The Science of Everything Podcast - Episode 160: The Evolution of Animals

Episode Date: May 1, 2026

A journey through the evolutionary history of animals, beginning with the split of the metazoans from protists about 760 million years ago and ending with the rise of the amniotes about 340 million years ago. The focus is the development of key morphological traits necessary for more complex animal forms, including the origin of true tissues, bilaterial symmetry, the complete gut, the coelom, internal skeletons, and limbs. We conclude with an analysis of key adaptions that allowed amniotes to life in a wider range of terrestrial habitats. Recommended pre-listening is Episode 155: Embryology and Development and Episode 157: The Geologic Time Scale. If you enjoyed the podcast please consider supporting the show by making a PayPal donation or becoming a Patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

You're listening to The Science of Everything podcast, episode 160. The Evolution of Animals. I'm your host, James Fodor. In this episode, we're going to discuss the evolutionary origins and development proliferation of animals. So we're going to start at the very beginning with the split of animals from other types of initially single cellular life forms and then talk about the different key traits that have evolved over time in order to, that are necessary for increasing the complexity of animal life and expanding into new niches and new environments. So in particular, we're going to be talking about the development of bilateral symmetry, the complete gut, the sealum, which is the internal body cavity, the backbone, which is

common to chord dates, going to talk about the evolution of jaws, moving from jawless to George Fish. We're then going to conclude by talking about how vertebrates moved onto land with the evolution of tetrapods and then gradually developed more and more adaptations for land with the evolution of amniotes. Recommended pre-listening for this episode is episode 157, the geologic timescale, which will give you some historical context to fit in the different evolutionary stages that we're going to be talking about. And also, episode 69 and 7, on animal diversity would also be useful because that will provide a context for different types of extant organisms, animal life that exist today, whereas in this episode we're going to be focusing

mostly on extinct form. So all that being said, let's make a start and we'll begin with the very origins of animals. So where did animals come from? In previous episodes, we've talked about the origin of life from non-life, so that happened about four billion years ago. And in the geologic timescale episode, we talked about different events that happened, particularly in the 3 billion years or so, after the origin of life and how different geological forces as well as climactic forces shapes the Earth during that period of time. But I emphasize that up until 1 billion years ago, and even sometime after that, say maybe 800 million years ago, life on Earth was entirely single cellular. For the first two billion years or so, it consists mostly of bacteria or similar organisms, and then about two billion years ago, eukaryotes evolved. But still, life was only single cellular or most maybe colonial organisms.

What we're going to be focusing on today is how that changed. We're going to start with the split of animals from other types of protests and go right up until the evolution of amniotes. We're going to focus this discussion around key evolutionary steps in that process. Now, in doing so, it's important to address a potential misconception here that evolution is not directional. Evolution doesn't have an end goal. It's not trying to achieve anything. We shouldn't think of evolution as a process of developing more and more sophisticated life or life becoming more and more human-like or progressing towards humans. Really, evolution is just the process of life changing over time in response to environmental pressures.

Now, because life began very, in a relatively simple form, single cellular life, I mean, even that is very complex in an absolute sense, but compared to some of the organisms that are alive today, like say humans, it's much, much simpler. Because life began there, there were many opportunities to expand into niches that were progressively more complex or that required more complex, more derived organisms with more specialized traits and body parts and systems in order to exploit those. So over evolutionary time, we do see a gradual increase in the maximum complexity and sophistication of the organisms that exist.

I say maximum, because very simple organisms still exist today. There's niches for those. It's just because there are some niches that can only be exploited with the increase in complexity and sophistication, we see those niches being filled over time, and it happens later rather than sooner in evolution because you have to develop that complexity progressively bit by bit. So that can give the illusion that evolution is sort of progressing towards more complex forms. It's really just because we started only with the simple forms and it takes longer to get towards the more complex forms. But the simple forms are still very much around and are just as

adapted to their environment as the more complex, more derived organisms like humans are adapted to their environments. That being said, we're going to look at the evolution of key traits sort of on the direction towards humans or mammals. Not because evolution is directed towards that goal or it's like trying to get there, but simply because we're interested in our evolutionary history and how we acquired the traits that we have. So we should understand the exercise in that context that we're not sort of thinking of evolution as directional, but simply asking the question looking backwards as to how did we acquire the traits that we did and when did they happen, when did they arise and why? What were the pressures operative there? And what were the stages necessary in

order to achieve the level of complexity that we have today. So that's the way we're thinking about this. So let's start then with the origin of animals. So animals belong to the traditionally kingdom called Metazoa, and it's thought that Metazoa originated around 760 million years ago. I should say about these dates, particularly anything prior to the beginning of the Cambrian, that those are highly uncertain and depend a lot on the different assumptions that one makes and the different researchers come to different conclusions about these dates. So you will see quite different dates and different sources. The order of the evolution of key traits that we're going to discuss is largely agreed upon, but the exact dates are quite variable. So that being said, I've picked a set

of dates that seem plausible to me based on recent research, but you will see others. So it does seem generally agreed upon that, you know, somewhere around 700 to 800 million years ago, let's say 760, metazones first originated. And at this point, the metazons that existed at that time were single-celled organisms that basically they split off from a similar group of protists, so single-celled eukaryotic organisms, called coanoflagellulets. The coanoflagellulates are a group of free-living single-cellular organisms. Some of them are also colonial, meaning they sort of live together, but they're separate organisms. And the name refers to a funnel-shaped collar that they have of interconnected microvilli,

which surround the flagellum, which is basically like a whip-like organelle device that organism uses to move about. The funnel-shaped collar that it has actually serves to trap food and other detritus against the microvili, as the flagellum moves about, thereby helping it to collect food, which is then digested, engulfed and digested. Now, coanoflagellular are particularly interesting because they look very similar morphologically to the koanocyte, which is a type of cell found in sponges. Now, there are some differences here. The koanoflagellates are generally free-living unicellular organisms, although there are some

chemical signals that can trigger colony formation, so they can trigger them to clump together and form a collective. We also see colonial living in other similar related. organisms to the koanoflageolids. The difference, of course, to the koanocytes in sponges is that they are part of a single organism and are obligate multicellular organisms. So sponges are always multicellular. It's thought that metazoleons evolved from coanoflagellates becoming obligate colonial organisms, which then saw progressive cell differentiation leading to the development of sponges around 750 million years ago. Spongers are quite simple organisms. They don't have distinct tissues.

There are different types of cells, but not tissues, and they don't have any type of symmetries, and they're just filter feeders, you know, they're sessile, they don't move. Although there has been a little bit of controversy about this, it seems to be generally accepted these days that sponges or periphera, is the term for the clade, forms a single clade and is the most distantly related to any of the other type of animals. So, in other words, they split off earliest. So it's plausible. No one knows for sure because there's not a great fossil record for sponges, especially going back this far.

But it's generally thought that the very earliest animal ancestors originated from kohenoflagellates, gradually differentiated and formed organisms that in some ways resembled contemporary sponges. They wouldn't have been exactly the same, in some ways resembled those. Now, the next key stage in animal evolution was the development of true tissue. So this is specialized collections of cells that perform a particular function. Be in mind that sponges have specialized cells, but not tissues. They don't group together and share a sort of a developmental history and so forth as true tissues do. So eumetazoa forms a clade of organisms that have true tissues.

