Transcript of Episode 116 – Doug Erwin on the Cambrian Explosion

The following is a rough transcript which has not been revised by The Jim Rutt Show or by Doug Erwin. Please check with us before using any quotations from this transcript. Thank you.

Jim: Today’s guest is Doug Erwin, who is currently senior scientist and curator of paleobiology at the National Museum of National History of the Smithsonian Institution. I think that’s the one with the big, old elephant in the lobby, as I recall?

Doug: You’re right, Jim.

Jim: Yeah, been there a million times when I was a kid, grew up in the D.C. area and was shocked shitless when I went out into the world and found there were museums that charged for admission, how scandalous. Go to some third-tier regional museum, and they’d want six bucks and I’d say, “I can go to the best museum in the world for free, goddamn it,” but being that as it may, shall we continue?

Jim: His primary research interests are in evolutionary novelty and innovation across biological, cultural and technological domains, the evolution of animal regulatory genomes and the origin of early evolution of animals and the end-Permian mass extinction. In addition to his work at the Smithsonian Institution, Doug is also on the external faculty at the Santa Fe Institute. Welcome, Doug.

Doug: Hi, Jim, it’s great to be here.

Jim: Yeah, it’s really good to have you here. We haven’t chatted in a while. We’re going to talk today about one of my favorite topics, and we’re going to talk mostly about material from Doug’s book, which he co-wrote with James Valentine, and the book’s titled The Cambrian Explosion: The Construction of Animal Diversity. This has been a domain I’ve been fascinated with as long as I’ve known about it. I can’t even remember when I learned about it. I might have been 22 years old, something like that.

Jim: It’s just one of the coolest things in the history of life on Earth. So Doug, before we get down into excruciating detail, super duper high level in 30, 90 seconds, something like that, what happened in the Cambrian explosion? Why is it so very important to our understanding of life on Earth?

Doug: So life evolved probably three and a half billion years ago, say, give or take. And then, eukaryotic cells, which are cells that have a nucleus, appeared maybe two billion, well, .8 billion years ago and that gave us a diversity of microbial life, but nothing like plants or animals. And it wasn’t until somewhere about 800 million years ago, we think, that the first animals evolved. Those are, we think, animals with very few cells. They had multiple cells, but they were not the sort of thing that we see in the fossil record.

Doug: It’s not until about 570 million years ago that we see the first macroscopic fossils that look like animals, and then about 540 million years ago is the midst of what we call the Cambrian explosion, and that’s the appearance of essentially all the different types of animals that we find in oceans, so everything from sponges and corals to various kinds of worms and predatory arthropods, and then trilobites all the way up to the first fish. So we even have verb that says part of this explosion of animal life has been downhill ever since.

Jim: Yeah, exactly. We’re two fine examples of that. Yeah, that’s my thing I just found so amazing. Correct me if I’m wrong, which I may well be. Essentially, every one of the major groups of animal life that exist today existed by the end of the Cambrian explosion, which was, what, about 10 million years in duration, something like that.

Doug: Yeah. So all of the groups that we call phyla with one exception, which we call bryozoans and basically nobody cares about them, anyway. So it doesn’t matter. Bryozoans were probably around, but we don’t see them in the fossil records. So every one of these phyla that appear in the fossil records are part of this Cambrian explosion, and we also know that many of the poorly fossilized groups also originated in the same time, as well.

Jim: And that’s amazing, and we still think the Cambrian explosion took about 10 million years, something like that?

Doug: Well, it depends on what you pick as the beginning and the end, but it’s certainly not more than about 20 million years.

Jim: Yeah. To put that into perspective for our listeners, the time since the last common ancestor between chimps and humans was about seven million years ago, so something on the order of a little bit more than to maybe, at most, three times the time it took to go from proto-chimp to human, which ain’t much change. That’s the period in which every phylum of life, from insects to worms to crabs to mollusks, et cetera, all came into being. So this is a radically rapid evolution and diversification of life, again correct me if I’m wrong, and the world has never seen anything like that, either before or after.

Doug: No. So the one important but slight change I’ll make to what you said, Jim, which is that this is when they appear in the fossil record. One of the things I think we’ll probably get into in a little bit is that many of these clades may have originated earlier. So the Cambrian explosion is real, but it turns out to be an even more complex phenomenon than we thought a few years ago.

Jim: Okay, that’s a good clarification and it does make sense, which actually gets us to a couple of preparatory questions, I think, before we dig into this thing in more detail, which is some real basics on how is it that we tell time in paleontology? I don’t think there’s any old calendars or watches that you find down there in the rock layer. So how do we make these estimates of the time?

Doug: We go walk the rocks. That’s the most fun part of all of this for a geologist is that it means you have to go out in the field, and what we look for are basically fossilized volcanic ash layers. Layers of ash, if they come from the right kind of volcano, will have a mineral called zircon. And zircon, when it’s formed during a volcanic eruption, takes into its lattice uranium. That uranium decays, and then we take it back. I did this for years with my late colleague, Sam Bowring, at MIT.

Doug: Take it back to Sam’s lab at MIT, and they do a lot of analytical techniques, and they analyze the amount of uranium and the amount of lead. There are actually two decay systems. So they cross-check each other. And with that, we can get very, very precise dates on these events, down to the order of hundreds of thousands of years. And by going around the world to China, to Namibia in Southern Africa, to Russia and many other places, Sam and his group and a number of other labs around the world have refined this timescale for the interval that we’re talking about. So we often know these events very, very precisely. Well, precisely to a geologist is a few hundred thousand years, but for something 500 million years ago, that’s pretty remarkable.

Jim: It really is when we think about things like carbon dating for much earlier human-scale stuff where we can get down to a few hundred years, but over the range of half a billion years or a hundred thousand’s pretty damn remarkable, actually.

Doug: For work later, about 215 million years ago, we actually have resolved to hone down to 30,000 years, plus or minus. That’s pretty phenomenal.

Jim: Cool. Okay, next staging question before we dig into the details. You talk a bit about the difference upfront between micro and macro evolution. Maybe tell us a little about that distinction.

Doug: So one of the most pressing questions in evolutionary biology, actually since the beginning of the 1800s, even before Darwin was born, is whether or not evolution occurs on two levels. And micro evolution is the normal adaptive changes that happen with the evolution of different kinds of flies that occurs within populations and is often responsible for the evolution of new species. Macro evolution is larger-scale patterns that are associated with evolutionary novelties, with the appearance of major evolutionary changes, for example, the origin of birds or the movement from fish to land, from the first tetrapods.

Doug: Those are the kinds of macro evolutionary changes. Many evolutionary biologists would claim that micro evolution plus time equals macro evolution. So there’s nothing different about it, but one of the questions that has been raised for years by paleontologists is that it appears that there are some events that we have in the history of life that can’t be explained simply by micro evolutionary processes, and that’s what we call macro evolution.

