June 17, 2022
The Cambrian explosion is commonly labelled as the time in Earth’s history when animals suddenly appear. But research from geoscientist Rachel Wood and her team turns this explanation on its head. We talked with Rachel about how this research changes our understanding of how animals arose on Earth and about whether we can even call this event the “Cambrian explosion,” anymore.
Shane Hanlon: 00:00 Hi, Vicky.
Vicky Thompson: 00:01 Hi, Shane.
Shane Hanlon: 00:03 Question for you today. So let me back up. Do you visit the zoo? Is the zoo something that you do? Am I Dr. Seuss?
Vicky Thompson: 00:13 Dr. Do little. Yes, I like the zoo. I don’t get to go as much as I would want to, but I do like the zoo.
Shane Hanlon: 00:21 What’s your favorite animal at the zoo?
Vicky Thompson: 00:23 I love to visit the meerkats.
Shane Hanlon: 00:26 The meerkats.
Vicky Thompson: 00:27 Yeah. You know the little, they pop up.
Shane Hanlon: 00:32 Oh yeah.
Vicky Thompson: 00:33 With their little hands.
Shane Hanlon: 00:35 Did you watch the show back in the day, Meerkat Manor? Do you know what I’m talking about?
Vicky Thompson: 00:39 No.
Shane Hanlon: 00:40 It was an Animal Planet show. Oh, man. All right.
Vicky Thompson: 00:41 Oh yeah. Because I love their… I bet I would like that because I like their little families.
Shane Hanlon: 00:49 It’s interesting. I mentioned this before, but in a different context I used to volunteer at the zoo. And the specific program I did was it was called snore and roar. And so basically families, mostly families would camp out in tents and stuff in the zoo overnight. And part of that experience was a behind the scenes tour at one of the exhibits.
Vicky Thompson: 01:12 Was it always in the same spot as the zoo? Or in the zoo, did you get to stay by what’s your favorite animal? Did you get to be by it?
Shane Hanlon: 01:22 Yeah. We always camped out at the same spot. It was such a surreal experience because it’s right outside where the lions were. And so in the middle of the night, in the middle of DC, you would hear lions, wolves, and ambulances, all at the same time.
Shane Hanlon: 01:44 Science is fascinating, but don’t just take my word for it. Join us as we hear stories from scientists or everyone. I’m Shane Hanlon.
Vicky Thompson: 01:53 And I’m Vicky Thompson.
Shane Hanlon: 01:55 And this is third pod from the sun. All right. So before we figure out exactly why we’re talking about zoos, I want to bring in producer, Molly Magid. Hi, Molly.
Molly Magid: 02:10 Hi, Shane.
Shane Hanlon: 02:11 So Molly, what’s your favorite zoo animal?
Molly Magid: 02:13 I don’t know if it’s a favorite animal, but it’s more of an exhibit. Like when you go into the reptile and amphibian house, and it’s all sort of like dark and Misty and you’re turning the corners, and you’re seeing things. It’s such a great experience.
Shane Hanlon: 02:28 So that’s so funny. So I’m a herpetologist by training. So I studied amphibians, reptiles, that type of thing. And I want to love the reptile exhibits, but they’re always okay, not always, but a lot of them are kind of sad.
Molly Magid: 02:42 Oh.
Shane Hanlon: 02:44 I mean, maybe that’s unfair. I just feel like they kind of get the short end of the stick. It’s like, okay, here’s these tiny creatures, and now here’s these tiny spaces. Maybe it’s unfair for me to want them to have spaces that are as big as elephant ranges, or something like that.
Molly Magid: 03:00 Yeah. Maybe we should appreciate them more. Turn the lights on.
Shane Hanlon: 03:04 Oh yeah. Yeah. See, I like this. We’re going to start a campaign after we record this to get better reptile exhibits. Okay, Molly. So why zoos? Why are we talking about zoos right now?
Molly Magid: 03:17 Yeah. Well, today we’re talking about how animals appeared on earth. It took billions of years for the ancestor of modern animals to appear and even longer for creatures we see now at the zoo to evolve. In Carl Sagan’s famous calendar metaphor, which showed Earth’s history compressed into just a single calendar year, animals don’t even appear until late November. And that’s what we call the Cambrian explosion.
Shane Hanlon: 03:43 So I remember learning about this in school at some point in K through 12, but I don’t remember the specifics. The word explosion is in it so I assume that means that there was a ton of life just popped up kind of all at once.
Molly Magid: 04:02 Yeah. I would say that’s a pretty typical explanation of what the Cambrian explosion was, but today we’ll learn it might not be that simple. So we talked with Rachel Wood, a geoscientist who studies the Cambrian explosion. And Rachel’s been interested in the mystery of how and when animals appeared on earth for quite a while.