And basically it's all animals except for the sponges. So one of the key aspects, as I said, is that there are true tissues that are organized into germ layers. So this is something that we're discussed in the episode on embryology and development, so 155, if you're not familiar with that. But eumatozoa always originate from at least two different, there's two different germ layers, and they pass through a gastrolation stage. So the simplest form of eumatozoa are the diploblastic animals, Nideria and tenethora, and they only have two germ layers, the endoderm and the ectoderm. Most animals are triploblastic, so they have three layers of germ tissue, the ectoderm, the endoderm, and the mesoderm between the two. And triplablblblbastic animals have recognizable organs. So here we're seeing a progressive increase in complexity. So the coanoflagellulates are just single-celled organisms and have no differentiation, even when in a colonial form, there's little to no differentiation between cells.

Next step are sponges, which have differentiated cells, but no differentiation. tissues and no distinct germ layers. Next step again is the earliest type of eumetazoa, so nideria and tenethora. These are jellyfish, comb jellies, and other related organisms. A small note here, I will say jellyfish even though they're not actually fish, just because if I say jellies, people may be confused, but just bear that note in mind. Anyway, these organisms only have two germ layers, the endermermen ectoderm, but they don't have true organs. So there's specialization at the tissue level, but not the organ level.

The next stage of specialization is to have, with the three germ layers, specialized organs, that level of complexity begins at the grouping called bilateria, which is bilaterally symmetric animals. But we'll get to those. Just understand that as we develop progressively more complex, sophisticated and differentiated morphological features, this is also replicated and sort of originated and then manifested at the developmental level, so in germ layers, and also in gene expression, which we'll talk about a bit more later.

So there's still a fair bit of disagreement as to the exact ordering of which the next three major groupings of animals split off. So tenophora are comb jellies. Nideria, as I said, those are jellyfish and related organisms like anemones and various corals. often just talk about them as jellyfish just as a shorthand. Different authors have put them in all sorts of different. And the third type is called Placozoa. So this is probably one that many of you may not have heard of. It's not very well known as far as I can tell. Placazoa is a phylum of free living organisms, which are blob-like animals. It's really just an aggregation of cells. It doesn't look like much of anything. Honestly, it looks a little bit just like a pile of vomit. Excuse me

in saying so. But they are. They are animals, and they do move around. They move in water by ciliary motion, and they eat food, normally just different organic debris, by engulfment, and they're reproduced by fission or budding. They're often described as the simplest animals on earth, and in some sense, they're even simpler than sponges, even though they are more closely related to us than sponges. I'll talk a bit more about Placazzo in a moment, because scholars have often thought that they might represent the evolutionarily most basic form of animals, even though

genomic analysis indicates that, as I said, they are actually more closely related to humans and other chordates than sponges are. But coming back to the point about Tenophora versus Nideria versus Placozoa, there's no real agreement as to what order these three different groupings branched off from the rest of the animal kingdom. From what I've been able to tell, there's increasing evidence that tenethora, the comb jellies, may have been the first to branch off after the sponges. And then whether it was Placozoa and then Nideria or Nideria and then Placazoa is a bit hard to say. Regardless of exactly which of these branched off first, it's clear that this happened quite a long time after Sponger's branched off, but still a long time before the Cambrian explosion. So remember, the origin of animals, where sponges branched off.

ranch off from the rest of us is dated to around 760 million years ago in the chronology that I'm using. But the next step after that, the origin of true tissues, gastrolation, and the splitting off of the tenethora, or possibly nideria, or possibly placazoa, depending on who you believe, was around 650 million years ago. So about 100 million years after the sponges. So there's a long time for further evolution there. Now, because of the uncertainty, we can't resolve, as I said, sort of the relative dates of these three, they probably occurred within a relatively short period of time, maybe like tens of millions of years with each other. But a more interesting question for our point of view is how did this evolution occur? So how do we go from something like

a sponge to something like a jellyfish, which sort of morphologically in terms of its lifespan is very, very different? And what about Plaqueso? How does that fit into everything? I mean, it's just like a blob of a few layers of cells that moves around and, engulfs things. It doesn't really do anything else. So how do we link all those things together? Because there's really very little, if any, fossil evidence for any of these things this far back in the past, most of the theories have been developed on the basis of observing surviving animals in all of these groupings, which of course may or may not resemble the ancestral forms, as well as looking at hoax genes. So we've talked about hoax genes before. These are

regulatory genes which control the development of different, they control gene expression during development, so which genes are turned on in which cells at which times, which determine the developments of different types of tissues and specialization of functions in development. And it's proven to be very useful for understanding how different branches of the animal kingdom developed in relation to each other, because usually hox genes are conserved, or relatively conserved. So the closer are your hoax genes, the more closely related you are, in other words. That's a commonly used method to try to infer what the most ancestral forms would have looked like at different stages in the evolutionary branching. The third method is to look at traits.

And so to compare what traits different organisms have, both in the developmental stage and in the adult stage, because often many of these organisms have very different juvenile compared to adult life forms. For example, many Nideria have two-dural. different forms, one in which they swim freely in the ocean, that's like the jellyfish form, and then a polyp form where they later in their life settle on the ocean floor and become sessile and form, well, a polyp or something that looks like a component organism in a coral reef or something like that. So looking at these different morphological traits and lifestyle development traits is also used to try to determine what traits the ancestor must have shared. So by analyzing all of

these different sources. Scientists have come up with a wide range of different theories as to how these different forms of animals split off from each other and what the ancestral forms must have looked like. There's not a lot of consensus here. So I'm going to tell you about one set of theories that I find persuasive and at least gives you a flavor of what might have happened. In particular, I want to explain what's called the Placula hypothesis. So according to this theory, Nideria, tenophores, placozoa, and bilateria, all originated from a placozoan-like ancestor after the split with sponges. So remember, placosova, that's that mostly flat blob of cells,

the sort of simplest possible animal. So the idea, according to this hypothesis, is that when animals first differentiated from the co-wine flageolates, there was one branch that developed into, eventually developed into, sponges and a different branch that instead of continuing the sort of filter feeding approach developed into something like a placosoa which moves around and eats up the organic detritus by engulfing it and then binary splitting. So it kind of became even simpler as opposed to sponges which are a little bit more sophisticated than that. So the idea here is that there's initial split.

One goes in the direction of the sponges and the other branch becomes like a placazoa. and then progressively you have one or more branching episodes off that Placozoan ancestor. So because we don't know the exact ordering, this could have happened multiple times. So for example, the tenophores, if they did branch off first, the tenophores branched off and formed the comb jellies. But then the ancestral form of the Placozoan was maintained in some organisms, and then there were further evolutionary branchings off those. That's the ordering that I'm going to assume here, but this hypothesis can be modified in terms of the number of branching events and exactly which one evolved from which, depending on what we think is, you know, what phylogenate tree you find most plausible. But anyway, just to keep things simple, let's assume that the tenor four branched off first, and then next is the Niderians plus the bilateria. So those are the two most recent branchings.

