This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. The eon in which we live is called the Phanerozoic, which comes from the ancient Greek words for visible life. The eon starts with the Cambrian, which began 541 million years ago. But in recent decades, it has become increasingly clear from the fossil record that there was visible life before the Cambrian, and complex life at that. So what caused it to emerge then, and what caused it to proliferate and diversify so vigorously in the early Cambrian? Rachel Wood is Professor of Carbonate Geoscience at the University of Edinburgh. She and her team have uncovered fossils that suggest that the fuse of the so-called Cambrian explosion was lit in the Ediacaran, the geological period that preceded the Cambrian. Rachel Wood, welcome to Geology Bites.

Rachel Wood: Thank you very much.

Oliver Strimpel: How old is the oldest large fossil?

Rachel Wood: Well, the record is quite remarkable. We think the origin of life was anywhere between about 3.6 and 3.8 billion years ago, but the oldest large fossils we have are in rocks that are approximately between 570 and 575 million years old. And in fact, the oldest examples are probably found in Canada, in Newfoundland.

Oliver Strimpel: What did these large fossils actually look like?

Rachel Wood: Well, they were quite unlike anything that we know today. They were remarkably large; they could be up to about a meter in size, and they had a very, very unusual organization, in some cases a fractal organization. If you can imagine, some of them were like giant sea pens, almost like giant feathers. They had a stem and a feather-like part, but they were rooted onto the seafloor, probably associated with microbial mats and biofilms. When I evoke the image of sea pens, it's not to say that they were remotely related to sea pens, but they looked rather like giant feathers, or sea pens, on the seafloor. Others lived horizontally on the seafloor, again with this fractal organization.

Oliver Strimpel: Were they preserved because they had hard body parts?

Rachel Wood: No. These oldest biotas, as we call them, had no skeletons whatsoever. If they did have some sort of skeleton, it might have been an organic skeleton, perhaps of something akin to collagen, or some sort of gel that supported their tissues. But they were entirely soft-bodied.

Oliver Strimpel: What sort of rocks do we find these fossils in?

Rachel Wood: Well, the oldest examples are found in what we call clastic rocks. So these are shales and siltstones; in other words, rocks that are dominated by tiny grains of quartz and clay. In some of the most remarkable settings, these earliest communities, these earliest macrofossil communities, are preserved in situ, actually preserved by being buried very, very rapidly by turbidity currents. And these are very turbulent flows that start in shallow waters but eventually come to rest in very deep water. So we know that these communities were living in remarkably deep bits of the seafloor.

Oliver Strimpel: How can we get precise dates for the rocks in which we find these fossils?

Rachel Wood: Well, the most powerful and the most accurate method we have for dating fossils is to find ash beds that are associated with these fossil beds, either before or after, interleaved, if you like, with these fossil beds, and we extract a key mineral called zircon. And zircon is very, very good for measuring an isotopic ratio with radiogenic isotopes, particularly uranium and then its daughter, lead. So we can measure this ratio between radiogenic uranium and its daughter lead and extract a pretty accurate date for when these ash beds fell. And so if we can really bracket where these ash beds occur, we can really bracket the age of these communities.

Oliver Strimpel: Presumably these Precambrian fossil-bearing beds are mostly fairly rare and outcrop only in certain places on the planet. Where do we find them, then?

Rachel Wood: Well, what's extraordinary is that these beds, with these communities of soft-bodied macrobiota, are remarkably global. We find them in Canada, in North America, Siberia, Australia. There are remarkable populations in Namibia. So they are really widespread, far more widespread than you would expect for the preservation of soft-bodied forms.

Oliver Strimpel: Can you speculate as to why they might be so ubiquitous?

Rachel Wood: It is an area of really active research, because how could something that might have had the consistency of a jellyfish, say, how did that get preserved in these sediments, where it simply wouldn't be preserved today? So there are several hypotheses that invoke the fact that perhaps seawater at this time had very, very different chemical constituents from today. For example, one set of ideas proposes that we had elevated amounts of silica dissolved in seawater, and this effectively aided the preservation of these biotas. There are other ideas that suggest that these microbial films, which we know they're associated with, might have formed some sort of death mask that preserved these forms. And indeed, other hypotheses invoke the precipitation of key clay minerals; again, that could have been a mechanism by which the clay minerals coated these fossils and then aided preservation. But all these ideas really rely on non-uniformitarian principles. In other words, the present is not the key to the past; the past is a different place. Things happened differently there.

