot of detritus. from the ocean. And once the sponges did that, there was less for decay bacteria to decay. Decay bacteria suck up all the oxygen from the seawater. So without them, the seawater became more oxygenated all the way down, which was more space for large oxygen-breathing creatures to live in, to occupy. Another one which is related is perhaps there was more oxygen in the atmosphere, which tends to happen during ice-agey type times. But another one, which I think is the clincher, was that suddenly a lot of minerals appeared in the seawater, particularly calcium.

Now, calcium is the element from which you create bones, calcium carbonate, and invertebrates calcium phosphate. But certainly, all mollusk shells are made of calcium carbonate. The sugary skeletons kite of jointed creatures are reinforced with calcium carbonate. I mean, just think of, you know, big lobsters, that sort of thing. Where did all these minerals come

from? Well, it looks like there was some huge episode of continents banging into each other. The formation of Gondwana, the great southern continent. It seems that two large continents slammed into each other

creating the most enormous mountain range, 4,000 kilometers long at least, 1,000 kilometers wide, and who knows how high. I mean, perhaps the greatest mountain range that ever existed on the planet. I mean, the Himalayas today are still being created by the collision of India with Southern Asia. Now, this started 50 million years ago,

and it's still going on, and it's created these enormous mountains. But mountains can only go so high because of gravity and rocks and everything. So what happened when these enormous mountains were created on land, 5, 50 million years ago or so, was they eroded really quickly, in geological terms, until, you know, it wasn't that long, a few tens of millions ago. They were eroded flat. Now, where did all that stuff go?

It went into the sea. So in a relatively short order in terms of geology, all this mineral stuff was in the sea. And this happened, it was a perfect storm that happened alongside another major episode in evolution. And you're going to ask me what that is, on you? I guess I should. Go on, Henry, what is this other major event?

Well, this is a family podcast, isn't it? This is a family podcast. It was the evolution of the anus. Ah, you know what? Because I know I asked that question earlier, and it might have come out to the blue, but there was reason for it. Go on then, so the evolution of the anus. Well, the evolution of the anus. Well, what happened was most early animals and some primitive animals, they kind of absorb and excrete all their stuff through the skin because they're very small.

Now, some creatures, such as jellyfish and hydra, they have one opening that leads to a bag. like gut. So everything that goes in comes out the same way. And it's usually a kind of dissolved wash of stuff. But a major innovation was the through gut, where there's a mouth at one end and anus at the other end. Now, of course, biology loves its exceptions. They're these amazing things called comb jellies, tinafores. They have four anuses, which is amazing. But we'll forget about them. They're another, like chickens, they're another whole thing. But so there was mouth at one end and an an anus at the other. And so animals for the first time had a direction of travel.

So animals started doing things that they hadn't done before. Most of the time, previously they just stayed in the same place, waving tentacles in the air. I mean, Ediacaran animals were rather like that. They just stayed in the same place. They started burrowing. Now, as we've talked about, the origin of the Cambrian is marked when burrowing animals happened as a particular kind of burrow called treptychmus that was made by

a burrowing animal. Now there are many ways to borrow. One way is to inflate yourself and to make yourself kind of hydrostatically rigid like an earthworm, but another way is to clothe yourself clothe your body in

armour. So because of these things happening at the same time, the bilateral body plan with the mouth and the anus and all that calcium coming into the sea, and if animals are moving in a particular direction of travel, eyes evolved in the Cambrian. There's a guy called Andrew Parker

who's written and talked about this a lot about how eyes happened. So when animals have got eyes and they're moving in one direction, they're usually looking for something, and what that something is is food. So they start to eat each other. And of course, what with all the calcium, that led to the evolution of teeth and the evolution of armour. Now, one of the very earliest Cambrian fossils

as opposed to just a burrow, which is what we call a trace fossil, is an animal called Cloudiner. It's very, very small, and it looks like a stack of ice cream cones. And one of the very earliest Cloudiners has got a bite crunched out of it. So even then, back at the very earliest Cambrian, there are signs that animals were taking bites out of each other. So from that fossil record, we have an example of that particular animal with a bite taken out of it.