Jim: And we’ll get into some of those examples later. And then, now the framing that you put up front in the book, which we will refer to at least a couple of times during the book, is that you point out there are three networks or three things to consider when thinking about the Cambrian explosion. Could you go through those for us?

Doug: Sure. So when I think about these large evolutionary events and what all of us do as paleontologists, there are three different aspects of it that we need to consider. The first is the condition of the environment, what we might call environmental potential, whether or not the environment and this includes temperature, climate more broadly but also things like the amount of oxygen that’s available in the oceans, whether or not that has the potential for further evolutionary success.

Doug: And then, there’s also ecological opportunity. What’s the network and interactions among organisms, and does that allow certain types of evolutionary changes to occur or does it limit those? And the third is genetic or developmental changes, so the network of interactions between genes that allow you to produce, for example, large-scale organisms.

Jim: Gotcha, that’s very helpful. And of course, there’s effects that transcend across those three levels as well, right? So clearly, the nature of the food web’s going to look different in a world with lots of physical capabilities, just as one example.

Doug: Right. And one of the things that I’m most interested in is how the evolutionary process itself has evolved over time, and part of that is because the environment is more permissive of complex organisms now than it was a billion years ago. Part of it’s because the ecological interactions are more permissive, but part of it’s also the changes in genetic and developmental potential, as well. It’s the interaction between all three of those, and one of the things that we see in mass extinctions is that the environment changes in such a way that you can’t have sophisticated ecosystems for a while, and then everything dies.

Jim: Makes sense. One last preliminary question before we dig in, always what I’m interested in, which is what did the continents look like at the times we’re talking about, 700, 600, 500 million years ago? They didn’t look like they look today. I know that.

Doug: They didn’t look anything like they did today. Most of the continents were in fairly equatorial and southern latitudes. So many of the continents, including North America, didn’t exist in the form that it is today. North America was created by a lot of island arcs, like Japan, being plastered against the side of it to form, first, the Appalachians, and then eventually the Sierra Nevada on the other side. There were many more continents that were much smaller and were spread out around the equator, and then down into southern latitudes with far fewer continents north of the equator.

Jim: Okay, there’s a really nice drawing in the book. And by the way, I didn’t mention this on the intro. I should. I’ll just mention it now. The book is beautifully made. I was in the publishing business for part of my business career, and so I got a bit of an eye on what a quality book looks like. This thing has really nice paper, beautiful typefaces, astoundingly nice four-color plates, maybe even more than four-color plates. I pulled out my loop and actually looked at them, and the color separations are perfect.

Jim: If you’re a person who appreciates a book as an artifact, this book is actually truly beautiful, and there’s some very nice drawings of continental change, et cetera, around the time Doug is talking about. So now, with those base things behind us, let’s take us on a little evolutionary journey. Why don’t we start with the last known unicellular ancestor of our animals or metazoans, as y’all call them, and maybe take us up through the whole period before the Cambrian. Was it the Ediacaran?

Doug: Ediacaran, yeah.

Jim: Ediacaran period, and then stop at the Cambrian. We’ll get rolling again. So Doug, ready, set, go.

Doug: So we’ve learned a lot since the book was published. Some of what I’ll say right now is different than what’s in the book because of new discoveries over the past eight or nine years. We start, perhaps, 900 million years ago with a number of unicellular groups that are collectively called the Holozoa, H-O-L-O-Z-A, and that’s a bunch of groups that turn out to have some but not all of the same capacity as animals in the sense that they sometimes made small multicellular entities.

Doug: We don’t see any of this in the fossil record, and we see nothing in the fossil record until about 570 million years ago. And the best locality, if you want to go see these animals, is up in Newfoundland at a place called Mistaken Point. It’s just down the road from where the telegraph signal from the Titanic was first picked up when it was sinking in the North Atlantic. And at Mistaken Point, we have some meter-long fossilized fronds and some round discs, a whole variety of different organisms that are on flat, on bedding plains.

Doug: They were discovered decades ago and have been intensively studied for many years. That’s the first occurrence of this Ediacaran biota. It’s named after the Ediacaran Hills in South Australia, but it’s also found in England, in China, in Australia, in Namibia, in Russia and a number of other places. All of these organisms are frond-like. Some of them later could move. These are disc-like forms that have left ghost-like impressions in the sediment, but none of them have a mouth.

Doug: None of them have appendages. None of them have eyes. A couple of them might have a gut, but even that’s not found in most of them. So we have this Ediacaran biota that lasts from about 570 million years ago until 539 million years ago. So just over 30 million years with these impressions of soft-bodied organisms, some of them more than a meter-long. So these are not insignificant organisms. They’re more complicated than a jellyfish, but exactly what they are is something that’s still debated. In fact, with one of my postdocs, we had a paper out last week that tries to examine the developmental tools that might have been required to build some of these organisms.

Jim: And even though they’re frond-like, we’re pretty clear they’re actually animals rather than early plants?

Doug: Yes, for a variety of reasons. There are some things that we have no idea what they are, but there are actually lots of fossil algae in the record, and these are not anything like fossil algae. They’re not lichen or anything like that. And for a variety of reasons, we’re fairly confident now that most of these groups are somehow related to animals.

Jim: Very interesting. And you referenced it earlier and during this period, especially in the late part of the period, the levels of oxygen in the water started to increase. Tell us a little bit about that.

Doug: So the environments of the time were completely unlike anything today. During this interval between 800 million years ago and 540 million years ago, there were two extensive glaciations called Snowball Earths, where the Earth basically seems to have frozen over, like a giant snowball. And in the aftermath in the second of those glaciations, we think there was initial increase in the oxygen, but exactly when that happened is very contentious. In general, the oceans at that time had very low oxygen levels.

Doug: How low is still unclear. Was it 1% or 5%? That actually makes a big difference. We don’t know and one of the most contentious issues in this field of geology is what kind of geochemical tools you can use to infer past oxygen levels. All of them point to very low oxygen levels. Some of them suggest that oxygen may have begun increasing 800 million years ago. Others say maybe it was 600 million years ago. So that’s still an area of very active research.

Jim: Okay. That’s a little bit of a sidebar, but it’s something I’m personally interested in. You mentioned this Snowball Earth. How the hell did we get out of that?

Doug: Probably because if you sit in a Snowball Earth long enough, eventually enough volcanoes will go off and spew CO2 into the atmosphere. Things will begin to heat up. So it’s probably basically waiting for enough volcanic activity, the increased CO2 levels, that the ice begins to melt. And climate models are highly complicated, but basically once you get glaciers down to about 30 degrees latitude, the climate system flips and the oceans freeze over down to the equator, but conversely once the things begin to heat up, they shift back very rapidly on the timescale of thousands of tens of thousands of years. Everything melts, so you can flip from one of these systems to the other very, very quickly.

Jim: That must have been stressful for life.