Oliver Strimpel: You're a field geologist, so you actually go and hunt for these fossils in the field. Where have you been to get the fossils that you've just been discussing?

Rachel Wood: Well, the areas I've spent most time in, in the last five years or so, have been Namibia, particularly southern Namibia, but also southeast Siberia. And actually, one of the most remarkable expeditions I've been on recently was to a very remote part of southeast Siberia, on the Yudoma River. It was an area that really hadn't been studied extensively since the early 1980s, and I was very privileged to go with a joint Russian, Chinese and UK expedition. We were out in the bush for five weeks with no connection to the outside world, living off the land: berries, pickled mushrooms, beautiful, delicious freshwater fish. We lived like kings, but it was a very, very remote existence. Enjoyable, but very challenging.

Oliver Strimpel: You've mentioned that there were probably special conditions that prevailed at that time to enable these soft-bodied creatures to be so well preserved in the fossil record. Does that information, or any other information you've been able to extract from the rocks of that time, give you some clues as to what it was in the environment that triggered the emergence of complex life at that particular time?

Rachel Wood: Yes. I mean, this is a very complex problem. Trying to understand why something was preserved may or may not be related to why the animals (if they were animals; they may not have been animals), these biotas, were there at all. So you have to think about the prerequisites for complex life, and very much the flavor of research in the last 20 years or so has been the idea that we needed sufficient oxygen dissolved in seawater to allow the emergence of complex life. And we know that the origin of animals in particular certainly requires some amount of oxygen. Exactly how much oxygen is very much debated, because we now know that animals with very, very low metabolism, so for example sponges, which cannot move, have very few, if you like, metabolic requirements. These need much, much lower amounts of dissolved oxygen than, say, a predator that actively moves and has a high-energy lifestyle. So there are all sorts of ideas and hypotheses out there that invoke the stepwise, if you like, or perhaps episodic injection of oxygen as being a key requirement to allow increasingly more complex life, with increasingly more metabolically demanding lifestyles, in particular animal lifestyles.

Oliver Strimpel: Okay, so is the idea that we started off with an anoxic, or largely anoxic, planet, and eventually, in the late Ediacaran, the dissolved oxygen levels reached a critical point which enabled complex life to emerge?

Rachel Wood: Yes, that's it. There isn't a huge amount of consensus for these key bits of geological time, particularly the Ediacaran and the early Cambrian. But if you'd asked people this question maybe 20 years ago, they would have suggested two episodes of increased oxygenation: one, the Great Oxidation Event, which is approximately 2.1 billion years ago, and then another one, perhaps close to the Precambrian–Cambrian boundary, and that was the trigger, perhaps, for the Cambrian explosion. But now we don't see a narrative of a single increase. We see these pulses of oxygenation coming and going, and these may well have started in the Ediacaran, and possibly even earlier: pulses of oxygenation where the shallow seafloor became oxygenated, but then anoxic waters impinged again on the shallow shelves. So in other words, it wasn't a stepwise incremental increase. We just have these episodes of oxygenation coming and going, but with periods of more dominant anoxia in between. The other theme that's really emerging is that our shallow marine shelves, and possibly the deep waters as well, were incredibly heterogeneous at the same period of time. We can go to Namibia, for example, and say, well, our shallow shelf seas were oxygenated at this time, but going through exactly that same interval elsewhere, for example in China, in fact they were anoxic. So in other words, we have this very, very heterogeneous distribution of dissolved oxygen in our seawater at that time, and this, in fact, may be the key to why the fossil records of these places are very, very different.

Oliver Strimpel: I'm curious how it is you can reconstruct what the oxygen level was in the Ediacaran.

Rachel Wood: Yes, there are various geochemical proxies, and they fall very simply into two categories. One set records a global oxygenation level. So these would be things like uranium isotopes. We can interrogate uranium isotopes, particularly in limestones and dolomites, and these can then be used to reconstruct the degree of the seafloor globally that was covered with anoxic waters. They're rather tricky, because you have to make all sorts of assumptions about the Earth state, and they also need modeling. So those are our global proxies. But potentially more powerful is the use of local proxies. You can actually look at the conditions just in a very small area, maybe a basin, or even one part of a sedimentary basin, and one of the key proxies used here is called iron speciation. It's a very simple aqueous geochemical technique where you extract all the iron from a rock, but you divide that iron into different types of iron, broadly into the amount of highly reactive iron and the total amount of iron, and it's this ratio that then allows you to infer that that sediment was deposited in the presence or absence of oxygen. So this is a powerful technique, which we can use to interrogate, at a really fine scale within basins, individual sections within the basins, and then compare the local redox dynamics through time of one area, for example Namibia, with another area, for example Siberia or South China.