Yes, and it's one of the earliest Cambrian fossils. And there are plenty records of tributtal. trilobites with bites taken out. We're getting trilobites very quickly. They're nice hemispherical, you know, semicircular bites. So whatever they were, they were very tidy eaters. Well, shall we get to trilobites now? Because they're perhaps the most iconic fossil from the Cambrian period.

And they also exemplify these new changes that you've just described that allow for this development of life at the time. Yes, trilobites are absolutely beautiful. And I would say that most fossil collectors will have trilobites in the... their collection. Trialibytes evolved early in the Cambrian. Their acme was maybe in the Cambrian or the succeeding Ordovician. After the Devonian period, they went into a bit of a decline, and they finally petered out during the end-permian mass extinction of the Great Dying. But with the trilobites, that was really a departure in a minor key. They were, by that time, quite minor

components. But earlier, there were trilobites everywhere. I mean, some of them were, are virtually microscopic. Some of them are a foot or two long. Some of them had enormous eyes, compound eyes like insects do. Some of them were blind. Some of them borrowed. Some of them skittered along the surface and some of them swam. So there were trilobites for every occasion. Right. So trying to get an idea of what exactly a trilobite

was, we can get a general sense with the shell and that kind of woodlice look. But there is also, once again, harkening back to why this is such an important time in the explosion of life. There is a lot of diversity in the shape and size of the trilobuses. Oh yes, they were a marvellously adaptable form. I mean, if you can think of a pillbug or a woodlice, the reason they're called trilobites is they were divided into three parts longitudinally, a bit like a church with a nave. Oh, true. Yeah, the side bits at each side. And at the front, they had a very particular head. But also like jointed-limbed animals, like insects and crustaceans today, they molted.

So there are even fossils of trilobites molting or the actual malted skeleton of the trilobite when it grew to become bigger. And very occasionally there are trilobite fossils with soft part preservation. So the underneath of the trilobites, you can see the gills that are attached to the legs, very much like crustaceans.

They weren't particularly closely related to crustaceans like woodlite. lysosocrossations, they were the kind of their own thing. There's a lot of debate about which jointed-limbed animals they were most closely related to, but they kind of exemplify that kind of spiny, leggy creature with a skeleton that they could shed with big eyes. They're even fossils now of the internal digestion of Trialabites showing what they ate. They kind of them snarveled up things and digested them. They don't really have to be. have a mouth with jaws, crustaceans. They do have a mouth, but it's surrounded by all kinds

of mouth parts, and they just stuff stuff in and grind it up. They were great because, you know, like one of my favorite fossils, Leicestosaurus, which was a go anywhere, eat anything fossil from the Triassic. Trialibites were a bit like that, only in the Cambrian, which may have been why they were so successful. And so, I mean, from the surviving fossil record, then it seems that trilobites are front and center, you can normally recognize if you're at the Cambrian layer because you'll find trilobite remains. Are there any particular sites in the world where we do have a really, apart from right outside Lester, are there any particular examples, like particular rock faces, where they are just full to brim of different Cambrian animals,

trilobites, but also all these other animals that emerge at this time? Well, the Cambrian is named after Cambrian whales. So that's full of, the firm Cambrian corners, but there was a lot more to the Cambrian than trilobites. A century ago or so, there was a fellow called Walcott, who was a geologist, and he used to take his family on vacation to British Columbia. And high up on a mountain in British Columbia, he and his family discovered what are now known as the Burgess Shales, which are actually a series of quite small exposures, very high up. I mean, I've not been there.

