Animals - Chapter 7

- Hello, my name's Lindsay Turnbull and I teach biology at the University of Oxford. In this episode, I want to explain how the animals leapt onto the Earth stage 541 million years ago and took the world by storm. (birds chirping)

(frog croaking)

(birds chirping) If we squeeze the entire four and a half billion years of our planet's history into a single calendar year, then this episode is really all about the 18th of November because that is the day when the animals really spring onto our planet in this great profusion. Now, all animals are made from eukaryotic cells and they themselves arose on around 7th of August, but they spent about a billion years not creating larger beings, just living alone.

And in fact, that period is sometimes called The Boring Billion by geologists. But once they started to gang up and make multicellular beings, then boy, did they get going quickly. It's like they went into some kind of creative frenzy, creating animals like this incredible Anomalocaris, which terrorised marine ecosystems around 500 million years ago.

Now, Anomalocaris is an animal, and although you may never have seen this animal before, you probably don't have too much difficulty recognising that it is an animal. So let's just stop and think about what an animal is. If I asked you to name a few animals, what might pop into your head? Maybe something like this, or this, or this?

Sure, all those things are animals, but actually they all belong to the same group of animals, the same one that we belong to and zoologists recognise at least 30 different animal groups. They're called phyla. We talked about the animal phyla, and a single phyla is called a phylum. So what about this, or this, or this? Well, these are also all animals, they just belong to different phyla. So what exactly is it that makes an animal an animal?

So what makes an animal an animal? Well, the first and most important thing is that all animals are descended from a single common ancestor, and that ancestor didn't give rise to any other type of organism. In other words, the animals can claim this ancestor as exclusively theirs. And what did it look like? Well, it probably looked a bit like this. So it's just a type of eukaryotic cell, rather unremarkable.

It has a long twirling flagellum that it spins around to reel in bacteria, which it then engulfs rather like the amoeba, so it can extend little blobby extensions of the cytoplasm and engulf these bacteria whole. Why do we think this was the ancestor of the animals? Well, because today, there are still cells around that look a bit like this, and they do something odd.

So when there are certain kinds of bacteria around in the water, as this cell divides, the daughter cells stay together and they form a small colony, a little like a miniature organism. But it's not a true animal because those cells can also break apart again and go their own way and a real animal can't do that. But one of the simplest kind of animals that we know about is called a sponge. So there is a phylum of animals called the sponges. You may not think they're animals, but they are.

And if we slice through the sponge, we see it just has these thin walls and the inner wall is formed from cells that look very much like these cells. So they also have these long flagella that they twirl around and they pull water through the sponge and the sponge cells eat all the bacteria in the water. Now, there are other things about sponges that make us sure that they are also animals.

All animals eat other cells, none of them can photosynthesize or do anything like that, so they rely on eating other cells to get the food they need. Sponges also produce egg and sperm cells. So when a new sponge forms, it's formed from the fusion of an egg and a sperm cell, which then starts to divide to form a multicellular body. And that multicellular body is made up of several different cell types.

So in fact, we could almost say that all animals that have ever lived are little more than glorified sponges.

Now it is a bit hard to believe that a sponge is an animal. They really look so different from all the other animals, and we think they're one of the groups that evolved first. And there's something unusual about the way that they feed. Sponges just eat individual cells. They can't tackle bigger prey, but all the other animal groups feed on multicellular things. And if you're gonna do that, then you need a new innovation.

You basically need a body cavity into which you can put that thing that you've caught and then you can pour digestive enzymes onto it and break it down into the molecules that you want to build your own body. Now, the next two groups of animals to evolve both have that innovation. They're both different kinds of jelly animal. The first is called the cnidarians. So those are corals, jellyfish and sea anemones. And they are fearsome predators, and they have a special adaptation.

They have a kind of cell that contains a little venomous harpoon that they can fire out to sting their prey to death, and then they bring it into the body cavity and digest it. The second group of jelly animals are very beautiful animals called comb jellies. They're not as successful today as the cnidarians. There aren't so many species. They don't fire little venomous harpoons.

They fire little glue covered nets that also can trap their victims, but they're far too weak to capture a human, so you don't need to worry about them when you go swimming. But the jelly animals, as well as being able to capture larger kinds of prey, also evolved a couple of other key innovations shared by all other animals, and that is muscles to allow bigger movements and also nerve cells to coordinate those movements so they don't just wobble like a...well, like a jelly.