Doug: It was and that’s one of the big problems. If these animals actually evolved by 800 million years ago, which is what we have some evidence for, then how did they get through these two tremendous climatic disruptions? That may actually be telling us something about the presence of refugia at equatorial low levels that allowed some of these organisms to persist.

Jim: And so, maybe it wasn’t entirely a Snowball Earth. Maybe there was a hot water spot or something that allowed some of them to survive.

Doug: Yeah. And if you think about it from a geometric standpoint, even if you have ice down to the poles, because the Earth isn’t destroyed, you’re still going to get a lot of tensions between ice flows and cracking the way you see ice in polar latitudes today. So there’ll still be things opening up that will allow life to go on for a while.

Jim: Okay. So let’s make sure we know we’re on track here. This life is Ediacaran life. It was multicellular.

Doug: Yes, it was absolutely multicellular, but almost none of them have any evidence for a mouth or appendages or anything like that.

Jim: Was it the same multicellularity that you and I are, or at least I am, goddamn it, or was this something that was invented later during the Cambrian explosion? Is it a disjunct multicellularity? Because quite interestingly, you point out in the book multicellularity’s been evolved. I think you said, at that time, your best estimate was 20 times. Was they us or are we something, an innovation, since then?

Doug: Yes, probably both. So some of these organisms may actually be an independent development of multicellularity. Most of them though, we think, are probably related fairly closely to animals. For example, there’s a little three-inch long thing, found up in Russia and Australia, called [cambriella 00:22:03]. It was probably related to mollusks. So some of them, we can at least figure out who their cousins are. Others are sufficiently weird that we don’t really know where to put them in our evolutionary tree.

Jim: Okay. And of course, that was a long time ago, and a lot of them didn’t have much in the way of structural elements to be preserved in the fossil record, as I recall you writing.

Doug: Right, these were all soft-bodied. None of them had skeletons. Skeletons don’t appear until the very end of the Ediacaran. When we go to Namibia, which we have done quite a lot since the middle 1990s, we see reefs developing just before the Cambrian that have some of the first skeletal elements in them.

Jim: Okay. So now, let’s take ourselves up to the transition between Ediacaran and the Cambrian, and let’s revisit what the world looked like in your three lenses, the three networks.

Doug: Okay. So by this time, we think there’s at least some oxygen in shallow water ecosystems and probably some oxygen in the atmosphere but far, far less than what we have today, maybe 1% of what we have today. The environments, we’ve gotten through these thick glaciations. So the world was warmer, but the oceans probably, at least in the deep ocean, was probably still anoxic. So oxygen may have been limited to the shallow waters, so very different than today. The ecosystems, just before this Cambrian radiation started, probably had no predators.

Doug: So there were these organisms, some of them a meter, a meter and a half, long, laying around but fairly few things eating them. One of the things that I haven’t mentioned yet is that the ocean bottom was covered with microbial mats and most of these Ediacaran organisms, many of them, probably fed on these mats, digested the mat as a source of food. So the ecosystems, the networks are very flat in terms of hierarchical complexity of them and the developmental systems, the third of the networks, is also relatively simple.

Doug: So we have a number of cell types, but we don’t have very sophisticated patterning mechanisms to produce the complexity that we see in many modern animals. And except for the Ediacaran, we think that many of the animals at this time were still very, very small. That’s a important clue that we’ll get to in a sec.

Jim: Okay. So to keep us on our timeline, we’re now here about 540 million years ago. Is that about right?

Doug: That’s exactly right.

Jim: Okay, then what happened?

Doug: Well, then, within the span of 10 to 15 million years, basically everything appears in the fossil record. So many of these groups of organisms, what we call a clade of related organisms, may have already been present, but what happens at the Cambrian is that they achieve larger body size. They develop new ways of cell interactions and regional patterning to produce large organisms, trilobites and sponges and worms and things like that, and all of those things interact in new ways ecologically.

Doug: So they build new ecosystems that have lots of predators. They have organisms with eyes that can find things to eat or figure out how to flee from a predator. So all of what we think of as a modern ecosystem in the ocean appears during this time, and all of the major groups of animals appear during this time. So it’s a remarkable event in the history of life. There are only about five or six really interesting events in the history of life, and this is certainly one of them.

Jim: Yeah, you mentioned in our chat just then and the book mentions it as well in references, other theoreticians, that the emergence of predation had a big impact. Maybe you could tell us a little bit about how that worked and when it occurred and what the impact was.

Doug: Actually, a paper that came out about two weeks ago suggests that the origin of predation may have been slightly before what we thought it was three weeks ago. So this is still something that is debated, but as far as we can tell, most of the organisms in the Ediacaran did not have the capacity to eat other animals, but at the base of the Cambrian explosion, we see the first mouse. We see a lot of organisms that are clearly preying on other organisms, and that predation allows and indicates that ecosystems are becoming more complicated, because once you can prey on something else, then you can prey on your young.

Doug: Animal A preys on animal B, but then animal B responds, right? It either figures out how to get away from animal A or builds a hard part so that animal A can’t eat it, and that’s part of what we see in the Cambrian. The reason the Cambrian explosion has been known since about 1830 is that that’s the appearance of durably skeletonized shells and bones and things in the fossil record. Darwin knew about the Cambrian explosion. He worried about it a lot in, I think, chapter nine of the Origin of Species, because it was a real challenge to his ideas.

Doug: How do you go from these long sequences of rocks in South England and further north and have no fossils, and then all of a sudden, almost on a single bedding plain, a single horizon in the rock record, you find all of the major groups of skeletonized fossils? So the record is certainly more complex now than it was when Darwin was examining it in the 1850s, but the basic picture is still very similar.

Jim: Yeah. So it sounds like what we could say is that predation, as it usually does, generates an arms race, right? One of the results of that arms race was armor, as happens in human arms races in history and armor, titanous bodies or mineral shells, et cetera, get preserved in the fossil record a hell of a lot more than the soft-bodied creatures from before. So perhaps, it looks maybe more stark actually than it actually was in terms of biomass. Is that reasonable?

Doug: That’s exactly right. And predation, of course, is also one of the easiest things to pick up. There were probably lots of other things going on, as well. So building a skeleton is one way of dealing with a predator, but the other way is swimming fast. And so, you also see an increase in organisms that could move fast or could burrow down into the sediment. If you’re a worm, one way to get away from a trilobite that’s trying to eat you is to burrow yourself down in the sediment.

Jim: Yeah. So all these kinds of dynamics really fit into your ecosystems, species versus species, second network, what I colloquially call who ‘et whom.

Doug: Exactly.

Jim: Yeah, that’s a huge driver. You mentioned the burrowers and the sediment and all that. That also has an effect, alluding back to your first model, where now we’re getting to the point where the creatures are starting to modify the environment itself. Tell us a little bit about that.