Oliver Strimpel: Is oxygen the main driving factor in the environment that affects the evolution of life at this point?

Rachel Wood: I think it's likely to be one of the key drivers, but it's also likely not to be the only key driver. We have other effects. We have global temperature changes, climatic changes. And another idea that is really gaining a lot of traction now is the availability of nutrients, so phosphorus and nitrogen, for example. These are key building blocks of life. And we also know that as well as having a very heterogeneous and very, very dynamic oxygen history in our shallow marine seas during this time, we also have very, very heterogeneous nutrient input into the seas. And we know from the modern that in areas where there are a lot of nutrients, particularly nutrients either coming off land or nutrients upwelling from cold waters, so for example the Benguela Current that hits the southern African coast, this creates enormous productivity of life. So we think that these things couple together: climatic changes, changes in oxygenation and changes in the availability of nutrients. These all play a role in determining the evolution of these early complex ecosystems.

Oliver Strimpel: So I'm wondering if it's possible to tease apart these environmental factors that you talk about, such as oxygen and the availability of nutrients, from internal biological factors that may have been at play here in the emergence and subsequent flourishing of life.

Rachel Wood: Yes. These so-called intrinsic characteristics are very, very important, and they should be seen in parallel with these attempts to interrogate the physical and chemical world, which we can do from geochemistry and sedimentology and so forth. And there's very, very interesting work taking place on trying to understand the evolution of gene regulatory networks, conserved versus non-conserved parts of the genome, and trying to put these genetics into the context of evolution. So yes, this is a very, very important part of trying to understand the emergence of complex life.

Oliver Strimpel: Can we use some of these intrinsic considerations to understand what led to the emergence of biomineralization, of hard body parts, that is, shells and skeletons?

Rachel Wood: Yes. So one of the key, incredible innovations in the history of complex life is this rise of skeletons: the ability of life to co-opt minerals that are already there in the environment and create a skeleton or a shell out of those minerals. One really interesting thing that's emerging now is when you look at the biological affinity of these earliest biomineralizers, which appeared in the Ediacaran about 550 million years ago. I should say these earliest purported animal biomineralizers. These were the beginning of this sort of root of the Cambrian explosion, and then the numbers diversified incredibly in the Cambrian, both in terms of the mineralogies they co-opted, the types of skeletons, and the sheer complexity of these skeletons. But one idea is that these skeletons appear independently in unrelated lineages, and this tells us that the biogeochemical machinery, if you like, all the genetic machinery used to create biominerals, is a conserved feature, long predating the history of metazoans, of animals. Indeed, it may be common to all eukaryotes. And one of the prevalent ideas is that there was this pre-existing machinery that was simply co-opted for another function, co-opted for the function of producing skeletons. So this has led to the idea that something must have happened, perhaps in the environment. And one aspect of that is that the selection to have a skeleton may have simply changed. In other words, the fitness gained from having a skeleton might have increased, coincident with the appearance of all these skeletons. And the key idea there is that we can invoke a rise in predation pressure, and the rising predation pressure then gave added fitness to animals that bore a protective skeleton. And maybe this is the trigger for why all these independent groups appear to produce skeletons independently, but broadly around the same time.

Oliver Strimpel: When we spoke earlier, you mentioned the idea that the rise in oxygen enabled the biota to have a higher metabolism, more energy, enabled them to move and therefore to predate. And so the ability for one animal to eat another was originally triggered by the environment and then led to this evolutionary change.