So let's look at how muscles work. Remember, animals are made from eukaryotic cells, and one of the key features of eukaryotic cells is that they have this cytoskeleton. These are the dynamic filaments inside the cell, and muscle cells have a very special arrangement of those filaments. They have them in parallel rows and two sets of filaments. So if you put your hands up, stick the thumbs up, and have your fingers aligned like this to represent the two types of filaments.

So one hand is one kind, and your other hand is the other kind. And that your thumbs here represent the ends of the muscle cell. Now, when the muscle cell contracts, it needs to get shorter, and that happens because the filaments slide between each other. And if you notice, your thumbs have moved closer together so the cell has contracted. The problem for the muscle cell as it's actually stuck like that now, so it can't pull itself back again. So muscles always have to come in pairs.

So there would be a second muscle that would work in opposition to this one that would pull the filaments apart again and re-extend the cell. So we always find muscles working in opposite pairs. So for example, in the body of this little hydra, which is a type of sea anemone, you see there are muscles in the body wall and there are two sets of muscles. There are the ones that run top to bottom. And when those contract and get shorter, then the animal becomes short and squat.

But there has to be a second set of muscles or it gets stuck like that forever. And those go in rings around the body, and when those contract, they squeeze the body in and it becomes long and thin again. And so by having these two sets of muscles, this hydra can be as long and thin or as short and fat as it likes.

All of the remaining animal phyla are what we call bilaterians. And that means they have a single plane of symmetry that cuts down the middle of the body, giving them near identical left and right sides, but meaning that the back is different to the front and crucially, the head is different to the tail.

And this means that around the head, a whole bunch of sense organs have evolved, things like eyes, ears, and whiskers to allow the animal to sense the environment well and to move purposefully forwards. And they often have additional appendages to aid movement. So what are some of these bilaterian phyla? Well, we don't have time to look at them all, but there are three important ones. One is a group of worms called the annelid worms. That's the biggest worm phyla.

These worms have segmented bodies, so an earthworm is a very familiar example of an annelid worm. And by using the segments and contracting the segments in coordinated ways, a worm can move purposefully through the soil. The problem is though that if you are moving across a surface, dragging a heavy body around is not very efficient. It would be much better to sprout legs. And that brings us to the second, the most successful phyla of all time probably, and they're called the arthropods.

That's things like insects, spiders and crustaceans. They have a hard exoskeleton and they have legs, and some of them even have wings, And they're hugely successful. The crustaceans dominate the oceans and the insects dominate the land. Insects have an extraordinary array of mouth parts so they can eat just about anything. So there are far more species of insect on land than just about any other phylum. The last phylum that's very important is called the molluscs.

They're also a very diverse group. There are snails, which you're probably quite familiar with, but there are also bivalves, things like oysters and mussels that people eat. And the third type of mollusc is called a cephalopod, and those are octopuses and squid. And humans are really fascinated with those because they're really very intelligent animals. In fact, the closest thing we have on Earth to a true alien-intelligence.

So let's bring back Anomalocaris, that predator from the marine environment from 500 million years ago and have a better look at it. We can see that it does have a single plane of symmetry running along its body. It's got near identical left and right sides, but the back and the front are different, and certainly the head and the tail end are different.

And at the head end, we can see powerful sense organs, large compound eyes, and these downward curving appendages with which it rootled around in the sand or mud, hoping to kick up some prey that it could hoover up with that mouth. We can also see that it was an arthropod. It has a hard exoskeleton, a segmented body from which appendages sprout, similar to other arthropods today, but also of course, very different.

We don't have anything exactly like this, but we can see the similarities with modern arthropods. And in fact, there are incredible fossil beds from this time in which we can see all kinds of modern phyla represented. There are cnidarians. There are arthropods. There are annelids. There are molluscs. So it seems that many modern animal phyla simply sprung into being in a very, very short space of geological time. Why did that happen? How did that happen? Well, we don't really know.

It's still a very exciting area of research. What we do know is in amongst all those animals, there were tiny little things that looked a little bit like a primitive fish, and they came from our phylum, the chordates. And over the next tens of millions of years, they would super-size and become animals that would dominate the oceans, but also go on to conquer the land. And we're gonna look at those in the next episode.

Well, I do hope you enjoyed that episode and you found it useful. And if you did, then please consider sharing the link with friends and colleagues. There's a lot more information and detail in the book, of course, about animals, including a lot more about other animal phyla and also some speculation about what did cause that incredible explosion of animal life 540 million years ago. There's a link to where you can find the book below.

Otherwise, join me next time as we explore that phyla, the chordates, to which both you and I belong. (birds chirping)