There was a field trip I didn't go on because I knew I was not going to be fit enough to get there. I mean, it's like mountaineering to get there. We're high up in the Rockies. It just shows you the power of the earth. There are these deposits that happened under in the deep sea, 5008 million years ago, something like that. So picture the scene, continental shelf, a mudslide, buries, all these animals all at once, and they go down to the deep sea

and because of not very much oxygen, they're preserved perfectly, including their soft parts. And what a menagerie they are. There are lots of spiny-skinned animals of various sorts. There are some famous ones like hallucigenia. Yes, what's this? Heluciginia was named after a friend of mine,

Simon Conway Morris and the University of Cambridge who I don't think he'd mind me saying he's a bit of an old hippie. he named it and it says something in the paper on account of the strange and dreamlike appearance. Polycinogenic, okay. Yes, so it's basically a worm with enormous spikes dipping out of its back. And it's since been discovered that there are quite a lot of these.

They're called lobopods. They're kind of closely related to arthropods, these armoured worms. And there is a relic of those animals living today on land, the onycophores or velvet worms, which are strange little like worms with mishland mount stumpy legs that go around forest floors. So there was that. But there were a lot of sort of shrimp-y like things, but there was a big, big predator in time.

We're going to tee up that one because I've got two names here. I'd like to do the smaller one first. And I hope I'm on the right track when I say the word opabina. Opabinia. Opabinia. Opabinia. Opobabina. It's very hard to describe what Opabinia looks like. If you've got small children, the closest thing is like the Nunu from the telitub.

From the teletub. Okay, yes, the sucking up. The other thing, yeah. It was a, the oppabini was a strange shrimp-like creature. It was a swimming creature. It had lots of fins. And it had five eyes on stalks. And it had this long hose pipe mouth with jaws at the end. That was a real weirdo. I mean, but that's obscurely related to all these jointed limbs. So that was one of them. The other one, though, and I think this is the big predator.

I hope I get this pronunciation right. Anomolocharis. Anomolochorus, yes. Now, Anomolochorus has a very checkered history. Charles Walcott described a lot of different fossils that turned out to be different bits of what we now know as a lomelocaris. Now, it had a circular garbage grinder mouth with circular plates, but Walcott described a fossil which was just the circular mouth as a kind of jellyfish called Paitoya.

It also had segmented, spiny, pincer-type things at the front to stuff things into its mouth, and these were described as some kind of shrimp, and it wasn't until people found the whole thing. so this enormous shrimp-like thing with big googly eyes and this circular mouth and pincers to shove things into its mouth they could be a metre long which was huge back then that is like the T-Rex that was a big mama yeah that was the big predator of the time

Anomala caris means kind of weird shrimp really and the Anomalacharids in the Cambrian were very successful In fact they survived the Cambrian because there has some wonderful fossils coming out of Morocco, more recently the Fezhuata formation in Morocco, which is not Cambrian, it's Ordovician, it's the succeeding period, and that includes the latest known anomalous carids. They were also anomalous carers that were filter feeders, a bit like the versions of basking sharks. So they were big, but were filter feeders. And so they specialized in various ways, these anomal carriages. And you mentioned earlier those trilobites which had had bites taken out of them, circular bites. Yeah. Does that align then with that kind of circular mouth? Oh, possibly.

Possibly, but there were lots of other things there at the time. I mean, there were all sorts of arthropods which are known as great appendage arthropods. These had, these were more like, if you can imagine lobsters, but with immense, much more immense claws, sticking right out from the front end of the animal. These have been found. These are a very kind of Cambrian arthropods. So there are loads and loads and loads of arthropods from the Cambrian from which evolved from a small selection.

The arthropods we get today, such as collicerates, which is scorpions and spiders and crustaceans from which the insects are actually an offshoot of crustaceans. They're the kind of land-living crustaceans in the same way that tetrapods are land-living fish. And things like horseshoe crabs that look kind of prehistoric. These were all offshoots,

but a lot of different kind of arthropods became extinct. They didn't survive the Cambrian. They maybe lived a bit longer. But there were also a lot of amazing worms in the Cambrian. In Cambridge, some time ago, the great student of the Burgessels was a guy called Harry Whittington, and his students, it were Simon Conway Morris, who I mentioned and also Derek Briggs.