Doug: Organisms probably modified, certainly modified, the environment before this, but one of the things that happens as worms begin to burrow in the sediment is that that changes the geochemistry of the sediment. It creates a habitat for those organisms to live in. It creates a habitat for other organisms to live in. If there’s a big burrow there, you don’t have to build your own burrow. You can go live in a burrow that somebody else already built for you. So that creates new habitats for other organisms.

Doug: And even if there hadn’t been this increase in shells and other skeletons at this time, we would know that something profound had happened at that point in the rock record, simply because the sediments change. The sediments before this in the Ediacaran, they look like one pancake after another. They’re very well-layered sediments, but then once you get to the Cambrian, this burrowing activity destroys those horizontal layers of sediment. You get everything mixed up, and that tells us that something has happened, even if we didn’t have the other fossil record.

Doug: Because those events happened so closely in time, that also tells us that we didn’t have a lot of large worms that weren’t preserved as fossils 10 or 15 million years before the Cambrian explosion. So it gives us a check on what’s happening biologically by looking at this other kind of record.

Jim: That makes sense. And also, there were second-order facts, like the amount of sediment in the water changed, which had some impact in the nature of the niches that could emerge. And as I recall, in the book, there’s also some expectation that these burrows would have brought oxygen down into the sediments, which would again provide opportunities for animals to evolve for those niches.

Doug: Right. There are a couple things going on at this time. The activity of the organisms is increasing the amount of oxygen that’s in the water column. So progressively, deeper parts of the ocean are available for organisms to live in, because there’s more oxygen in the water column, but the burrowing activity also brings oxygen down into the sediment, and that means that more of the sediment is oxygenated and that creates a habitat, as well. And it also stimulates the growth of algae, so it’s like farming. You’re mixing up the sediment, and that actually allows these microbes and these microbial mats to increase their rate of production, as well. It’s like plowing a cornfield in Iowa.

Jim: We’re starting to get all these new phyla that are growing out of these new opportunity spaces. Do we have a sense of what some of the earliest phyla to emerge were?

Doug: It would be nice if the first phyla to appear were sponges, and then sea anemones and jellyfish, and then eventually we get to arthropods and vertebrates and things like that. The record is complicated by the fact that not all groups of organisms are easily fossilizable. So we have two remarkably well-preserved windows into the environment of this time. One is called the Chengjiang fauna, which occurs in the Yunnan Province in China, about 60 different localities.

Doug: And the other one, which is slightly younger, is the Burgess Shale fauna that occurs up in British Columbia in Canada, and that was discovered by my predecessor, Charles Walcott, at the Smithsonian in 1909. The Chengjiang fauna was discovered in 1985. And both of these deposits preserve soft-bodied organisms that otherwise wouldn’t be preferred in the fossil record, but it’s a very different kind of preservation from this earlier Ediacaran biota, but we have lots of different kinds of worms and arthropods and sponges and algae and many things that we don’t still entirely know what they were, but unfortunately some of this Cambrian explosion, some of these lineages first show up in the Chengjiang biota in Lower Cambrian in China all at the same time.

Doug: So it makes it much harder to tell who came first. We’re pretty sure that trilobites come after some other arthropods, but it turns out to be very difficult to really pull apart what is occurring before what because of these preservational problems.

Jim: Interesting, and I guess it happened so fast, too. It tends to blur the record.

Doug: Yes, we have to have very precise age dating, and then we have to be able to correlate between Mongolia and South China and Russia and the US and Morocco. And when you’re doing that on the scale of a few million years, that can become very difficult.

Jim: Interesting. Now, one of the big emergences, which is of interest to us personally, is the bilaterians.

Doug: Yes.

Jim: Where do they come along in the story, and who are they?

Doug: We are bilaterians, because we are bilaterally symmetrical. We have a head at one end usually, not always, and then a tail or appendages at the other end, and we move preferentially in one direction. So sponges and corals and various other groups like that are pre-bilaterians, but then everything else, flies and vertebrates and fish, arthropods and mollusks and brachiopods and everybody else is a bilaterian. So the most important branch in the tree for understanding this event is the node that is the last common bilaterian ancestor is the node where all of these different groups split.

Doug: And one of the things that I’ve been most involved in is trying to estimate what the age of that ancestor was and how complicated they were, and it turns out that there are several different ways of doing this, because it relies both on evidence from the fossil record as well as utilizing something called a molecular clock, which we use molecular sequences, usually DNA, from living organisms. And then, we use some statistical techniques to compare those sequences of living organisms and estimate when the last ancestor of those diverged.

Doug: So we might take a clam and an arthropod, and go back and see where the last common ancestor of the clam and the arthropod was, and then we can take a fly, which is an arthropod, and compare that to a chicken or to a fish, and that would give us the last common bilaterian ancestor. Last time there was a plant which there was a split that led to, on one side, arthropods and, on the other side, fish and other vertebrates, including us.

Jim: Interesting, yeah. So that was a big, big important split in the world history of life.

Doug: Right. Most of the interesting things that happened is part of this Cambrian explosion is the appearance of these large-bodied bilaterian organisms that burrow and eat things and have great eyes and all that sort of thing, and there’s a difference of opinion among many of us about whether the bilaterians evolved close to the Cambrian explosion or whether they actually had existed for some tens of millions of years already before the Cambrian explosion. And then, something else, probably in ecology or the environment, changed at 540 million years ago that allowed these larger organisms to appear in the fossil record.

Jim: You mentioned a couple times larger organisms, and one of the things I took away from the book is that, until there are circulatory systems, there are some essentially chemical, physical, constraints on size. Could you talk us through that a little bit and, in fact, there was a quite cool little equation that related the size that an organism could be relative to the concentration of oxygen in the absences of a circulatory system. So maybe you could take us through the history of that, and where do we think circulatory system first came into being and what impact they had on things?

Doug: Okay. So imagine that you’re a little worm. If you don’t have a way of moving oxygen around in your body, you’re relying completely on diffusion of oxygen in through the outer layer of your body into your tissues. If you have a circulatory system, the oxygen either can come in through lungs or through other ways of getting the seawater in, or it just diffuses in through the outer layer and is then absorbed, but a circulatory system, of whatever means, is a way of taking oxygen from the outside, and then distributing it through the body and because we need oxygen for metabolic systems.

Doug: Having a circulatory system allows you to get much larger and the more efficient your circulatory system is, the larger you can get. So lots of organisms have what are called closed circulatory systems, like we do as vertebrates, with arteries and veins and hearts. And having a circulatory system also requires a pump, in many cases requires a pump, to keep that flow going and make sure that the oxygen gets out to the tissues, and then the blood or other sorts of fluids that now lack oxygen come back and get recharged with oxygen so that they can then flow out and give the oxygen to the tissues that need it.

Doug: And systems are absolutely essential to getting organisms of more than a couple of centimeters’ length. You just can’t build a large organism, unless it’s a completely flat pancake, without some kind of circulatory system.