But how does the branching actually occur? This is the key to the hypothesis of the placular transformation. So the idea is you start with a very simple flat disk of cells, which is the form that the placazoa has. Now, when a placosovoa feeds, it moves over an area of organic particles and sort of bunches inwards a little bit. Like imagine a pancake that you sort of press inwards a little bit from the sides, and then it's sort of bulges out just a little bit in the center. That's kind of what we have in mind here. This forms a digestive cavity that surrounds the particle and then it externally digests it. So the idea of this hypothesis is that eventually as the Placuzzo becomes more and more

protuberant in the middle, it sort of pushes up further and further, it begins to adopt the appearance or form of essentially like a bag. And here the cells that were formerly part of the upper epithelia, so facing up, They form the outside of the bag, which eventually goes on to form the ectoderm, whereas the low epithelial tissues that previously faced the bottom of the sea, now become the inside layer of the bag, and they form the endoderm. And so this then also gives rise to the two different layers, the two different germ layers, the endoderm and the ectoderm, which are common to all diploblastic organisms.

So this then gives the basic body plan of the nideria, something similar. could have also happened for the comb jellies. So this is just one hypothesis. There are certainly others, but it does give you some idea about how you can go from something that is very, just basically a flat pancake of cells, the placarzoa, to something that more closely resembles a jellyfish

and that then swims freely in the ocean. Now, the next step on our evolutionary journey is probably the one that's attracted most attention. and this is the origin of bilateral symmetry as well as a complete gut. So bilateria, bilaterally symmetric organisms. So bilateria is a clade of animals that has bilateral symmetry at least at some point during embryonic development. So parenthetically, this does include a kinoderms, many of which have radial or five-point symmetry, like starfish, for example. But here I'm just going to talk about it is bilateral symmetry, because all of them have at least bilaterally symmetrical embryos at some point.

development. Almost all of the bilaterians are triploblast, so they have three germ layers, and the mesoderm, so that's the most recent layer, forms a complete digestive tract, so it has a separate mouth and anus. So the tenophores and the nideria have a mouth, but it also doubles as an anus, so that's called a blind gut, so it's the same way in and out, and that's implicit in the body plan we talked about with regard to the placards. hypothesis where it's sort of the flat pancake sort of folds up and bulges outwards and then forms a bag but the bag only has one hole right so that's the blind gut the next level of complexity is to develop a separate mouth from anus which allows for a much more sophisticated

digestive process to work out which is going to be necessary in order to take advantage of a wide range of different forms of nutrition so again continuing on from the placular hypothesis is, and we're talking about 630 million years ago, at this point, we have an ancestor of both Nideria and biliteria, which resembles something like a jellyfish, or a simplified form of a jellyfish that's formed from a placuzzoa-like organism on the basis of the placula hypothesis of sort of bulging outwards and then swimming freely around the oceans, as we discussed. How do you go from that to a bilaterally symmetric organism? The idea of the planular hypothesis,

don't get confused with the placular hypothesis, these are different, the planular hypothesis says that bilitarians could have originated from the free-swimming planular larva of something like a jellyfish. The planula is a bilaterally symmetric larval form found in many types of jellyfish. Remember I mentioned that jellyfish have a complicated life cycle where they have both free swimming

and then sessile forms. And they change morphology many times during their lifespan. And one of those forms is a bilaterally symmetric planular organism, which is its larval form. So the idea of the planular hypothesis is basically that the bilaterally symmetric animals derive from a planular larvae, which then just, instead of developing into either a jellyfish form or a polyp or something else, developed into a bilaterally symmetric or. organism that retains that bilateral symmetry over its life one. And as we'll see later, it probably developed into some sort of primitive worm-like. So this would be an example of what's

sometimes called pedomorphosis, which is a common technique in evolution. Essentially, it refers to the retention of juvenile characteristics into adulthood. So the idea is that you have this planular form, which has some of the characteristics of bilaterally symmetric organisms. And in Nideria, then goes into develop other forms, but in, according to this theory, the bilaterally symmetric lineage, it just retained that symmetry and general form throughout its lifespan into adult form, and then gradually migrated from just being free swimming back onto the ocean floor again and started crawling and became something like a worm. Now, an alternative hypothesis is that instead of developing from the planular lava,

the ancestor of bilaterally symmetric animals originated from something like a jellyfish form, which then settled onto the ocean floor on its oral surface, so that's like the opening of the bag that we talked about, and then gradually became sort of more flattened and elongated and started to move around. And one direction of that lineage may have led to Niderians, where that form becomes stationary in as a polyp, in part of its lifestyle, whereas in the Bilitarian branch of that lineage, the free swimming form may have been lost,

and then what you went up with is the common ancestor of Bilitaria, which is essentially like a very simple worm that can crawl along the surface of the ocean floor. So these are two different hypotheses here, the planular hypothesis, and then a hypothesis based on a, deriving more directly from a, jellyfish form. Either of these are quite plausible and consistent with the evidence that we have.

It's generally thought that the ancestor of all bilaterally symmetric animals, sometimes called oer biliteria, looked somewhat like a worm. It crawled along the ocean floor. It probably moved about through rose of cilia that beat and allowed it to trap food particles as well as move about. It probably had some simple eye allowing it to see, probably retained for a long time a free swimming larval form as well, although this was then subsequently lost in many descendants. So at this point, we've seen the development of animals from the coanoflagellulates through the periphera with the, with the development of obligate colonial living and then the filter feeding specialised cells of the periphery. We then

hypothesized that the something like a placazola-like ancestor, the flat pancake kind of glob of cells developed into something like a jellyfish through the placular hypothesis, where it sort of bulges outwards and then becomes a jellyfish. And then we discussed how this jellyfish-like organism could have been the ancestor of bilaterally symmetric animals, either through development of the planula and planula lava retaining that bilaterally symmetric form into adulthood and gradually settling on the ocean floor, retaining that for a longer phase of its life, or alternatively by a more jellyfish-like form settling on its oral surface on the ocean floor and then gradually becoming flattened and more elongated. So either way, what we have is a

picture of how we go from colonial single-celled organisms to something like a Placazoa to something like a jellyfish and then to something like a very primitive worm. By this point, we have the origin of bilateria around 630 million years ago. And hereon we have a relative increase in our confidence about what happened. At least the order of the splits is pretty well agreed upon from this point. And very shortly after the origin of bilateria, we have the development of most of the remaining major clades of animals, including the split into deuterostomes and protostomes.

which I've discussed in the previous episode on the embryology and development if you're interested. But we need to pause here and talk a little bit about the fossil record during this time. Now, I mentioned earlier that there is very little fossil evidence for these early periods. We only get really good fossil records from around 540 million years ago with the beginning of the Cambrian. However, we do have some fossils prior to this, and in particular, the period immediately, prior to the Cambrian is called the Ediacaran, which goes from about 635 to 540 million years ago. So around the time, according to the dating way, using of the origin of bilateria, right up to the Cambrian explosion where we have the origin of vertebrates.

So during this time, the Ediacaran, we actually do have some fossils that are relevant to the story here. There are no generally agreed animal fossils to my knowledge prior to the Ediacarron. So the previous period is called the cryogenian, and before that we have the Tonnian periods. So the 760 million years ago, that's during the Tonian, and we don't have any fossils from then. After that is the cryogenian when it's thought to be large-scale freezing of the ice on the surface of the earth, and that's significantly disrupted animal life. And there were also other processes there that we think inhibited fossil formation. So we don't have much of anything there.

I think there's maybe a couple of putative ones near the end of that period. But generally, the fossil evidence that we have begins in the Ediacaran. So around the time that bilateria evolved. However, it's not hard-bodied fossils. Those only start with the Cambrian. It's soft-bodied impressions. So it's a much weaker fossil evidence. And what we do have is very confusing and hard to interpret.