They're both still active. Derek mostly does the arthropods and Simon did all the worms, but there are lots of worms. Now there's an obscure group of worms now called Priapylids, which the classically trained listeners of the ancients will titter because it means basically penis worms, and they look like penises. and these were major, major players in the fauna digging stuff in the seabed and filter-rout gung from the seabed. They're not many of them around now, but they still do exist, but there were many more of them back then.

So it seems like this is very diverse, very, quite frankly, bizarre world on the seabed at the time of the Cambrian explosion. All these different arthropods and worms and so on. I've got one other in my notes because it seems really interesting because of the quality of the fossils that are survived. Fuxian Hu Huia. Yes, I'm never quite sure how to pronounce this. It's Chinese. Now, the Burgess Shales was, it was for a long time the poster boy of these Cambrian deposits,

which preserved creatures, including their soft parts in incredible detail. A later one was discovered in China, in South China, the Chengjiang, Now, if you want to go and discover new faunas for things, go to China. It's just amazing. So in the southern China was the Chengjiang fauna and their other sins. The Chengyang fauna is older than the Burgess sales by a few million years. And that included lots of similar things to the Burgess sales. But the other ones, Fuxian Huir, I apologize.

I don't know how to pronounce it either. Now, some of these are preserved so beautifully that you can even see the nervous systems. Wow. That's amazing. Yeah, you can reconstruct the nervous systems of Fuxian, Huya, to see how their brains were similar or different from modern arthropods. And you get an idea. Now, this is quite important for anatomy geeks because students of insects and arthropods are perplexed by what's known as the arthropod head problem. Now, arthropods are segmented animals

And nobody's quite sure How many of the original segments Went to make the head Now, this is a problem that perplexed people back to Gertor But once you've got the nervous system Inside, as well as the segments on the outside, You can get a better idea I think people are made inroads into the arthropod head problem

But I sincerely hope they never solve it Because it's a wonderful problem It's a nice problem to have Well, we've covered all these various types of arthropods so far. And so far, Henry, we've covered they've all been in vertebrates. But this is also such a big time because this is when we see the emergence of vertebrates. And what do we mean, first of all, by vertebrates. Pertebrates are the animals, the group of animals to which we belong.

These are animals with backbones. The backbones. So that rather than have an external hard skeleton, like the arthropods and invertebrates, they have, well, vertebrates have an internal skeleton, the backbone. Now, the origin of vertebrates is a subject that is close to my heart because when I was a graduate student in Cambridge,

I had to teach early vertebrate history to undergraduates. Now, in Oxford and Cambridge, most of the actual teaching is given to the graduate students, so I had to teach these undergraduates. But I found that all the lecture, the notes from the lecture, were very, very old-fashioned. So I had to do a lot of work to try and understand this myself, and I got sucked in down that rabbit hole. So it became a, it's a subject that's very close to my

heart, and I've written now two books about the origins of vertebrates. I can't help myself. I don't want to write another one. I did write one, and then people kept asking me to write another one, and I said, no, I'm not a scientist, you scientists, you write one. And then I was cornered in a room by two of these scientists and my publisher, so there was no excuse. So I had to spend 16 months writing another book about the origin of vertebrates because a lot has been discovered in terms of genetics, in terms of molecular biology, and of course in fossils. Now, it was the Changsang Fauna of China that produced the things that were, that kind of

solve the problem. But I'll have to go back to one of these strange worms that Simon Conway and Morris can't help yourself, can you? You've got to go back to the strange worms. Well, there was this kind of strange worm thing, including Picaya, which looked like a segmented fish fillet. Because it wasn't an arthropod, Simon got to study that. He'd written an initial description a long time ago. Many more fossils had been found since. And he kept not writing the definitive monograph and I asked him why he hadn't done it he said because the more you look at it the

weirder it gets. The less and less like a vertebrate ancestor. Now but in the Chang Chang, so Piccar is not the earliest vertebrate. It's an offshoot somewhere

because there were all sorts of weird things that weren't vertebrates. But the earliest vertebrates, and we could call them fish, were at the Changchang fauna there's one called Hykoichthis

and there's another called Milo Comingia and these were the earliest known fish. These are the first fish in the world they had no paired fins

they had if they had a backbone it was just cartilaginous they had no hard parts and this is why it took exceptional preservation in places like

the Changsang Fauna to show them up at all they were, you know, about the size of an anchovy or maybe small, real tiddlers like a, you know, wide-bate thighs. Sadines, simply. Yeah, or even smaller. And they had eyes, but more than that, they had four eyes. They had two pairs of eyes. In addition to the regular pair of eyes, they had another accessory pair next to them of smaller eyes.