Jim: And when do we think the circulatory system entered history?

Doug: Well, we think the circulatory systems entered probably well before the Cambrian, but they were really simple, because the pump was really simple. So you have to get a more sophisticated pump in order to allow a larger body size. The circulatory systems may have been there, but they were pretty unsophisticated. And curiously enough, one of the ways we look at this is by comparing the regulatory genes that are responsible for things like a heart. There’s a network of genes that we call a kernel that is responsible for controlling the development of a heart in vertebrates as well as in flies and echinoderms and things like that.

Doug: So that means that the last common ancestor of bilaterians had this network of genes that controlled this activity. It gets complicated, but we think that the initial role of these genes was just for a few cells that had a contractile muscle function. So it wasn’t a heart the way we think of it in ourselves. It was just a few muscles that would contract and allow blood to be circulated around the body of a worm or a simple arthropod or something.

Jim: Okay. So we had primitive circulatory systems. So was there a substantial upregulation in circulatory capacity with the bilaterians?

Doug: Yes, and that goes along with a lot of other developmental changes, as well. So you have to be able to produce different parts of the body, different segments of an arthropod or limbs or eyes or a gut. And to build all those different parts of an animal requires the circulatory system to bring nutrients and oxygen to those tissues. And of course, the more you move, the more oxygen you need. So if you’re just sitting there, like the Ediacaran organisms, you don’t have nearly the oxygen requirements than you do if you’re scuttling around on the seafloor or burrowing or trying to swim.

Jim: Yeah. When I think of a nonbilaterian, I think of something like a starfish. Is that right? Does a starfish have a circulatory system?

Doug: They do, they do. It’s a simple circulatory system. So a starfish, it’s not bilaterally symmetrical, but it’s an echinoderm and echinoderms are within the bilateria. So they’re after that bilaterian split. They just went the other way and became radially symmetrical or pentaradial in the case of echinoderms rather than being like a fish or a mollusk or something.

Jim: Okay, well, that’s interesting. I learned something. The day is complete. That’s a good thing, all right. So circulatory system upgrades, big important thing, and it allowed the animals to grow considerably bigger as they continued to ratchet up, presumably under some arms-race dynamics, the power of their circulatory systems.

Doug: Right. So there’s feedback between all of these systems. You get one thing in development, and then you do something else ecologically. And then, you want to do something else ecologically, because the environment changes. So there’s constant feedback between all these things and because we’re now able to resolve the timing increasingly better, we’re beginning to puzzle out what led to what, but the causality is still very complicated.

Jim: Very interesting. In the book, you do talk about circulation quite a bit, but one of the things you didn’t talk much about… In fact, maybe not at all. I looked in the index and the word didn’t even appear there is when did neurons appear? Didn’t they also appear around the same time?

Doug: Yeah. So the reason we didn’t talk about neurons, because it was getting very complicated then. There has been a fairly substantial debate about when neurons evolved and when neurons coalesced into nervous systems, and that’s because there’s a debate about the actual structure of the evolutionary tree of animals. There’s one group of people who believe that sponges are the least primitive organism, and that means that nervous systems evolved once. There’s another group of people that believes that a very unusual, very predatory group called ctenophores, which are more complicated than sponges, are actually older than sponges.

Doug: And if that was true, then nervous systems would have evolved twice. So the evolution of nervous systems is actually a really critical issue, because they transfer information. They control muscular activity. They allow you to do all sorts of different things, but it’s very hard to infer when those appeared in the early evolution of animals. And I’ve been to several meetings in London actually at the Royal Society, just about the evolution of nervous systems. And this is an area that attracts paleontologists and developmental biologists and philosophers.

Doug: There are a whole bunch of philosophers that are interested in when did the first nervous system appear, and what does that mean for being an animal? But when we wrote the book, it was so contentious that we skipped that, hoping that nobody would notice-

Jim: I noticed, goddamn it.

Doug: … which you just did.

Jim: Do you have a preferred guesstimate? You made it clear that nobody knows the answer truthfully, but you’ve wallowed in this evidence for much of your working career. What’s your thought?

Doug: So my view is that the sponges are the base of the animals rather than ctenophores and I think, for a variety of reasons, that’s fairly clear at this point. And that means that neurons evolved once, and they coalesced into nervous systems maybe twice, but I actually think nervous systems probably evolved about 750 million years ago. So if you look at modern cnidarians, sea anemones and jellyfish, there’s a new technique used by developmental biologists called single-cell RNA sequencing, which means that you take an embryo of a jellyfish and you break all the cells apart, and you look at what genes are active in each different cell type.

Doug: And then, you do a bunch of statistical stuff, and you come out with these beautiful colored plots of the number of different cell types, the number of genes that are active in each cell type. And when you do that, you discover that cnidarians actually have far more neurons than anybody ever guessed, just from looking at a sea anemone. And if these molecular clocks that we use are correct, that probably means that neurons were relatively diverse by 700, 750, million years ago, so even the Snowball Earth events.

Jim: Interesting. So neurons go way back.

Doug: Neurons go way back, but remember, I also said the other question is when there’s a brain. When do the neurons coalesce into a central nervous system? And I think that much happened much later, and it happens independently in these different bilaterian groups. So the brain of a fly is not the same as a brain of a fish to my mind.

Jim: When you say it’s not the same, what’s that mean, that they have different evolutionary history?

Doug: Yeah, they evolved independently of each other.

Jim: When do we think that split occurred, or when did these mutual evolutions, these separate evolutions, happen?

Doug: So the critical question in all of this is when was that last common bilaterian ancestor that you asked about a few minutes ago, and my guess is that that’s around 600, 630, million years ago, so maybe as much as 100 million years before the Cambrian explosion. It’s possible it’s even younger than that, but that means that these different groups that already split into different branches long before the Cambrian. And that gives them the time that they needed to independently evolve brains and eyes and guts and limbs and things like that.

Jim: Okay, well, that’s interesting. So that’s really not part necessarily of the Cambrian explosion story, though obviously it’s part of the context once it gets rolling.

Doug: Yeah. Well, it’s critical to understanding what the relative contribution of these three different networks are. So remember that we had the physical network. We had the ecological network, and we had the genetic and developmental network. And part of what we’re trying to understand is when did the changes happen in each of the networks that allow animals to evolve. So one answer is, well, they all changed at the same time, and everything happened 540 million years ago, and it’s just a giant mess.

Doug: And that’s possible, but it’s unsatisfying. So part of what we’re trying to figure out is was it possible that there was enough oxygen to make a large animal 700 million years ago, but something else had to happen, or did the environment not allow big animals until 540 million years ago? And similarly, with development, we want to try and figure out how the tree branched and when those branches happened so that we can figure out whether it was, for example, the origin of a brain that allowed this Cambrian explosion that we see or whether brains maybe were already there, and then something happened ecologically that allowed the Cambrian explosion.