Rachel Wood: 04:29 Hello. So I’m Rachel Wood. I’m based at the University of Edinburgh Scotland in the UK. And I’m the Professor of Carbonate Geoscience.
Rachel Wood: 04:38 It’s been a particular interest of mine that I would say for at least 10, 15, maybe 20 years in understanding this phenomenon we know as the Cambrian explosion of Cambrian radiation. From about the age of six, I used to collect fossils, waiting for the school bus, finding them in the gravel. And I suppose I just simply never grew out of it.
Rachel Wood: 04:59 I mean, when I was an undergraduate about 100 years ago, the Cambrian explosion was described as the Cambrian explosion. That if you were walking up a succession of rocks from older rocks to younger rocks, all of a sudden you would be walking on the skeletons of animals and the fossils of animals. And there would be nothing or very, very little before and lots looking up the succession into younger and younger rocks. Of course, we did know about older fossils, but they were thought to be something totally different and totally disconnected and probably not animals. And if they were animals, not in any way related to the animals that are part of the Cambrian explosion, which are thought to be the origin, the ancestors of modern animal groups.
Molly Magid: 05:48 So I’d love to hear a bit more about some of your research into the Cambrian explosion.
Rachel Wood: 05:54 The number one question with trying to understand the Cambrian explosion is even framing the question, what is it? And if we can agree on what it is, when did it start?
Rachel Wood: 06:11 So if we take it in a really, really broad brush stroke, it is the appearance and rapid diversification of animal groups. And I suppose where I’ve got to in my thinking is that the Cambrian explosion was thought to be a Cambrian event, hence the name, because it was thought that the origin of all these major groups was indeed in the Cambrian, in the early Cambrian. But as we’ve discovered new fossils and in particular fossils much older fossils and particularly fossils with exceptional preservation, we’ve realized that it’s probably possible to make connections between Cambrian fossils and older fossils, fossils from the Ediacaran particularly the last few tens of millions of years of the Ediacaran. And they point to it to a connection rather than being two disparate sets of unrelated groups. Although this is all still very tentative, new work is suggesting that the origin of these major groups is simply not in the Cambria anymore.
Rachel Wood: 07:13 We can push it back, maybe in some cases, a few million years and other cases, tens of millions of years. So I think as we think long and hard and we put together these characters and more and more exceptionally preserved material comes together and put thinking of it as a whole, you can think of all these forms as being animals. And these are then just successive stages of the burgeoning of different types of animals appearing on the stage. And I just think it creates a very useful narrative to think that these things are successive radiations. It makes more sort of sense than cutting it up and compartmentalizing these separate bits of time.
Molly Magid: 08:07 So I’m trying to rethink about Cambrian explosion, given everything you’ve told me, and I’m curious if it was part of a larger range of events, and it wasn’t necessarily this larger range all in the Cambrian. Should we rename the Cambrian explosion or rethink it in some way?
Rachel Wood: 08:31 Well, some people really, yes, don’t believe it was a phenomenon at all. And it was neither an explosion nor was it Cambrian. Others do. I mean, again, it comes down to definitions. Now, if you want to define, for example, the Cambrian explosion as the origin of modern phyla, then you could argue that most of them are appearing in the Cambrian. The Cambrian is certainly distinctive with the record that we have, for example, the predation, we don’t have that sort of record deeper in time. But the way I prefer to think about it is life doesn’t appear from nowhere. It always appears with some precursor state. And if you take a wholistic view and sort of put all these fossils together that we see in the Ediacaran from about 575 million years ago until the Cambrian, some connections start to emerge.
Rachel Wood: 09:31 So for a long time, it was thought that these soft bodied fossils in the Ediacaran were totally unrelated to the Cambrian and indeed were not even animals. Now, some of them may not be animals, but there’s increasing evidence when you really pull apart their characteristics and detail that they are probably animals. They’re Eumetazoans, which are all the animals except the sponges. So the root, even of those animals that we always thought were absolutely Cambrian, they are turning up in the Ediacaran as well. So we’re doing two things. We are pulling those animals back into older and older rocks, but we’re also connecting the animals in those older rocks to the Cambrian animals. So when you stand back and you get a wider context, you can see that there’s a bigger picture emerging of putting together all these animals as a series of successive radiations, each one was fairly distinct. Bu if we just focus on the Cambrian alone, we lose all that context and we lose the context of understanding the origins and the harbingers of what happened in the Cambrian.
Rachel Wood: 10:48 So this whole interval of time, it’s probably only sort of 60, 70 million years or so, so much happens that is fascinating, but it doesn’t really just happen in the Cambrian. It is the Ediacaran Cambrian radiation that is so dynamic and so fascinating.