Rachel Wood: Well, now, as you say, we're trying to tease apart these intrinsic biological features from these extrinsic triggers. So this is an idea that is really prevalent in evolutionary studies: the relationship between environmental change, in creating environmental opportunity, and how life responds to that, in other words, evolutionary innovation. And this is a really constant dialogue, and it's very, very difficult to tease them apart, because they often go hand in hand and there are multiple feedbacks. I mean, one of the remarkable things about our Earth, the most remarkable thing about it, is that, as far as we know, it is the only place in the universe that harbors complex life, in fact any life at all. But this life co-evolved with the planet, and all the things that go to support this life are intrinsic to the workings of our planet: the carbon cycle, the sulfur cycle, the nitrogen cycle, the regulation of climate and so forth. So, teasing apart chicken and egg: in other words, what came first? Did the environment create a permissive environment that allowed life to respond, or did life innovate, and in so doing change the way our biogeochemical cycles operate, and in so doing create this extraordinary feedback on the planet? This is getting to the heart of what we're really interested in, and of course this is the hub of many, many research programs.

Oliver Strimpel: So does this mean that this period of innovation, which perhaps originally we thought was in the Cambrian, can be pushed back largely to the late Precambrian, and that we see evolutionary ancestors of Cambrian life forms in the Ediacaran?

Rachel Wood: Well, I'm certainly one of those that is convinced that we can see the roots of these innovations in the Ediacaran. Of course, we should always remember that the fossil record that we have is only a very incomplete record, and these fossils are certainly only recording the minimum age of any of these groups. And what is the origin of these groups? If we look at molecular phylogenies, for example, the proposal is that many of these groups have really old origins. For example, molecular phylogenies suggest that the origin of the animals themselves could be anywhere between 800 million years ago and 600 million years ago. We have no record of animal fossils then at all, but this is what the phylogenies tell us. So it should come as no surprise that we can increasingly make some links between Ediacaran animals and Cambrian animals. One example could be this Ediacaran animal called Namacalathus. It's known from Namibia, Canada, South America and Siberia. It's a goblet-shaped fossil; it has a goblet-shaped skeleton. But we've recently found some exceptionally preserved individuals that have soft tissue. Of course, it depends on how you interpret the soft tissue, but there is a curious structure inside this skeleton that is preserved in pyrite: it's a tube of pyrite. And we believe the best explanation for this tubular structure, which is asymmetric and slightly J-shaped, is that it's a gut, and we think it's the gut of a group known as the lophotrochozoans. This is a group that today represents about a third of all marine invertebrates, related to the mollusks, the clams, the brachiopods, the bryozoans, which are little tiny colonial animals. So we think here we have a link between the Cambrian and the terminal Ediacaran.

Oliver Strimpel: That's interesting. So from what you've said, it sounds like there was more continuity between the Ediacaran and the Cambrian than we first thought. Was there, in fact, a mass extinction or some kind of dramatic environmental event that actually did take place at the Ediacaran–Cambrian boundary?

Rachel Wood: Well, the nature of this boundary is really hotly debated. One of the issues is that many of these boundary successions globally are incomplete. Many of the ones that have the richest fossils don't have these key ash beds to date. So this is a real problem for dating the order of events, and also for correlating globally between sections that are remote from each other. So there are two schools of thought. One is that, yes, there was an extinction. We see at this time an incredible negative isotope excursion in carbon, which suggests there was a profound perturbation of the carbon cycle, and some have suggested that this coincides with large igneous province magmatic activity. So this is very similar to the mass extinction events that we see later in the Phanerozoic, many of which, but not all, can be associated with these large igneous provinces. These are provinces where you have a huge amount of continental magmatic production over a very long period of time, millions of years, and these create rapid and sustained global warming, and often this global warming can lead to the ocean circulation becoming sluggish and anoxia developing. One school of thought suggests that we had an event like this, a mass extinction event, so perhaps anoxia triggered by this volcanism, that created an extinction of the so-called Ediacaran biota and, in effect, paved the way for the flourishing of the Cambrian biota. The other school of thought is that there wasn't a mass extinction; there may have been perhaps a more modest turnover, and nonetheless we can determine transitional biotas across this interval. Indeed, we can find skeletal fossils which were previously thought to be entirely lower Cambrian; we can now find them in Ediacaran sediments. They are transitional, what we call small shelly fossils. So I slightly fall into the second category, and I believe that there wasn't a mass extinction. I may be proved wrong, but I think at the moment the evidence for mass extinction is not compelling; rather, we see a series of transitional biotas. So in other words, we see this slight turnover at the boundary, but we see many turnovers before that, and we see turnovers afterwards as well. So it may be possible to reframe the narrative of the Cambrian explosion as perhaps not being Cambrian at all, and simply being one of several radiations of complex life, animal life, indeed, that actually precede the Cambrian explosion, and the Cambrian explosion is one of the final manifestations of this event. However, having said that, what seems to be very characteristic of the Cambrian explosion is this genuine explosion of fossils of predators. I think that is a distinctive part of the Cambrian, but the Cambrian explosion couldn't have happened without all these other radiations of all these other transitional biotas before that.