Doug: So we’re trying to pull out the different threads of causality through all of this, which is why we all fight about things a lot, but it’s also critical to try to understand the contribution of these three different networks that we’ve talked about.

Jim: Very interesting. And so, what’s your view on when brains happen for the first time?

Doug: I think brains actually evolved very close to the Cambrian explosion. I think that’s one of the events that is certainly necessary before you can get to Cambrian, along with guts and eyes and things like that, but I think that’s one of the things that’s happening during the Ediacaran, but my guess is that both the developmental changes and the environment were probably permissive to make large animals before we see them in the fossil record, and the critical component may actually be ecology. It may be this network of predation and other ecological interactions that allowed the Cambrian to be so explosive.

Jim: Then, one could easily imagine how ecological who-ate-who niche could drive the formation and perfection of brains, as an example.

Doug: Right. And it’s not only who ate whom, but it’s also this process of niche construction of organisms building and changing the environment, both for themselves and for other organisms that provides a lot of this feedback.

Jim: You anticipated me by one topic. I know this is one of your favorite areas, niche construction and ecosystems engineering. What is that and why is it important to the story?

Doug: Okay. Niche construction and ecosystem engineering are the abilities of organisms to make habitats for themselves or for other organisms. Darwin’s last book was on what? Do you remember, Jim?

Jim: No.

Doug: It was on earthworms. His last book was on earthworms, which he was fascinated by. He actually had a stone out in the garden at Down House, and he measured the activity of earthworms in building the soil and burying the stone. And Darwin knew that earthworms basically build soil and that allows more earthworms to exist, but it also allows all sorts of other organisms to exist and for plants to grow, and that activity is not limited to earthworms in soil. It occurs with lots of organisms, but one of the things that we’ve argued is that there’s a big increase in this niche construction activity that occurs right at the Cambrian boundary.

Doug: Ediacaran organisms don’t seem to engage in the same level of niche construction activity as organisms in the Cambrian. There’s a big change. That’s part of what we see in the change in the amount of burrowing that happens. I usually focus on ecosystem engineering. The two things are very similar concepts that were promoted by two different groups. Niche construction technically requires the ability to detect changes in what evolutionary biologists call fitness and as a paleontologist, I can’t do that.

Doug: I can’t tell fitness for a trilobite 530 million years ago. So we focus on ecosystem engineering, which is just the ways in which organisms change their environment that is advantageous either to that species or to another species. I mean, niche construction is what you’ve worked on your entire life, Jim. I mean, that’s what technological evolution is about, creating environments for various kinds of technology to appear.

Jim: Hell, just our houses and our clothes, right? We wouldn’t be living up here in mountains of Virginia if humans hadn’t created a niche for that.

Doug: Exactly.

Jim: Okay, I think that gets the idea across very well. Well, now let’s turn to the third network, the genomic networks and the very closely related developmental machinery. So maybe if you could make the distinction between those two to start, and then hop in from there.

Doug: Great. So our genome is not simply a sequence of bases of DNA that get transcribed into RNA and then translated into proteins, and then you have a fly or me or Jim or something like that. The genome is composed of a whole lot of interactions between different bits of DNA. Some of those things control the activity of a gene. Others change the structure of chromosomes. They uncoil and coil and do all sorts of things, and it’s the sum of those interactions really is what is involved in the genome.

Doug: And one of the things that we’re trying to understand in the early evolution of animals is how these changes in the genome allow a sequence of more complicated entities to exist. The first one are different cell types. So how do you get a muscle cell that’s different from a nervous cell that’s different from a skin cell? Then, how do you put those cells together to form tissues or organs? You need bodies of cells of the same kind that interact, and then how do you pattern those cells during the growth of an organism so that you get the gut in the right place and the appendages sticking out on the side and the brain and the eyes at the front end and so forth?

Doug: So you have to not only get different cell types. You have to get those cell types into tissues, and then you have to have the tissues patterned in the growing embryo, the early animal, in the right places to produce the morphology that we see in the fossil record. Actually, a lot of us want to understand not just what the genes are that distinguish flies from fish or whatever, but how those genes act to produce tissues and different parts of the growing embryo.

Jim: Yeah. So we actually have the machinery that creates things like torsos and appendages and such, and then we have the genes that drive the creation of that machinery. Is that pretty close?

Doug: Yeah, exactly. I mean, you need all the heart cells in the right place to make a heart, but then if you’re making muscles, you need muscle cells. So it’s different parts of the body to act in the right way so that appendages can work and things like that.

Jim: One of the things that you talk about quite a bit in the book, and I know I’ve read about it elsewhere is the importance of Hox genes in this work or what you call Hox gene clusters. Maybe if you could tell folks what Hox genes are and what role they play and how they vary between species.

Doug: So one of the most exciting developments in the past for 30 years in biology has been the discovery that all animals share far more genes than anybody imagined in even 1990 so that there is some genes that are active in the developing eye in a fly. And the homolog, the ancestral equivalent of that gene is also active in the developing eye of a vertebrate, like us. And that’s true for many different parts of the organism, and one of the earliest examples of this that was discovered are these genes called Hox genes and those pattern the body from in the developing embryo.

Doug: Those genes come on in a sequential order with the first gene in this cluster comes on toward the head. And then, if you look at a developing fly embryo, they come on in sequence telling this part of the body, “You’re up by the head. This next part is the thorax, and then this part is the abdomen,” and so on. And so, using some laboratory techniques, you can actually get these wonderful color maps, essentially, of the embryo with different colors showing where the first Hox gene is turned on, and then the second Hox gene and the third Hox gene.

Doug: And the same Hox genes are found in both vertebrates and flies as well as lots of other animals, as well. And so, now by looking at lots of different living organisms, you can tell how many Hox genes they have, when they’re turned on and look at when this cluster of Hox genes was assembled, and the sequence of Hox genes evolved by duplications. So initially, you had one or two Hox genes, and then those get duplicated. And so, then you have four, and then you can duplicate that and you have eight, right? By gene duplication, you can add these and by adding the number of genes, you can then pattern the body from the front end to the back end.

Jim: And do we believe that the, more or less, same mechanism was what was driving the amazing body plan diversity back in the Cambrian explosion?

Doug: Yeah, I mean, the wonderful thing about being able to compare living flies and living fish today is that you’re peering back to the last common ancestor of flies and fish, right? So by comparing the development in modern organisms, and this is this wonderful field called comparative evolutionary developmental biology or evo-devo. We’re actually looking back through time. So when I go [inaudible 01:02:51] in October to a meeting in Naples with a lot of other evo-devo people, you’ll have lots of developmental biologists and evolutionary biologists and paleontologists, all of whom are interested in the same questions but using different kinds of data. So this comparing the development of living organisms allows us to look back to the ancestors of the groups that we’re comparing.