The fossil evidence that we have from this period is called the Ediacaran biota. Basically, this is just any sort of animal fossils or putative animal fossils. We don't even know if they are even all animals that date from the Ediacaran period. So roughly 100 million years prior to the Cambrian explosion. The fossils that we have from this period are highly varied morphologically. So there's tubular shaped, frond-shaped organisms. most of them seem to be sessile, although there are some that have a more like worm-like or even hinting at like a jellyfish morphology.

But many of them sort of look vaguely like a sponge or a coral, but also very different from anything that currently exists. Many of the fossils are quite large, so up to two metres in size. So we're not just talking microscopic at this point. There's clear differentiation of different cells. So they're at least at the level of complexity of a sponge. They appear to be soft-bodied. We don't have any hard-bodied fossils from this period.

And mostly they are found as impressions on external surfaces that have then become fossilized. So not mineralization of the tissues themselves, but impressions that they have left on other surfaces. This also means that it's often very difficult to interpret what we're looking at, because we don't know if it is a flattened version of a, essentially like a sort of a silhouette of a much more complex, complicated shape that we're just seeing the impression of. We also don't know if it's an entire organism or only a part of an organism. So there's a lot of debate about what these fossils represent exactly and what they mean. Initially it was disputed as to whether any of these were even animals at all. At this point it seems pretty widely agreed that many of them, if not all of them,

are animals of some form, rather than say plants or fungi, as some had thought. But there's not really much agreement as to where they should fit into the tree of life, into the animal kingdom. So I've been talking about the different stages of evolution and split of the different groups in terms of the extant organisms that we know of, periphera, the sponges, tenophora, that's the comb jellies, placuzoa, the pancake ones, and Nideria, that's your jellyfish, and then bilateria, including the proterostomes, judoen, and so forth. So those are all living organisms. But certainly there are many organisms and probably vary, especially from the early period, highly varied types of organisms that don't have contemporary descendants. And so we wouldn't have space for those in our tree of life that's built around the taxonomy of contemporary organisms.

So the question is, where do these Ediacaran biota fit into our taxonomy, into our tree of life? are they more related to sponges or tenethora or nideria or bilateria or something very different? Where do they fit in? And there's, as I said, not been very much agreement about this, particularly because it seems that this time was, this time we're looking at the Ediocharan, was particularly poor for fossilization. And there are many different ways to interpret what we're seeing in these fossils.

I looked at a number of papers on this, and it's sort of hard to summarize. what I would say broadly is that scientists have attempted to place the Ediochari and Biota into classification alongside known taxonomic groupings generally put them as somewhere between around when the around the divergence of periphera up until the origin of bilateria so some put them between Periphera and Nideria and some put them a little bit before Periphera and just after Nideria and well before bilateria.

But what this would mean is that the organisms that we see in the Ediocaran biota that fossilized to give these structures that we have now originated around the same time as the split between periphera and the rest of the animal kingdom, so periphera and eumetazoa. And some of the other forms continue to then split off in the interval between when periphera split off and probably up to the time when bilateria emerged and split from nideria. So this would be between 760 and about 630 million years ago. So that's not when these organisms existed. They existed during the Ediacaran period.

But I'm talking about when they diverged from other animals. So you have the divergence and then there's a period of evolution and then they appear in the fossil record. So this can get a little bit confusing. But the idea is that the forms that we see in the Ediocaran biota represent quite distantly related forms of animals that are about as distantly related to us humans as sponges are up into, and some of them as as distantly related as comb jellies or as niderians, and probably all more distantly related to us than any bilaterally symmetric animals that exist today than any bilateria. Now, there are a few exceptions.

There are some organisms from the, from the Ediacarambiota, which are thought to be a bit more closely related. One of them is called Kimberle. And this is generally thought there was some controversy about this, but it seems from the more recent papers, as far as I can tell, that this is now generally thought to be bilitarian, which means it's quite a bit more closely related to us, although possibly not any known form of bilaterian, but more similar to us than any of the forms that branched off between periphera up to Nideria. The main reason for this is because the fossil impressions do appear bilaterally symmetric, and some have thought that it even resembles a gastropod, so it's a little bit like a slug. Although it's hotly contested as to whether

it can reasonably be called a mollusk. My suspicion personally is that it probably isn't a mollusk, but it probably is something like the ancestral form or the basal form of bilateria. So it may be relatively close morphologically to the early mollusks, but perhaps just is a little bit older than that. So this is quite interesting because it gives us an insight into what the earliest bilaterally symmetric animals may have looked like, the ure bilateria. And as I was indicating, ubaletaria from genetic and other considerations, probably looked like very simple worms. And that is what Kimberle looks like, at least as far as we can tell. We only have impressions of it. But the reconstructions indicate that it probably looks like not exactly a worm, more like a sort of a fat slug

which is sort of like an oval that protrudes a bit in the middle. So relatively simple morphologically, but still has, as far as we can tell, a number of the hallmarks of more sophisticated organisms. So it has bilateral symmetry and it has, well, we think it would have had triploblastic differentiation, so three different cell layers and a complete gut, although we can't tell that directly from the fossil evidence, but based on its other characteristics, we think that it probably has, these derived characteristics. So that's important because it indicates a significant development

evolutionarily, even compared to the, say, the jellyfish organisms that existed also around that time. This gives us a picture where around 630 million years ago, or perhaps slightly after that, bilaterally symmetric ancestors already had diverged from the jellyfish and from the tenophores and so forth, and existed as the sort of fairly primitive slug-like, worm-like or slug-like organisms that moved about on the ocean floor. Now, the next key stage in evolution is the development of the sealum. So the sealum is the main body cavity in most animals, and it surrounds and contains the digestive tract.

So you remember that the diploblastic animals, like the jellyfish, only have two germ layers. The endoderm and the ectoderm. Triploblastic animals, so that's all the bilitarians, they have a third layer, the mesoderm, which forms a complete gut. So that is basically a tube along the inside of the body. So the earliest animals, a sort of worm-like, the body plan there is called a tube within a tube. It's the complete gut that runs from the mouth to the anus, inside a tree. generally sort of tubular body plan. However, there are important limitations of this body design, which is just basically your tube surrounded by mesoderm tissue, where then the ectoderm surrounding that.

The main limitation is that there is no protection or limited protection for the internal organs and the digestive tract. If pressure is placed on the outside of the body, then that pressure will be transmitted directly onto the organs and the digestive tract itself. So that limitation means that for organisms that don't have acillam, or acillamates, they are quite limited in how large they can become and in how sophisticated they can get, because there's this limited protection of internal organs. And in order to have sophisticated specialization of a function, they have some specialization of function, but to have a greater level of sophistication,

you really need these larger and more complex organs, but they need protection. And so the sealum provides for that protection. So the sealum is a cavity filled mostly with fluid, which forms within the mesoderm tissue, I should say. It's sort of like a cavity within the mesoderm, which fills with fluid, that surrounds and protects the internal organs and the digestive tube. This was a very, very important evolutionary development. So as I said, it allowed for the development of more sophisticated organs because they have the protection from the compressive forces that are applied on the body.