So, Milo conmingier, the earliest vertebrates had four eyes. what happened to these other two they eventually went inside the head and became the pineal gland which is still we have pineal glands in the middle of our heads but they're connected by nerves to the eyes and the optical centres of the brain

and these are the glands that help us regulate our biarrhythms they produce melatonin and they keep our day and night cycle and they are what goes out of sync when you fly a long way and get jet lag. So, but originally these were eyes with lenses and retinas, and there were four of them in the earliest fishes. So these were the earliest vertebrates from Cheng Chang.

They weren't armored at all. There had been an idea that there were armored fishes in the Cambrian, but that has been debunked. The problem with the armoured fishes in the Cambrian was they're just shown from fragments, tiny fragments, is if you took an armoured fish and you stomped on it and rolled over it with a steamroller and scattered all the bits, and then you found one bit. But of course, these tiny fragments of armour, you could study under a microscope. The problem is they're indistinguishable from arthropod skeletons.

So these fragments that had been attributed to fossil fish were probably, some kind of arthropod. So the fish that we see in the Cambrian are unarmoured. It was only later in the Ordovician that they became armoured. And so with the evolution of the backbone, to get to those earliest fish, is this the thing that people are still debating how you got from that strange worm to the earliest fish with a clear backbone? Yeah, the origin of vertebrates has been a really big problem for a long time

because vertebrates are so different from invertebrates in many, many ways. This internal skeleton is very, very unusual. So the history goes back a long way, and that's another episode, the origin of vertebrates. Oh, yes, let's do that things. But our closest relatives are things called, believe it or not, sea squirts. These are creatures that live in one place squirting seaweed of seaweed. they live in one place squirting seawater in an hour. So did they write,

our closest ancestors? No, closest relatives. Our closest relatives are sea squirts. Yeah. Okay. Well, these sea squirts have larvae, and these have little heads and tails like tadpoles, and the tails have the notar cord in.

The notar cord is the precursor of the backbone, and they have the muscles on the other side. And then what these things do, in their heads, they've just got one. eye spot, not four, just one, and all it can do is detect light and dark. And it's got a little gravity sensing organ. And all it does, when it hatches, it goes on a very short journey with its tail just to find somewhere that's deep and dark. And then it sticks onto the rock with its head end, and then it resorbs the tail and then balloons out into this giant

pharynx, that's this giant balloon mouth and stomach, which is the sea squirt. So the Vertebrates are basically these two things in one. The mobile tail and the jaws and the viscera behind it. So a famous paleontologist called Al Roma, one of his last papers in 1972, conceived as vertebrates as a mixture of two animals, the somatico-visceral animal, that we have the muscles and the brain and the backbone

of the mobile animal, animal, which is represented by the little tadpole larva, and the visceral part, which is the stomach, the intestines, all the squishy mixed grill parts, which are represented by the adult tunicate. So it could be that the common ancestor of vertebrates and tunicates was an animal which was motile, and it had a segmented swishy tail, and it had a front end with jaws and our pharynx, but then we went our separate ways. Vertebrates have integrated the two, so they're almost seamlessly joined, but tunicates have kind of deconstructed themselves, so the somatic part is the larva and the visceral part

is the adult. And on my desk several years ago appeared a paper morphem Changsang of something called the two Licholians, and I looked at these and I thought, goodness me, expletive deleted. these look like romosomatocomatico visceral animal. And these Vatulocolians do indeed have a blobby head with little gill gillslits and a kind of circular mouth and a segmented tail. And these, not universally, are generally seen as somewhere in the ancestry of vertebrates and tunicates together. So, and there is another animal called the amphioxus, which is a swimming animal with gillslits. That used to be seen as the closest relative to vertebrates, but now it's been demoted further back because of all sorts of genetic things.