Jim: And so, what we see then is that we find Hox genes in both flies and fish, so we presume that they split apart somewhere in the Cambrian explosion. So it’s reasonable to assume their common ancestor also had Hox genes. Is that the line of logic?

Doug: That’s the line of logic, but that’s where it gets contentious too, because just because, for example, I said a few moments ago that flies and fish use the same genes for eye development. Does that mean that the last common ancestor of genes had an eye? My argument has been that the genes were there, but they weren’t doing exactly the same thing. They probably had a simple photo receptor. So a cell that was responsive to light, but not an image for the eye. And of course, if you look at the eye of a fly, it doesn’t look a whole lot like the eye of a fish.

Doug: You have more facets in the eye of an arthropod than you do in the single eye of a fish. So simply because the genes are present doesn’t necessarily mean that we can say that the same morphology was present. So that’s where it gets more complicated. Fortunately, there are more sophisticated ways of looking at these genetic interactions that allow us insight into how eyes and brains and appendages work in arthropods and vertebrates, whatever. And we can begin to pull apart some of these complexities.

Doug: So we’ve gone from the early days in evo-devo, where we were amazed by the presence of Hox genes at all to being able to interrogate these developmental processes in far greater detail to understand the relationship between the genes and the morphology that they produce.

Jim: That’s really interesting, actually, and they all use probably related machinery all the way back.

Doug: They do, but if you look in detail, for example, at eye development, you can see that although there’s the same gene at the top of the hierarchy, as you go down, that gene turns on more genes and more genes and so forth. And if you look at the whole hierarchy of genes, there’s a great deal of difference between the whole network of genes that are responsible for eye development in fish versus the network of genes responsible for eye development in flies.

Jim: And hence, they often say the eye was evolved multiple times. One could argue that that level may use the same machinery, but it developed multiple times perhaps.

Doug: Yeah, this is actually one of the most fun things to challenge creationists with, which is to ask people, “What’s the best designed eye in the animal kingdom?” And the answer is not humans and it’s not birds. It’s squids. From a design standpoint, the cephalopod eye is far better designed than any vertebrate eye. So apparently, God was a cephalopod.

Jim: Ah, well, doesn’t surprise me. I’ll have to remember that as a good trick question next time I run into a intelligent so-called design person. That’s interesting. Now, we got to work on the topic that will boost our ratings off the chart and that’s sex. I think I know the answer to this, but were all these Cambrian explosion beasties sexual?

Doug: Oh yeah, yeah, yeah. I mean, a lot of them were just broadcasting sperm into the water column, sperm and eggs. So they didn’t necessarily have internal fertilization, which is probably what you’re interested in, Jim.

Jim: Well, not really. I’m more interested in the genetics. I read the genetics for the stories not the pictures.

Doug: So sex goes way back to the origin of [inaudible 01:07:29]. Even the single-celled ancestors were still engaging in sex. It gets far more sophisticated, and there are a lot of developments or novelties that are introduced with the origin of bilaterians, and you get much more sophisticated sexual practices in bilaterians than you have in sea anemones or sponges or something like that. And sponges, they make more sponges, but they don’t spend a whole lot of time worrying about how they do it.

Jim: Interesting. And of course, sex is a big advantage in working with the genetic diversity that a species has that do recombination and gene dupes and all that good stuff, right?

Doug: Right. Well, but also there’s a trade-off too, because if you’re broadcasting eggs and sperm into the water column, you put a lot of energy into making the eggs and sperm and less energy into making sure that you fertilize an individual mate. So the population sizes are often much larger, and that changes the evolutionary patterns, as well. Once you get to being a snail or something, where you’re interested in actually fertilizing males and fertilizing a specific female, you put a lot more effort into making sure that you’re alive for the next breeding season rather than producing a whole lot of sperm.

Doug: And that means the population sizes were a lot less. So you invest more energy in each offspring than you do with some of these other strategies. So the life history strategies change phenomenally at the Cambrian explosion as well, because of this changing investment strategies.

Jim: What do we call that, the K and r strategy? Do I remember that correctly?

Doug: Well, yeah, they all have been characterized as an r strategist is something like a sponge. The K strategist is something like an elephant, to investing more in each individual offspring. It turns out that life history strategies are actually more complicated than the r and K strategies that we were taught in school.

Jim: So say a little bit about this.

Doug: Well, I mean, so one of the strategies that’s particularly relevant right now is that if you have a bad environment, like we’ve had the past year, sometimes the best thing to do is simply hunker down and make sure that you survive so that when things get better, you can reproduce again and that’s actually neither an r or a K strategy. It’s actually technically called an anti-Z strategy.

Doug: Z, in these equations, is population size. So you want to make sure the population size doesn’t go down so that when life gets better, you can pick things up from there. So animals turn out to have a much greater variety of life history strategies than one might imagine.

Jim: Cool, all righty. Let’s move onto something. I think it’s amazing. The plates in your book just highlight which is the unbelievable diversity of beasties that were created during this explosion. How the hell does that happen?

Doug: Oh, it’s fun, isn’t it? I mean, it’s such an incredible variety of things, and some of them are things that you can’t imagine, I mean like the bar scene in the first Star Wars movie where Spielberg and Lucas have all these very bizarre animals sitting around in the bar, and that’s what the Cambrian explosion was like. My favorite organism pivot species in the entire history of life is something called Opabinia, and Opabinia was about three or four inches long, and it had five eyes on stalks up at the head.

Doug: It had a long proboscis, one long proboscis with a claw at the end, and then seven or eight flaps along the side that it used to swim with and a mouth underneath. So this long proboscis could grab a worm or something. We don’t quite know what it ate, and then it would bring it back, like the trunk of an elephant, and stick whatever it was into the mouth. And then, it would glide along after something else, and it’s just a wonderful organism. And when not Charles Walcott but one of his successors first showed a reconstruction of this, apparently to the zoology department at the University of Oxford…

Doug: He was at Cambridge at the time. They all burst out laughing, because they couldn’t imagine that this was a real organism. And he was actually, I think, quite affronted to have the zoology department at the University of Oxford laughing at his animal. But yeah, that’s one of the bizarre creatures that appeared during this time, and we have worms that have respiratory trees that come out of their anus. We have tons of trilobites. We have a variety of things that almost certainly had backbones.

Doug: So what we call a chordate, they’re not yet quite a vertebrate, but they’re very experimental, and these are what my colleague and my PhD advisor, Jim Valentine, who was the co-author on this book has called weird wonders. So they’re really unusual animals. They worked perfectly well. Many of them lasted for, in some cases, tens of millions of years. So they’re a perfectly good animal, but they’re nothing like anything that we are familiar with today.

Jim: So not only did we create all the phyla that we have today, we also created a bunch of stuff that didn’t make it.

Doug: Yeah, and this, of course, is what led to Steve Gould writing his book, Wonderful Life, arguing that contingency played a much greater role in history than people often tend to think, that evolution may not have been solely a function of natural selection as Darwin argued, causing adaptive improvements to things, but that some organisms may have survived just through sheer luck rather than because they were necessarily better adapted.