Also, it meant that diffusion is no longer sufficient for a provision of oxygen. So acilamates largely rely on just diffusion through the external surface of the body, through the cells. And that's sufficient if you are relatively small. But for larger organisms and also organisms that have this fluid layer, that protects the internal organs, that's no longer sufficient. So there needs to be the development of specialized respiratory and circulatory systems. So the development of the sealum then facilitated the development of more sophisticated methods for bringing oxygen throughout the body, which in turn then allowed the body to get much larger

than was possible in, than is possible in acillamates, which rely on this diffusion. That serves as a real cap on the size that the body can attain. Another important factor is that retention of more fluid is required in a sealomate because it needs the fluid to fill up the cavity. This then increases the demands on having a more complex system of osmotic regulation. So that leads to the development, for example, of a urinary system. So these two things go hand in hand. The development of the selam allows for more sophisticated organs because of the protection it provides to those. But it also kind of acts as an impetus for that because you need a respiratory system, you need a

circulatory system, you need a urinary system now because of the new requirements of having a sealum. So in terms of exterior morphology, this doesn't change too much, not as much as, say, the development of bilateria does. We have contemporary examples of acilomates as well as sealamated organisms. So analyds, common earthworms, are silumates. So it's probable that their first silumated organisms looked more or less like these sort of earthworms, although analids are segmented and it's not clear whether the first silumates were also segmented. Segmented means that the body is comprised of many subunits which have a repeating pattern. You see segmentation in anales as well as in many arthropods, well all arthropods at varying degrees. Segmentation is a

further evolutionary development. So my guess would be that the earliest silamates were not segmented, but that is contested. But at any rate, putting aside the segmentation, a contemporary garden worm is perhaps not too far off what the earliest silamase would have looked like. So they evolved from asilamated flatworms. So we have flatworms still exist, and we know from genomic studies that they branched off from the rest of the bilaterally symmetric animals called nephrazoa, around, according to the dating. I'm used around 620 million years ago, though again you get somewhat different dates. Neferozoa, because all of these more sophisticated, silumated animals have kidneys of some form,

or at least the tissues to form a kidney, whereas the flatworms, they don't need that because they can just diffuse the oxygen as well as wastes in and out across the skin, or the outer layer of the organism. They're much smaller, and so don't have the same need for filtering, and, as I said before, they don't have the same, they don't need to retain as much fluid because they don't have the internal body cavity. A sealamates branched off about 620 million years ago, so not long probably after the bilateria emerged initially.

I should also mention that I've been talking about flat worms and acilomates as if they branched off from the rest of bilateria. That's not quite true because there are some organisms which subsequently then lost the ceilum and have reverted to a sort of simpler more basal flatworm acillamated morphology. So not all flatworms branched off at this time, but just to sort of keep things simple, at least it's the ancestral form, the more basal form further back in evolutionary time, was a flat worm that was acilamated, and then some aselamated worms branched off, and then the rest

of the animal kingdom, the ancestors, our ancestors, and the ancestors of most other types of animals developed sealums. Some later organisms then lost those, but that's not the focus here. So we go then from a flat worm to a rounder and larger, more sophisticated, sealomated worm that now has more complex internal body organs. So the next major split in evolution is the divide between the deuterostomes and the protostomes. This is one of the most important sort of branches or distinctions in the animal kingdom. The original basis of the distinction between protostomes and deuterostomes is that in protostombs, the blasterpore, which is like an indentation in the developing embryo, becomes the mouth while the anus forms later,

whereas in deuterostomes it's the other way around. And that is true for many protostomes, although it's now known that there's actually quite a lot of variation in what happens to the blasterpore in proserdoms. So that distinction is actually not the most important thing. However, it's still true that protostomes and judoenostomes form clades. So that means that they form a single branching episode in the tree of life. And they do differentiate two quite distinct groupings of animals. So protostomes include all of the arthropods, mollusks, anilids, as well as many types of flatworms and nematode. So those that subsequently, the flatworms that subsequently lost their selim, for example.

Deuterostomes, on the other hand, include chordates, which is where we fit, as well as echiniderms and hemichordates, which are special types of worms that we'll discuss again later. So most types of animals fall into the protostome category, but we're particularly interested in deuterostomes because that's where we as the vertebrates fit in. So it's thought that this split between protostomes and deuterostomes probably happened not too long after the split of bilateria. there probably wasn't any very dramatic body plan change that was required here. We're probably still talking about a primitive worm-like organism, but in this case that had a sealum. So we see that that initial body plan or something close to it was probably laid down at the time of the origination of uwe bilateria, or the common ancestor of all bilateral symmetric organisms, which is what we suspect that idiocaran fossil, the Kimberlella, may have resembled. the sort of oval slug-like organism or worm-like organism.

That then progressively developed, becoming a bit more sophisticated, developing Ascelum, for example, and then splitting off into two separate lineages, the protostomes and the deutro-storms. But at the time of the split, it's probably the two lineages looked little different from each other. It was just only subsequently that they became much more differentiated. Now, the next big step is the origin of the chordates.

Here we skip a few tens of millions of years forwards. The origin of chordates is thought to be about 550 million years ago, so just a little bit before the Cambrian explosion. The chordate ancestor is thought to resemble actually an extant organism, or type of organisms, called the hemicordates. Hemicordates are also called acorn worms, because they have sort of a little acorn-like structure around their head. Although it looks fairly primitive,

it actually has most of the components that will become the core organs and organ systems in more complex forms of chordates. It has a nervous system, although lacking a brain, it does have sort of anterior ganglion, which sort of serves a similar purpose. It has a complete digestive system. It has a simple respiratory system based on gillslets. It has a circulatory system, although one without any capillary, so an open circulatory system. So this is thought to resemble the ancestral,

of form of the chordates as of around 550 or so million years ago just before the Cambrian explosion. Interestingly, many of the major phyla in bilateria have relatively closely related phyla which have a worm-like morphology. So in the case of chordates, we have the hemicordates. In the case of mollusks, they have analyds, which are quite closely related. And in the case of arthropods, they have velvet worms, which are quite closely related to them. There's also quite a number of other different phyla of worms of slightly different forms, which, and while this is interesting, it doesn't prove that the sort of most ancestral forms of bilateria were worm-like, but that I think combines with many other pieces of evidence to indicate that the forms of bilaterally

symmetric animals that existed in the late idiocharan were likely slug or worm-like of some form, which then progressively differentiated into a much wider morphological range of animals over the course of the, or during the Cambrian and subsequent periods. That's what we see the proliferation of hard-bodied fossils as these sort of worm-like creatures gradually developed more specializations. In the case of the chordates, this took the form of the emergence of an organism which the earliest chordates almost certainly resembled lancelets or amphioxus. These are small, slender and mostly transparent little fishes, jawless fish, which do not have any eyes or definite heads, but they do show all of the key chordate features.