Now, medical scientists loved tunicates because they had a perfect little heart, but only made out of a few cells, so they were excellent for experimenting ideas on the development of the heart. And the amphioxus doesn't have one. and medical scientists said really the tunicates should be closer to vertebrates than they are, but the genetics now reveals that they are. We are closely related to tunicates. We have many of the same genes, many of the same processes. It's just that tunicates have evolved in the strange deconstructing direction

and have also cut down on how many cells they've got so they don't have actually many cells. Just to refresh, when you say tunicates, what kind of animal should we be thinking? These are the sea squirts. These are the sea squirts. My apologies, but some of them, some of the sea squirts live in, as I was living in one place. Some of them float around in huge colonies in the sea.

There are some called pyrosomes that have these huge colonies that are kind of made trumpet-shaped. I mean, and divers can swim inside them. So they emerged at the time of the Cambrian explosion, and they're still visible today. Yes, there are fossil tunicates from the Cambrian explosion. But some of the tunicates have evolved to be really, really strange. I mean, they're these tiny ones called larvations, which are still tiny the size of a grain of rice. And they are still divided like a tadpole into a switchy tail and a head. And they filter feed from the sea.

But to do this, they secrete this enormous mucus structure called a house, which is unbelievably intricate. and it's made out of mucus that they secrete in huge amounts using genes and proteins that have seen nowhere else in nature. And they use these for a few hours and then shed the lot and grow another one. And these creatures have a life cycle of a day, a week or so. And these mucus houses drift to the bottom of the sea and they are a major, major part of the carbon cycle of the earth.

And yet not many people know about these because all this happens way, way out in the open sea and these things are very fragile. So most tunicates, most of these tiny tunicates, they're the size of a grain of rice, and their house is the size of a walnut. But there are ones that are maybe fossils that are known where they were, you know, fairly big animals and the house was the size of a football.

And you can imagine shoals of these. So our nearest relatives, the tunicates, have gone on to be an amazingly diverse and weird set of creatures and they go back to the Cambrian explosion as well. I'd never heard of them before. I'm just trying to picture

so you've got trilobites on the floor, Anomalacharis once in a while hunting, then you've got these tunicates there, and then at the beginning, you know, the first vertebrates, these very, very small fish, maybe in shoals altogether, you think? Probably, yes.

Probably, yeah. And big shoals are these small earliest fish. With four eyes. With four eyes. As the Cambrian goes on, millions and millions of years, more and more oxygen, do these earliest fish, do they get bigger?

Yes, they do, and they also acquire armour. Armour, right, so this is when we get the first armoured fish. Yes, now, in the succeeding Audivision period, from the Ordovician through to the Devonian, they're more and more armoured fish. Mostly these are jawless

fish. They just had mouths, they didn't have, you know, up and down crunching jaws. Some of them they had very boxy armoured skeletons, so the front end was pretty much solidly boxed, and the back end had a swissy tail. Sometimes they had fins on each side, and sometimes they didn't. And do we think this is an evolutionary upgrade to help fend off things like Anomalacharis?

Yes, but also a later arthropods, which were even more nightmarish. These were called the Eryptorids, which were relatives of spiders and scorpions. So these are the original scorpions and scorpion kind of friends. And these could be, you know, six to eight feet long, enormous googly eyes. And they really did have snapping pincers. And Al Roma, the same guy who came up with the idea of the dual origin of vertebrates, he published a paper in science in 1933 that's a classic, which is he said that maybe vertebrates evolved armour to escape the snapping jaws of Euryptorids.

Now it's kind of fanciful. but the unusual thing about vertebrates is the armour is not calcium carbonate, it's calcium phosphate. It's called hydroxyapatite, and it's the same arm, our bones are made of hydroxyappetide. Now, strangely enough, even though vertebrates had the notacord, we had external armour more like in vertebrates. Only later did the backbone become mineralised. and the armour evolved in various ways. There are some of these fossil fish where the armour is broken up into scales, a kind of chagreen of scales.