Jim: Yeah, though I think your little favorite animal survived, it was actually my senior year in college girlfriend.

Doug: I will not address that further, Jim.

Jim: Okay. Now, let’s see if you can pull all this together. In the last chapter of your book, you guys take at least a little bit of a swing at it. In as short as you can do it, how did all these various forces, the three networks, all that stuff, how did it all come together and cause this astounding explosion of diversity?

Doug: Well, I mean, the real answer to that is we don’t know. So what I think happened is that the developmental processes created the potential for the organisms tens of millions of years before the Cambrian explosion. Oxygen levels had increased, but it looks like they were still highly unstable. So they were going up and down on the scale of tens of years to hundreds of years, short enough so that getting a persistent large-bodied animal of the Ediacarans was very difficult to do, but that began to stabilize.

Doug: And I think there’s a feedback. Well, there’s clearly a feedback between all three of these systems, stabilizing each other so that the change in the environment allows organisms to grow larger. That requires changes in development and allows changes in ecology, and the feedback between all three of these systems drives further change. So once you get predators, for example, that gives the impetus for creating skeletons and also gives impetus for better eyes and a better brain so that you can flee from the predator.

Doug: And so, there’s feedback between all three of these systems, and that becomes a fairly sophisticated problem in itself, as you know, Jim, because anytime you’re trying to understand interactions and feedbacks between networks, whatever kind of network it is, it becomes a really difficult problem.

Jim: A couple networks, nonlinear systems, all that good stuff.

Doug: Right, yeah. And so, pretty soon, you’re off and you’re telling stories, and part of what we’re trying to do is constrain the storytelling with evidence as best we can.

Jim: Well, I think you did a good job there. Wow, thank you a lot, Doug, for a really amazingly good and clear run-through one of the more interesting and complicated parts of the history of life. Before we go, as we arranged in our pre-show chat, I’m going to ask you one of my favorite questions, as listeners to the show know well, particularly because of the work that you do, which I should give you some insight into it. I’m utterly obsessed with the Fermi paradox. As a reminder for our listeners, the Fermi paradox is named for something that supposedly happened at Los Alamos during the building of the atomic bomb.

Jim: No one knows for sure if it really happened. Supposedly, a bunch of young physicists were at the lunch table gassing about, “Oh, there’s got to be at least 10,000, 10,000, intelligent species in the galaxy,” blah, blah, blah, making all these arguments on why that had to be. And Enrico Fermi, one of the most senior and famous of the physicists, walked over to these guys and said, “Well, where are they?” And hence, that’s been the Fermi paradox, which has set off the whole steady search for extraterrestrial intelligence, the Drake equation, which is a rough-and-ready way to estimate how many such species might exist right now, et cetera.

Jim: And I’ve long thought that whatever mechanism that created the Cambrian explosion might just be the trick answer on why we don’t see any aliens. What do you think about the Drake equation, the Fermi paradox and, you’re interested in talking about it, what the Cambrian explosion might tell us about that?

Doug: So one answer to this is that I hope intelligent life does evolve eventually, but that might just reflect the fact that I’ve lived outside of Washington D.C. for 30 years. The NASA astrobiology program, actually, has funded my research for about the last 25 or 20-so years, which is wonderful because of exactly the question you ask. And the short answer is that I think the evolution of life is actually pretty easy, and work that I’m currently doing with a colleague in England suggests that evolving eukaryotes, the cells with a nucleus that encapsulates the DNA, isn’t actually all that hard.

Doug: It’s harder than evolving life. It was certainly difficult, but it’s not tremendously difficult. Whereas, evolving animals may actually had been a very hard thing to accomplish. So my guess is that there’s probably lots of life out in the universe in various places, but most of it’s probably microbial. In fact, almost all of it’s microbial, but almost all of life on this planet is microbial, too. The hard part is actually getting something like a central nervous system and appendages and the whole ecological interactions, but if anybody had visited Earth five million years ago, they would’ve found some sophisticated vertebrates but nothing with technology.

Doug: So the window, even on this planet for the kind of organisms that can produce technology, is incredibly brief, given the three and a half billion years of history on the planet. And who knows how long we’ll last?

Jim: One of the terms in the Drake equation, which is how long does an intelligent species last on average or a technological species last on average. And so far, you could say we’ve made it 200 years or so. I don’t know how much longer.

Doug: You can have incredibly intelligent species that don’t necessarily produce technology. Technology or certainly the technology to go exploring other planets may not necessarily be something that itself evolves all that rapidly, but certainly one of the things that I’ve been very interested since I wrote this book on the Cambrian more broadly is how novelty arises and not just in biological systems, like the Cambrian, but in cultural and technological systems, as well.

Doug: The further down you go along that pathway, I think, the more difficult the steps actually become. One of, I think, the interesting questions in a technological sense, so I’ll throw this back to you is once you start getting cumulative evolution, as you did in humans 3,000 years ago or whatever, does the rest of technological growth become inevitable?

Jim: Yeah, it’s an interesting question and I would say no, at least not in any kind of short time, because think about it. I had a really interesting guy named Michael Strevens on the podcast recently and his very interesting, new, modern take on the philosophy of science from NYU Philosophy Department, and one of the things we talked about quite a lot is it’s amazing how late in the day and what an odd set of circumstances it might have been that allowed for the emergence of real science in the late 17th century.

Jim: And one could imagine playing the tape again and it taking another 100,000 years, for instance, right? No guarantee that that particular strange set of circumstances, which is his argument, that it was a particular odd set of circumstances that happened in Western Europe and in England, in particular, that allowed modern science to arise and may even have been catalyzed by the unique personality of Newton, part of his argument. No guarantee that that was a foregone conclusion.

Doug: Right, yeah, it may have been a much more contingent event, as Steve Gould would say, than a lot of people are willing to admit.

Jim: Yeah, but I must say, when I was a 12-year-old nerd, oh, I was sure there was 100,000 intelligent species in the galaxy, and most of them were engaged in Robert Heinlein-style adventures, but now in my old age and my dotage, so to speak, I’m now agnostic. It may just be that these hurdles, there’s so many of them, and all you got to do is fail at one of them, and it may be that we are by ourselves. If we are, we have an incredible responsibility, not that we don’t have an incredible responsibility anyway, but in some sense, our responsibility goes up not to do something stupid and destroy ourselves and our planet along the way.

Doug: I agree.

Jim: Well, Doug, if you don’t have any other final thoughts, I’m going to wrap it up here and say this has been a great, really good episode. I think we’ve dug into an area of considerable complexity and complicatedness, but you’ve done a really masterful job of making it quite clear.

Doug: Thanks, Jim, it’s been fun.

Production services and audio editing by Jared Janes Consulting. Music by Tom Mahler at