So that's a notocord, gillslitz, and a dorsal nerve cord. The notocord is a rigid rod, which is found in all chordates, generally only in the embryonic or larval forms, although in some cases, like in the case of lancelots, it persists, it persists into, adulthood. It provides, well, in most organisms it only exists in development, but in the case of the lancelets, it provides sort of rigidity and a structure around which the vertebral column develops for most vertebrates. Lanslets don't have vertebra, but most chordates do. Lanslets spend most of their time buried in the ground on the bottom of the ocean or in mud, although they can also swim around, and they're thought to closely resemble what the earliest chordates looked like, so you can see how that could have developed from a worm-like ancestor, essentially with just the development of the

notor cord and the dorsal nerve cord being the key evolutionary developments. Lanslets are chordates, but they're not vertebrates, because they don't have a vertebra, as I just said. Vertebrae proper evolved with the first jawless fish in the Cambrian and Ordovician periods. So this includes extant forms such as the hagfish and lampreys, as well as various other extinct forms, such as the conodonts, which I mentioned in a previous episode, and have very intricate mouth structures which have fossilized. So the vertebral column along the spine originated as local densifications of the notar chord, the main functions being to protect the spinal cord from external forces and trauma,

and thereby reducing the strain on the spinal cord during body motion. So the spinal cord is critical for the development of the vertebrae line. notar cord and then later the spinal vertebrae developed to protect, largely to protect the spinal cord. But the vertebral column proved to be extremely important because it served as a sort of a singular platform for supporting the body's weight, enabling motion of the trunk, and then providing a location for the limbs to connect to, which then ultimately served to support the body's weight and help to move it around. Now, none of these were sort of the initial purpose of the vertebral column. It was just to protect the spinal cord, but it would prove to be essential for these later developments,

which were necessary for the evolution of much larger animals. And we see that the vast majority of animal forms outside of chordates are limited to being quite small. I think the largest are found in some now extinct forms of insects and other arthropods, which could get quite large. But really nothing compares to the size of chordates that many chordates have achieved, obviously not all chordates, but many of them are quite large. And that's largely possible because of the skeleton, because it gives support and structure to such a large body,

which is just otherwise not possible or very difficult. So this origin here of the spinal column, which then led to the development, or went along with the development of the nodocord, and then noticord then densifying to form the spinal vertebra, which then extended overtime to form a skeleton, was a critical evolutionary step. At this point now, we've transitioned clearly into the period of extensive fossil evidence. At this point, also the timing of the developments and ordering

is much more precisely agreed upon, although the exact mechanisms are still somewhat disputed. So the next major step is the origin of jaws. Nathostomes are organisms that have jaws, or chordates, I should say, that have jaws. These originated in the Ordovician, but diversified substantially during the Silurian and Devonian periods. About something like 460 million years ago, up to about 360 million years ago, the end of the Devonian. The Devonian period is often called the Age of Fishers,

because it's a time when jawed fishes differentiated and were very highly dominant in the oceans, but as yet no tetrapods had evolved. Although Devonian is traditionally called the Age of Fishers. I mean, you can sort of extend that to the Silurian as well, which is a shorter period that was immediately before that. So the origin of jaws dates to just before this, the late Audivision period. Prior to that, during the Cambrian and Audubition,

the oceans were dominated by jawless fishes. The lack of jaws imposes significant limitations on what you can eat. Obviously, having jaws allows you to be much more effective as a predator to hold onto and manipulate your prey in a variety of ways. and also allows you to eat much tougher food. It's thought that jaws originally evolved from the brachial arches, also called gill arches. So these are a series of pared, bony or cartilaginous loops

that sit behind the throat of fish, and they help to support the gills. They look a little bit like vertebra on the diagrams, but they're not vertebra, they're distinct. They specifically exist to support the gills and give them structure and help them maintain their shape. Now, it's thought that gill arches, particularly the first three or so gill arches, fused together and progressively rotated horizontally. So gill arches are sort of roughly vertical, you know, relative to the mouth and the anus of the fish, right?

The jawless fish, the gills are sort of vertical. But the idea is that the first few gill arches gradually fused together and became more sort of horizontal and sort of angle. and then formed into the upper and lower jaw bones, as well as the bones that immediately brace the jaws. There is an alternative hypothesis which holds that jaws arose not from the gill arches, but instead from the lips of jawless fissures. I find the gill arch theory to be more plausible, although that's not necessarily based on anything in particular,

but you'll see both of those theories mentioned in the literature. But whether it was from the lips or whether it was from the gill arches, either way, we see that jaws originated in the Ordovician, and quickly we see that jawed fishes or Nathustomes quickly came to dominate during the Silurian and the Ordovician periods. So during these times, Nathor stones diversified enormously. There were many armored forms, many of the early forms who had extensive armor. These were called Placoderms. They're now extinct.

I mentioned them in a previous episode. Slightly later originated the Condrictis. these are cartilaginous fish which include sharks, rays and skates as well as some organisms that are now extinct and the Osticthies which are all the bony fish so this includes the ray-thind and the lobe-find fish this then brings us into the origin of limbs and the evolution of the tetrapods so everyone knows about the idea that millions of years ago the ancestors of mammals as well as other land animals emerged as the sort of fish crawled up onto the shore and sort of poked their nose around and decided that they liked what they saw and moved onto the land.

This is more or less what happened, and this describes the origin of the tetrapods, the four-limbed organisms. So all tetrapods share a common ancestor. The evolution of tetrapods begins around the late Devonian period. So this is around the end of the age of fishes, you remember, when quite a long time after the origin of the nathostombs, the geord fish. That was late Ordovician. We're now talking about late Devonian, which is about 100 million years later. Around this time, we see the emergence of some interesting transitional forms, which are sometimes called fischopods,

because they have a combination of the traits of early tetrapods as well as the traits of fish. We actually have quite a few very interesting transitional forms that show different stages in this. The main requirements in order for life to transition from the ocean to land is that the skeleton must be significantly strengthened and modified to support the animal's weight against gravity. This is completely different to the oceans where organisms are usually neutrally buoyant, so they float around. They don't have to support their own weight most of the time. But you do have to do that on land. And so that requires a significant expansion of the requirements. This relates to what I was talking about before with the importance of the origination of the skeleton.

Because other than fairly small organisms like worms, there aren't many forms of life that can exist on land and become larger. The only other phylum that has a large presence on land of any sort of fairly sizable body plan, like bigger than a worm, are the arthropods. And they have a hard external skeleton, an exoskeleton. So you need to have that structure in order to support the body weight of any large size. So in the case of the chordates, that became possible with the evolution of the endoskeleton, the skeletal system. That progressively developed and became more differentiated with the tetrapods. And also it formed the anchor point where the limbs would attach, which is critical because you need to have some sort of anchoring point for the limbs to move about, to move the organism. And that's the second point.

In addition to just supporting your weight, you also need to be able to move around. You can't just flip fins and swim around by pushing water. You have to sort of drag yourself or lift yourself up, and that's why limbs are necessary. The third key element are lungs. So you must be able to breathe the air instead of extracting oxygen from the water, the dissolved oxygen in the water. So lungs must be developed to augment the gills for breathing in air. Now, what's interesting is that it seems that fish in the lobed fin heritage, that eventually be the ancestors of the tetrapods

had already developed all of these key characteristics before they started to emerge onto land. So in other words, lobed fin fished and the early fishopods developed a more robust skeleton. They developed lungs, or at least sort of primitive versions of lungs, and critically, they started to develop limbs before they even emerged onto land, or before at least they spent very long on land.