Vicky Thompson: 11:15 So the Cambrian explosion is a misnomer then? It’s neither in the Cambrian nor an explosion. So am I hearing that right?
Molly Magid: 11:22 Yeah, that’s pretty much the gist of it.
Shane Hanlon: 11:26 I feel like I’ve been lied to my entire life.
Vicky Thompson: 11:31 Oh, drama queen. How often have you actually been thinking about the Cambrian explosion?
Shane Hanlon: 11:37 Well, so as we heard earlier, evidently I don’t even remember it that much from school so, okay. I mean, I have been lied to my entire life, even though I don’t necessarily remember it. Okay. So using proper vernacular, the Ediacaran Cambrian radiation, what are folks finding as they look through the fossil record of this time?
Molly Magid: 12:01 Well, Rachel and her team, they’re looking for evidence of complex life during these periods, but that’s not so easy because there’s not just one definition of what complex life actually is.
Rachel Wood: 12:13 Well, you ask a different scientist, and they’ll give you a different definition. So many would say complex life comes with the eukaryotes, the complex cells which have a nucleus and organelles, and so on. Others might say, no, it’s multicellular eukaryotes. And then of course only a tiny subset of the multicellular eukaryotes are animals. But I suppose we all have a fascination with the origin of animals because we’re animals ourselves. And a lot of the world that we interact with closely is the animal world. And it’s a world that we have an affinity to. So this is where this is a sort of enduring mystery, really the origin of the animals.
Molly Magid: 12:56 How do you think the fossil record has shaped our understanding of the Cambrian explosion because of either what we’re able to find or what we’re not able to find?
Rachel Wood: 13:07 Yes. I think that’s absolutely critical, and therein lies a whole set of studies. Why are animals found in some places and not others? It’s just as interesting sometimes to realize that the rocks should be providing evidence of animals, but they simply don’t. Let’s just say they’re both lovely, shallow marine, warm tropical areas. Why should one have life not the other? And this leads me to another set of sort of ideas, which derive from geochemical analyses, which is that the planet certainly in the late Ediacaran and probably into the early Cambrian as well, was simply extremely heterogeneous with the distribution of oxygen. And we know that animals demand oxygen. It was certainly a necessity that animals must have arisen in environments with at least some oxygen. We still don’t know exactly how much or whether it was variable, but nonetheless, in the late Ediacaran and early Cambrian, this distribution of oxygen in the seas was very, very, probably very discontinuous, certainly heterogeneous.
Rachel Wood: 14:18 And there’s now an enormous amount of work going into try and understand where oxygen was, what was governing the distribution of oxygen in different continents. Because of course at this time we had a series of small continents, certainly by the Cambrian straddling the equator. And they all had their own regional redox dynamics in the shallow marine seas. In other words, the availability of oxygen seems to have been a regional or local phenomenon. So there’s a lot of work trying to understand where this oxygen was, how different was it in different places? How much was globally distributed? How much was locally distributed? And of course, hand in hand with that, how did that control, if at all, the distribution of animals?
Molly Magid: 15:13 So if, for example, there was more oxygen available in one place then maybe that created the conditions for more of this complex life or animals to emerge just from the fact that they could take advantage of the oxygen?
Rachel Wood: 15:31 Yes, it’s possible. I mean, obviously we’re not talking about changes in atmospheric oxygen here, which is global. But for example, the same bit of time in Namibia, we have dominantly oxygenated oceans with a slightly deeper ocean which had no oxygen. It was anoxic, but it was dominated by well, had a lot of free iron. It was called ferruginous. But exactly the same bit of time in south China, the conditions were far more anoxic, and we had an ocean with a lot of free sulfur in the oceans. It was euxinic. So in other words, a very, very different inventory and distribution of if you like habitable space. And not only that, the coming and going of oxygen was very, very dynamic in the shallow marine seas. And we don’t yet fully understand how that was being controlled and over what length scales the availability of oxygen was changing.
Rachel Wood: 16:29 And so what’s the relationship between innovation diversification and then any variable of the earth? Could it be climate, could it be redox? Could it be the availability of nutrients? So I don’t think we’re anywhere near really understanding in terms of a shared process of all the environmental variables, which we know were so dynamic during this bit of time. And how did they really, what is their real relationship, if any, to these extraordinary radiations? That’s what fascinates me. And I don’t think we really have the answers at all.
Shane Hanlon: 17:16 So a lot of this was actually happening at the local level, right?
Molly Magid: 17:20 Exactly. Living things had to adapt to their environment at a local level, which led to this more complex, diverse life when we zoom out to a global scale.
Vicky Thompson: 17:33 So then why explosion and why specifically in the Cambrian?