Oliver Strimpel: So does that suggest the view that the Cambrian explosion, when it actually did take place, was more of an intrinsic, biologically driven change?

Rachel Wood: Yes, it is a very interesting and fundamental question. There's no doubt that ecological drivers were very important in the Cambrian explosion. We know how powerful a driver the rise of predators is, and we have this phenomenon known as an arms race. It's really a form of evolutionary escalation. In other words, a new predator evolves with horrible new mouthparts or grasping appendages, and so the prey has to respond by in some way acquiring some sort of defensive skeleton, or changing its behavior, becoming nocturnal, for example, or hiding in a crevice. This ends up in this arms race; it's a constant arms race. So this is certainly important. But we also know that the lower Cambrian was still a time of very dynamic redox.

Oliver Strimpel: Can you explain what you mean by redox?

Rachel Wood: So by redox, I simply mean the distribution of oxygen, and I normally mean the distribution of dissolved oxygen in seawater. So in other words, where we had free oxygen, we can refer to that as oxic conditions; where there was no oxygen at all, we can refer to that as anoxic conditions. And all sorts of other shifts in the carbon cycle and other cycles were ongoing at this time. So I think, again, we can't just pin all this on one factor. All these factors are interconnected, and I think this is what makes the Ediacaran so phenomenally interesting: we have to unpick all these factors and really understand how they are interconnected, and understand the drivers and the feedbacks. It's many, many lifetimes' work.

Oliver Strimpel: I'm struck by the fact that although we've now pushed back the origin of complex life into the Ediacaran, a few tens of millions of years into the Precambrian, that's not much in the grand scheme of things. It still leaves well over three billion years of history without complex life. Can you speculate why it took so long to arrive?

Rachel Wood: So why it took so long for complex life to appear is unresolved, and it is a tremendous source of current research. One of the pervasive ideas is that we have this phenomenon called the boring billion, which is a billion years when not much seemed to have happened, or at least we thought not much happened. In other words, evolution just seemed to have the same fossils from the beginning to the end. However, this record is really starting to be interrogated in a lot more detail now, and without doubt we will be revealing exciting new fossil finds. But also the geochemistry of this interval is being interrogated, and it's not boring at all. There seem to be episodes of oxygenation. In other words, this pattern that we see in the Ediacaran, of these pulses of oxygenation, maybe hand in hand with pulses of nutrients, may well be a motif that we can trace back into this boring billion. So we're constantly pushing the origin of these key building blocks, if you like, further back in time. And I think that this boring billion will prove anything but boring in the years to come.

Oliver Strimpel: So if you were to speculate about exolife, or life on any other planet, would you expect, therefore, that there are some fundamental constants of nature, if you like, so that it really does take billions of years for the appropriate complexity to evolve, whether it's what can be permitted by the environmental factors or what is needed in the molecular biology to amass sufficient complexity?

Rachel Wood: Yes, this is a prevalent theme: that, simply, life needs a very, very long time to assemble the necessary genetic machinery, the gene regulatory frameworks and so on. But of course, we only have one example, Earth. We cannot rerun the experiment. And of course, we know that it's very, very difficult to define life, to define this elusive quality that we call life, and there are some wonderful definitions out there in terms of self-regulation and homeostasis. Yes, but we're in the midst of a pandemic, which is caused by a virus, and many would argue a virus isn't living. But look at the power of a virus. Look at the power of something that is not living, but has this extraordinary ability to replicate by co-opting human genetic machinery, and get tremendously better at it. So it comes down to, I suppose, our ideas about what life is, what complexity is. We place animals at the top of the tree of life because we're one, and we're big, and we appear to be in control. But maybe we should rethink the way we perceive the tree of life.

Oliver Strimpel: Rachel Wood, thank you very much.

Rachel Wood: Thank you very much, Oliver. It's been a pleasure.

Oliver Strimpel: For more about Geology Bites, as well as pictures and diagrams that illustrate this podcast, you can go to geologybites.com.