So the lung or the swim bladder originated as an outgrim. of the gut, which then started to form a gas-filled bladder above the digestive system. The swim bladder is an organ that contemporary bony fish used to modulate their buoyancy, so they can expel air if they want to sink, and then they can swim up and gulp some more air and store it if they want to float. The swim bladder originated long before the evolution of tetrapods, but it appears that lungs developed from the swim bladder as an outgrowth of the gut. There are even fish today that still use their similar organs for breathing in air. For example, lungfish. So it appears that the ancestral form actually had an air-breathing lung,

which then subsequently became a swim bladder in bony fish, and formed the origin of lungs in the tetrapod lineage. This indicates that these ancestral forms that developed the air-breathing lung probably evolved in fairly shallow, warm waters, and they used their simple lung when the oxygen level in the water became too low. They swam it to the surface and gulps the air. So lungs already existed prior to the advent of tetrapods. I've already talked about how we know that the vertebral column existed,

so they already had an internal skeleton. It would be modified as the creatures began to, as early tetrapods began to move on to land. But then the key element that people often focus on is the origin of limbs, giving tetrapods, their name. So as I said, a series of transitional fossils indicates that the front and rear paired fins developed bones that are directly homologous to the humorous ulna and radius of the four fins of tetrapods, and to the fema tibular in the pelvic fins, which are homologous to

bones in our legs. So you can see that these structures became progressively enlarged and more robust and more differentiated from each other over evolutionary time until they become very clearly distinct limbs. So basically what happened is that these front and rear fins evolved progressively into limbs. There was not a sort of discrete stage where they stopped being fins and started being limbs. It was a gradual. The backbone also became stronger to prevent the body sagging under its own weight when it was on land. Also there is a reshaping of the jawbones. which allowed for a rudimentary middle ear to begin to develop, which allowed it to then hear better in air, because sound travels very differently in air compared to water. So that was another important requirement.

Some of you may have heard of the transitional fossil Tectolic, which is right at the middle of this sort of development where you see very clearly, it still looks like it's a fish with fins, but the fins are also quite robust and noticeably tetrapod-like. It's thought that these early tetrapod forms were actually quite poorly adapted to life on land. They would only barely be able to pull themselves along land, and probably they mostly lived in shallow muddy water or swampy environments, and so they only very rarely left the water, and probably couldn't do so for any prolonged periods. For example, it's thought that Tictalik mostly moved itself with its fins along the floor of streams,

and it probably only pulled itself onto shore for a very brief period, like contemporary mudskippers. So these early forms like Tectalik probably were still mostly fish in the sense of where they lived. And it was only later on that the major adaptations for land were made after the sort of crucial adaptations that allowed them to move onto land in the first place had already taken shape. So these adaptions occurred first and then there was the move onto land. The fish didn't start to move onto land and then adapt the, say, limbs that they needed. Those already existed, at least in a primitive form, before moving on to land. In terms of why these fissopods did this, it's thought that the main benefit was that there was

more food available on land. There obviously initially would have been limited competition there. Terrestrial plants existed by this point, as well as insects and worms. So there was no other vertebrate competition, but lots of potential food sources. Moving on to land could have also provided more oxygen in the air than the water did, as we discussed previously, and also potentially provided a means to escape predators. The most important reason, though, probably was simply that there was more food available on land, and that this provided an opportunity for them to access food that others do not have the ability to do. So to conclude, the evolution of tetrapods occurred in, as evolution often does, in a sort of indirect and ad hoc way, in that the primitive lung

and the primitive limbs probably developed for entirely different purposes, not in order to develop, in order to move on to land, but instead in order to, for, say, Tiktalek and other fish to pull themselves along the bottom of the stream, and in order for the fish to augment their oxygen supply when the oxygenation of the water was relatively low. only subsequently as fish began to sort of realize that they had access to a greater supply of food on the land did they spend more and more time there and become more and more progressively adapted to that environment. And then we have the evolution of the first true tetrapods in the early Carboniferous period. So that leads us into the final section of today's podcast where we talk about properly adapting to life on land, the evolution of amniotes.

these early transitional forms, the first sort of fishapods, the transitional forms, they lived at the late Devonian period, and many of them didn't last very long, but we see in the early Carboniferous period we see the development of a variety of amphibian-like creatures that lived quite close to the water, but now definitively out of the water, as opposed to the fishapods, which were still mostly in the water. Amphibians are, although they can live on land, still quite closely connected to the water. They lay their eggs in water. They breathe partly through their skin, and they generally undergo metamorphosis from an aquatic larvae form that has gills into an air-breathing adult form that has lungs. So you can still see some of the very close connections to their fish ancestors in amphibians. So this ecosystem that developed in these probably mostly like swampy, and moist rainforest environments fairly close to streams and the ocean. This ecosystem that developed in the Carboniferous period led to the proliferation of many

different types of essentially early forms of amphibians. But this all came to an end, or at least was significantly disrupted, with an event called the Carboniferous Rainforest Collapse, which occurred around 305 million years ago. So this was a complicated process in which, which the wet rainforests, which dominated most of the planet at this time, became progressively fragmented as the climate cooled and became drier. These changes favoured the evolution and diversification of animals, of tetrapod animals, which were better adapted to drier conditions

and to living more independently from the water. This then favored the, led to the evolution and then favored the diversification of a clade of animals called the amniotes. So they originated in. in the sort of mid-carboniferous period, but really only differentiated and radiated substantially in the subsequent Permian period. So that's the, the Permian is the final period of the Paleozoic era. So amniotes, you may recall, are organisms that are more strongly adapted to life on land

independently of ready access to water. I mean, they still need some access, obviously. They still are made up of mostly water, but much less so than amphibians. Contemporary reptiles, birds, and mammals are all amniotes. And in fact, because although there are still amphibians around, they're relatively diminished in terms of the number of species and the environments that they live in and have been in decline for some time. Most of the land vertebrates that have existed since the Permian have been amniotes. There are a number of major adaptations for terrestrial life that amniotes developed. The first was internal fertilization.

So this was the development of a special organ to deliver the sperm directly into the female, thereby avoiding the need to use water as the sort of substrate for fertilization. The way most amphibians reproduce is that the female lays eggs in water, and then the male fertilizes those eggs externally. But this requires ready access to water, like an external water source. So key adaptation to move away from that is developing a mechanism to deliver the sperm directly inside the female. hence the development of a penis. A related development is the amniotic egg.

So this we discussed in the developmental biology episode. But here we have the movement away from the larval stage. Remember, amphibians have a larval stage where they live in water for a while with gills until they develop into the air-breathing adult stage. So here we have, with the amniotes, we have the elimination of the larval stage. Instead, the embryo develops inside a semi-permeable shell that provides protection whilst still allowing gas to enter and exit so they can still, have access to that. But these

specialized membranes help to protect and surround and protect the embryo. It's thought that initially these eggs were probably sort of like parchments covered, had the consistency of parchment so that they were soft but still provided somewhat of

a protection. And only later in the Triassic it seems that the full hard-shelled eggs developed. Another key development is waterproof skin and scales. So keratinized skin helps to prevent drying out and protects against environmental hazards. That's distinguished from the soft and water permeable skin of the amphibians. Again, best when you have ready access to water and

you can keep yourself moist. Whereas if you need to conserve a lot of water, then that doesn't really make sense. You want to have a much stronger and waterproof layer that prevents the animal from drying out. Another important change is the development of coastal respiration. So this means breathing by expanding and contracting the rib cage, generally with something like a diaphragm, that alters the pressure inside so that the air moves in and out passively. It's much more efficient for land animals than buccal respiration, which is what amphibians use, which essentially involves sucking in the air into the cheeks and then moving it down into the lungs. Clostal respiration is more efficient, but involves more evolutionary steps away from the