They're these beautiful little fossil jawless fish called thelodonts, which have all sorts of amazingly starless shapes. Some of them come from this romantic locality called moth, which stands for man on the hill. It's out in the northwest territories of Canada. But jawless fishes are known from all over the place. In fact, there are quite a lot from, quite a lot of being described in the UK, in Wales and the west of England, where they're Aldavision, Silurian, Devonian rocks. You know, the clues in the name, the Ordovician and the Silurian were named after the Roman tribes in Wales and the Devonian is in Devon.

So there were a lot from there. And they were escaping predators, but also phosphate is quite rare on earth. and it's important in the biology of all animals. So there is an idea that bone was a kind of accessory store of phosphate for very active animals, which vertebrates are much more active than most others. So that was the origin of bone and fishes. And then at some point in the Silurian jaws were invented. So that's yours.

That's almost the next stage in the story, though, because now we're going well past. Yeah, we're well past the ocean. And that's another episode, the origin of jaws. The origin of jaws, and it's not sharks, isn't it? No, no, no. But let's go back to the Cambrian. So how long shall we say the Cambrian explosion lasts? How long before we do have this real diversity of life that we've covered in this episode today,

the first vertebrates, an omelocaris, trilobites and so on? Well, when we call the Cambrian explosion and explosion, it's more of a kind of extensive. It's a more of a difficult title to sell, if we call it the Cambrian extended detonation. Yes, or the Cambrian slow digestive rumble. But in terms of geology, it was because that hiatus in the fossil record that's so worried Darwin is real. I mean, there was a step change in evolution, partly motivated by the input of calcium minerals into the sea. And the whole thing lasted about

50 million years, which is less than the time between the dinosaurs and now. And that was a time in which almost, in fact, all modern phyla, that is large categories, of animals appeared in the

Cambrian, except one, but even that's been cleared up now. So what's that? That's the bryozoa or moss animals. These are tiny, tiny, only the creatures that live in little boxes and tentacles coming out. And that was thought to be the kind of slacker that didn't appear in the Cambrian. But now some fossils were found and some people said, yes, these are Brazoa.

And somebody said, no, these aren't Brazoa. They're seaweed. And the first lot came up with some more and said, yes, they're Brazoa. So that's been settled, which is nice. So just how rich and teeming in life would the seas, I can say, and the oceans of the earth have been back during the Cambrian. And we should, as it's probably clear already, but I just state,

because I don't think I've stated once during this chat, at a time before life is on land. So how just rich and diverse and just flourishing would the seas have been? Yes, well, the difference between land and sea would be quite stark. I mean, there have been some suggestions that some life was ashore in the Cambrian.

In fact, before the Cambrian, there are some deposits in northwest Scotland, which are a billion years old, which are fresh water deposits, and there are signs of encrusting mosses, lichens in ponds, so on land but not above the surface of the water. There are in some Cambrian deposits, this was shown to me in the Smithsonian by a lovely fellow called Ellis Yockelson, he's gone over the rainbow bridge since. there are some Cambrian beach deposits with tracks on that look like motorcycle tire tracks. I mean, they're the same width as motorcycle tire tracks, and they look like motorcycle tire tracks.

It's as if some prehistoric motorcyclist came out of the sea and did wheelies and then went back under the sea. So nobody knows what these creatures were, maybe some kind of slug-like creature. and there are thoughts that some of the Ediacaran creatures were maybe intertidal, maybe on the beach occasionally, but really for animals and plants in those days to venture above the surface of the sea would be as inimical to life as going into empty space. So it was a very long time before life came on to land. That would be another 100, 150 million years after the Cambrian explosion. I love doing episodes like this, whether it is the Cambrian, as we've done,

we've done dinosaurs in the past, going to do terabirds in the future, and hopefully also the carboniferous, when insects got really big, because they're defined by all of these different types of animals that to us nowadays feel so strange, so weird. And yet, that was just the story of evolution, the creation of these animals to try and deal with their habitats, since there has never been, has there a time in the Earth's history like the Cambrian, where you see such an, I'm going to use the explosion word again, but you know what,

I mean, such a diversity of life emerging because of all of these, increasing calcium, increasing oxygen and so on, that you have at this one period in time, such an amazing combination of different, bizarre, weird and wonderful creatures that roamed the seabeds of the world. some 500 million years ago. I should say, though, that over time, biodiversity has increased. Now, that could be because of the pull of the recent, because the more recent the rock, they're more of them.