Molly Magid: 17:41 It does sound like over time as we find out more, the picture gets more complicated. Do you think, in some ways, the reason why we’ve had this narrative of things being very separate and having just the Cambrian explosion happen in one specific time period, is because it’s easier to think of it as just like one event rather than…
Rachel Wood: 18:10 Yes. I think it probably comes from the fact that, I mean, fossils are conspicuously much more abundant in the Cambrian. So anyone walking up through a succession of rocks would start to see more and more obvious fossils, particularly skeletal fossils. So I think it very often gives the impression that these things just explode into the record. They’re not there, then suddenly they are there. And they’re there sometimes in perfusion. But as I said, if you go to some of these key Ediacaran cavities, the fossils are there in perfusion as well, but they are perhaps a little bit more rare, perhaps a little bit more few and far between there may be aspects of the Cambrian explosion that we can cling to in terms of maybe it is the explosion of bilaterian animals, maybe it’s the explosion of the phyla.
Rachel Wood: 19:04 Maybe it is simply when things become more abundant. Maybe it’s when animals with skeletons become more abundant. These are all certainly more components. There’s no doubt that we have this sort of increasing complexity by any metric during this time. I mean, another fascinating thing that’s coming out with the Cambrian explosion, in itself, even if you take the Cambrian alone, it’s not just one event. There seem to be multiple events within the Cambrian certainly at least two waves when things rise and fall and rise and fall. So if you like, we can subdivide and subdivide and find all these little mini radiations. So this does beg the question, which one is the Cambrian radiation? Are they all the Cambrian radiation? And I prefer to think of it as the Ediacaran Cambrian. It’s all part of the same radiation, big radiation event. But within it, we have these multiple phases.
Shane Hanlon: 20:10 Henceforth, I will no longer refer to the Cambrian explosion as well, the Cambrian explosion, but I’m also not calling it the Ediacaran Cambrian radiation. That’s just a mouthful. So what do we call it?
Vicky Thompson: 20:27 I don’t think it can be relegated to one name, Shane. It’s so much more. I think it’s like something, something about things becoming more complex and abundant. It just has to be woven in as a descriptor.
Shane Hanlon: 20:39 You realize that’s almost literally killing me as a science communicator to make things more complex than less complex.
Vicky Thompson: 20:47 Well, isn’t explaining it just more better?
Molly Magid: 20:51 Isn’t it all about making things more complex?
Shane Hanlon: 20:54 Okay. That’s fair. So maybe we won’t be the ones to figure this out. But maybe our listeners can hit us up with some suggestions. But regardless of the name change, this isn’t necessarily uncommon for seemingly precedent scientific fact to be rethought, right?
Molly Magid: 21:14 Yeah, that’s totally right. And that’s one of the amazing things about science. Scientists are never set on anything. We always get to reexamine things and build on past theories. And Rachel says this uncertainty is part of what makes her research so interesting.
Rachel Wood: 21:31 Well, I think it’s one of these enduring mysteries of how complex life came to be on planet earth. It’s fundamental to our existence. I mean, the extraordinary thing about our planet is that it is supporting an extraordinary diversity of complex life. And we don’t even know the bare bones of how that came about. It’s just fundamental to our existence and to the existence of this extraordinary planet that we have, which as far as we know is unique. But I suppose it simply remains absolutely fascinating and enigmatic because it’s this extraordinary crescendo of the origin of animals and the animal groups that we know and love today, but it’s simply remains a conundrum.
Shane Hanlon: 22:31 So it sounds like especially when it comes to certain aspects of science or in this case, fossils, that nothing is quite set in stone. I’m happy that I at least crack myself up on this one. All right, folks. Well, that’s all from third pod from the sun.
Vicky Thompson: 22:55 Thanks so much, Molly, for bringing us this story and to Rachel for sharing her work with us.
Shane Hanlon: 23:00 This episode was produced by Molly with audio engineering from Colin Warren.
Vicky Thompson: 23:05 We’d love to hear your thoughts on this podcast. Please rate and review us. And as always, you can find new episodes on your favorite podcasting app or at thirdpodfromthesun.com.
Shane Hanlon: 23:15 Thanks all. And we’ll see you next week. Okay. So then Vicky, you start this one.
Vicky Thompson: 23:28 It’s Cambrian.
Shane Hanlon: 23:31 Well, so that’s the thing, is there a… I think you can say it both ways.
Vicky Thompson: 23:37 Cambrien or Cambrian. Cambrian’s not just like a weird accent thing, Cambrian?
Shane Hanlon: 23:41 So I just called it Cambrian.
Molly Magid: 23:42 I call it Cambrian as well.
Shane Hanlon: 23:44 Yeah.