ancestral form that made sense for fish, which also had gill-based respiration, that they initially augmented that with the sucking an air into the swim bladder or primitive lung. That then developed into bucule respiration in amphibians. So amniotes here are moving away from that by developing a more efficient form of respiration, but it's only effective on land. The need for greater water retention also requires more efficient kidneys that are able to extract more of the wastes while retaining water. And so this leads to the development of metanephric kidneys, so more efficient kidneys. There's also the development of the development of metanephyric kidneys. There's also the development of adrenocortical tissues near the kidneys and elsewhere,

which helps to control metabolism and, in particular, the water balance in a more sophisticated way, which, again, is obviously necessary in operating away from water sources. So you see that all of the key developments of early amniotes are really focused on moving away from water and being able to live in dry environments. And this seems to be partly explained by the climactic changes that occurred around this time around the sort of late carboniferous, in which the climate became drier, meaning that the organisms could no longer thrive so much in the wet rainforest conditions that previously had been very beneficial for the early amphibians. Now, of course, I've been talking about this

as if the animals were sort of like deliberately trying to develop these traits in order to exist away from water. But again, we're just talking about that in terms of, from the perspective of hindsight, these traits were necessary in order for the animals. to survive away from water and in drier conditions. And so the organisms that progressively developed these traits were ones that survived. This then gave rise to the amniotes. The amniotes then very rapidly diversified in the Lake Carboniferous into a range of different types of organisms.

In particular, the reptiles, the para-reptiles, and the synapsids, which eventually gave rise to the mammalian heritage. However, at this point we've reached the Permian period and the phase in the Earth's history where we have now large land animals that can move about and live in a wider range of environments. And so I'm going to discuss all of that in a subsequent episode. We'll look at the radiation of reptiles, including the dinosaurs and other forms of reptiles. We'll also look at lesser known clades like the parareptiles and eurapsids, for example. and we'll also talk about the synapsid heritage, which takes the form of a variety of forms that are progressively less reptile and more mammal-like until we finally see the evolution of mammals proper in the Jurassic and Cretaceous periods. So that will be the focus of the next episode, but let's, before we end, take a step back and summarize what we've discussed here.

So the focus of today's episode has been on tracing the evolution of the key traits and basic body plans of animals, beginning from the split with protests about 760 million years ago, right up until the evolution of amniotes about 330 million years ago. So we're talking about a bit over 400 million years of evolutionary history here. Particularly the early stages are not very well known, although there are a number of hypotheses which are consistent with the data. and so I've presented some of those to you here. Let me just go over very quickly the sequence that we've discussed and highlight the key traits and the different groups and when they diverged from each other. So we began 760 million years ago

with the divergence of metazoa, the origination of metazoa and its separation from the coanoflagellates, which are single-celled protists, which have these flagella, and they move about and collect food into their into their collar-like little pouch. It's thought that one of the earliest forms of animals was a form of commonoflagellates that then became obligately colonial,

so they could only survive together, and then gradually that developed into a single organism in which the cells progressively differentiated, eventually forming something like modern-day sponges, the periphery, the oldest branch of animals to split off. The next branching event occurred, a long time later, maybe 650 million years ago, this was the development of true tissues and the two germ layers, the endoderm and the ectoderm. And it's not entirely clear which ordering

this occurred in, but the one I'll go with here is that the tenethora branched off first, and then Placozoa, that's the sort of pancake-like, very, very simple organism, and then the nideria, the jellyfish. According to the Placula hypothesis that I discussed, the our ancestral forms maintained a placular-like morphology until until our ancestors gradually evolved a form in which they in which the organism sort of bulged in the center and folded up eventually forming a sort of like a bag which then was not relegated to the ocean floor but then could move about and swim about freely and this resembles something like a jellyfish that's the placular hypothesis the idea there is that potentially that bag

developed into a more sophisticated jellyfish, which eventually returned to the ocean floor, landing on its mouth, elongating and flattening and becoming something like a slug-like or a simple flatworm organism along the light of the Kimberella, which was the fossils that we talked about as part of the Ediocharan biota. The other hypothesis, or another hypothesis, the planular hypothesis, is that rather than originating from a jellyfish form, bilaterally symmetric organism, instead originated from a planular, bilaterally symmetric lava of a jellyfish-like ancestor, which then maintained that morphology into its adulthood,

eventually moving to the bottom of the ocean floor once again and beginning to crawl along on cilia. Either way, it's thought that the urobiliteria, or the ancestor of all bilaterally symmetric animals, existed about 630 million years ago, and had the morphology of something like a sort of a slug or a very simple flat worm. But at this point, it already had developed three different germ lays, so triploblast, as well as bilateral symmetry, and a complete digestive tract from mouth to anus. The next stage was the development of a sealum, which is the body cavity, which develops as a sort of a cavity that opens up inside the mesoderm and provides that's filled with fluid

and provides protection for the internal organs. The next major split, occurring sometime later, around maybe 610 million years ago, between Deuterostomes and Protostomes. It's thought that the ancestral Deutrostome was probably something very similar to a hemichordate, which is an is a scillomate worm, which has many of the important morphological characteristics of contemporary chordates, but it was only with the development of chordates proper that we saw the origination of the notor cord as well as the post-anal tail and gillslets, which are characteristic of contemporary chordates. And the ancestral chordate form seems to have been something

like a lancelot, which is a small slender fish that generally burrows into the ocean floor. Vertebra proper evolved with the first jawless fishes at the start of the Cambrian, as the spine originated as densifications of the notar cord and helped to protect the spinal cord from external forces. The evolution of the spinal cord was critical because it would be essential for the development of the support structure, the internal skeleton that was necessary for the evolution of limbs and the support of supporting the weight of the early tetrapods as they moved on to land. But before we get to that, the jaws evolve around the late order vision. So that's about 450 or so million years ago with the origination of napostomes. It's thought that those

jaws originated by a fusing and shift into a more horizontal positioning of the gill arches to form jaws, which gave significant advantages for accessing and manipulating prey. Next, then, we have the origination of tetrapods, which occurred as low-fined fish, gradually acquired a number of characteristics necessary for life on land, including more robust skeletons, primitive lungs that could use for gulping air, and primitive limbs, which evolved from the paired frontal and rear fins. It's thought that most of these developments occurred as the organism still spent most of their time in the water, but then gradually they were able to spend more and more time on land because

of these developments, which enabled them to gain greater access to food that had relatively little competition, because there were no other vertebrates on land at this point. The final adaptation was precipitated by the need for greater independence from water as the rainforests of the carboniferous gradually dried out. This led to the evolution of proliferation of the amniotes, which have many adaptations that allow for less dependence on water, including internal fertilization, the amniotic egg, waterproof skin, costal respiration, and metanephyric kidneys. This leaves us then with the development of the first amniotes, which you can think of as roughly lizard-like in morphology. We'll talk about their form a little bit more

later. At this point, leaving off about 330 million years ago, around the beginning of the Permian, our ancestors are sort of small lizard-like organisms, which have adapted to the drier conditions of the late Carboniferous and moving into the Permian, but have yet to differentiate into the different forms that we'll see with reptiles, parapetiles, and synapsids. And we'll discuss more in a future episode. So hopefully you found this interesting and gives an idea of how animals developed from very primitive single cellular forms right up to those sort of simple lizards. And in the next episode, we'll talk about further differentiation and development of, particularly differentiation between reptiles and mammals over the next few hundred million years. If you enjoyed this episode, consider supporting the podcast by leaving a favorable review or a rating on Spotify or iTunes or whatever other aggregator you use.

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