But it does seem to be a real effect. So just because the Cambrian was really weird and strange, it shouldn't blind us to the weird and strange things that go on around us today, the interactions between creatures. I mean, I've been learning a lot about parasitism and things. For example, one of the most successful and species-rich groups of animal today are what's called parasitoid wasps. They are tiny, tiny, tiny, and we don't notice them. And they lay their eggs on the larvae, the maggots and the grubs and the caterpillars of other insects.

And they develop inside the caterpillar or whatever and eat it from the inside. Sometimes two different parasitoids lay their eggs on the same caterpillar, and the larvae grow up in. to tiny little soldiers that actually fight wars inside caterpillars. But these things get their just deserts because they're all infected by different kinds of bacteria. And some of these parasitoid wasps have parasitoids that parasitize them called hyperparasitoids. So, you know, big fleas, have little fleas. And these things go on all around you in every garden, in every forest. in every woodland, even today, a teaspoonful of soil contains an unbelievable richness of life.

Life is absolutely everywhere from the depths of the ocean up into the atmosphere. So the Cambrian is where animal life really had its first flowering, but once life gets established, it's very, very hard to get rid of. I mean, the Earth has tried many times to get rid of life, and us humans are also having a good go at trying to get rid of a lot of life, but it usually bounces back and the amazing interconnections that are formed by creatures that we hardly know exist. I mean, everyone knows about tigers and panders and rhinos and dogs and cats,

but these tiny, tiny creatures like each fungus has a fungus fly that only lives on that fungus, and each fungus has fungi that grow on the fungi, and each of those fungi that grow on the fungi have their own fungi and fungus. flies. It's kaleidoscopic, man. Could there still be some Cambrian relics out there at the depths of the oceans? Well, who knows? I mean, there have been all sorts of changes that have been done. So back in the Cambrian, there were these shellfish-like things called brachia pods, which had two shells, but they were an up and a down shell. There were a dorsal and a ventral. They're called lamp shells because they look like the kind of lamps that Aladdin had.

and these were the shellfish type analogue. They had an amazing structure inside called the Lofo4, a beautiful structure that was a Greek liar, which was full of gills and that would filter stuff away. And their competitor were bivalves, which also had two shells, but their shells side to side rather than top and bottom. And these go back to the Cambrian too,

but were always quite rare, but it was only after the end Permian extinction when almost all the brachyopods were wiped out. But there are still brachyopods today. And there are some that are quite successful and some are only found in the deep sea. But there are also ancient echinoderms, the starfish and sea urchins and allies

also go back to the Cambrian where they had some really, really weird versions which we haven't had a chance to talk about. And some really ancient type of kinoderms have been found in the depth of the deep sea. Henry, you're teeing up so many more episodes we could do in the future about these weird and wacky animals of the Cambrian and so on. But it just goes to me to say now it has been a pleasure to see you in person for the first time and to have you on the show. Well, thank you. As always, it's a great pleasure and it's a lot of fun to be here.

Actually, in person for once, may there be many more. Well, there you go. There was the one and only Dr. Henry G. shining a light on biology's Big Bang on the Cambrian explosion. I hope you enjoyed the episode. If you want more episodes with Henry on the evolution of life on our very, very, very distant prehistoric past, then you are in luck. We have done episodes with Henry on the origins of life on earth and also several on dinosaurs as well. So we'll put a link to a couple of those episodes in the show notes if you want more of the legend. That is Dr. Henry G. In the meantime, thank you so much for listening to this episode of The Ancients.

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