Vertebrates - Chapter 8

(birds chirping) (frogs croaking)

So, who are the vertebrates? Well, vertebrates include a whole bunch of very familiar animals. In the oceans we have fish and sharks, and on land we have frogs, salamanders, lizards, snakes, birds, mammals, including us of course. The key defining feature of vertebrates is the backbone, which serves two important functions. It's a crucial part of the skeleton, but it also protects a bundle of nerve fibres that connect the brain to the body.

Now, vertebrates are all large, active animals packed with muscle. The ancestral vertebrate strangely may have looked just a little bit like this modern basking shark. The shark swims around with its mouth open, water enters through the mouth and then out through those slits in the side of its neck. It's a type of feeding called filter feeding in which it traps any particles in the water.

Of course, there's one really big difference between the ancestral vertebrate that lived around 500 million years ago and this modern basking shark. The ancestral vertebrate was tiny, just a few centimetres long, whereas this shark can grow up to 12 metres. So that's one of the big questions that we need to answer in today's video, how is it that animals become really large? What are the challenges they face in doing so? And what are the advantages to becoming a giant?

The second thing we need to look at is to consider, okay, so there's a load of vertebrates in the ocean, but there's also a load of four-limbed vertebrates called tetrapods on land. Just how did they get there?

So we know that the tetrapods evolved from a fishy ancestor, but which one? Well, let's go back 400 million years ago to a time when the oceans were teeming with fish of all shapes and sizes. There are sharks, ancestors of the modern sharks, but they're not the meanest fish on the block because incredibly there's another group of fish, the armoured fish, and some of those are truly massive, including this top predator, Dunkleosteus. It's the size of a modern great white.

Now actually, those armoured fish left no modern descendants, but swimming around lower in the food chain were a load of bony fish, and they come in two kinds. There are the ray-finned fish, which have small fine bones in the fins, and they've become very, very successful. So nearly every fish in today's ocean that you can think of is actually a ray-finned fish. But there was another kind of bony fish that was rather common back then, and they're called lobe-finned fish.

And in today's ocean, you'd have to look really hard to find one. In fact, this is pretty much the only species that you can find in the oceans of a lobe-finned fish. It's called a coelacanth, and it was only discovered around a hundred years ago. People call it a living fossil because it looks similar to many fossil fish, but nothing like most of the ray-finned fish around it. Now, this fish or something rather like it gave rise to the tetrapods, but how exactly did that happen?

So around 380 million years ago, a lot of fish are living in coastal waters and there are forests on land now, and some of that vegetation is coming into the water and rotting away. And when vegetation rots in water, it uses up oxygen. That means that for the fish in that water, they're struggling for oxygen. What they need to do is get their noses into the air, like many modern fish do today. They can actually breathe air to supplement the oxygen in the water.

But if you're gonna get your snout into the air, it would be really useful to be able to push up from the bottom with your fins. And the fish that are best suited to that are the lobe-finned fish, because in those lobes there is quite a significant, substantial arm bone. And so that's what happened. One group of lobe-finned fish started to push up and develop legs on the front of their bodies while they still had a fishy tail.

And then eventually, a four-legged tetrapod appeared with four limbs paddling around in those swampy coastal waters.

Now, the early tetrapod was actually a very large animal, more than a metre long. So is that coelacanth, that lobe-finned fish. It's two metres long! So these are large animals and they need some adaptations to their bodies. Those bodies are packed with muscles, and muscle cells are actively respiring. That means they need a lot of glucose and a lot of oxygen, and they're also producing carbon dioxide which they need to get rid of.

But the muscle cells are buried deep in the body, and that's a long way from the source of oxygen, which is either the water or the air, and a long way from the gut of the animal where the glucose is being absorbed. So what do they do? Well, they have to invent organs, specialist groups of cells that are going to provide resources to other cells in the body.

So one of the things they need is a circulatory system, and that's a set of pipes that's going to deliver oxygen from either the lungs or the gills to the muscle cells, bringing oxygen and taking carbon dioxide away. And those pipes can also pass by the gut and pick up glucose and transport that to the muscle cells. Now, the liquid flowing in those pipes is of course blood, and the flow is kept moving by another organ called the heart.

The heart is just a muscular pump that keeps blood flowing around the pipes. Now, the tetrapod circulatory system has two parts. There's the part that pumps the blood around the body, and there's the part that takes the blood just to the lungs. And the heart is in two halves. The left hand side of the heart is much bigger and more muscular, and that pumps blood around the body, while the right hand side of the heart is smaller and not as muscular, and that just pumps blood to the lungs.

So we've seen that as animals get larger they have to make fundamental changes to their bodies, developing new organ systems for example. But there are even more fundamental changes that occur as animals get bigger. So let's start by looking at a really tiny animal, this ant. Now, ants are famous for being able to carry heavy weights. These are leafcutter ants, and they can carry a piece of leaf that's up to nine times their own body mass.

But you or I can't carry half a tonne on our heads for a mile or two. Why not? Well, as animals get larger, their volume increases faster than their surface area, and there are lots of biological things that scale with volume or with surface area. So weight increases with volume, whereas the strength that an animal has is proportional to the cross-sectional area of the muscle. That means as animals get bigger, they're becoming more and more underpowered.

Their strength is not increasing as fast as their weight. And you can see this when you look at large animals moving around. Look at this elephant. Look how slow and ponderous it is just moving one heavy leg after another, whereas this hamster in a wheel looks almost bionic in comparison. Now, these kinds of facts can be used to help us reconstruct the way animals that are now extinct once moved.

So for example, this huge, heavy sauropod, we know that it must have moved like this and not like this.

So if the rewards of getting larger are just to get slow and ponderous, then why would any animal want to get large? There must be some advantages too, and of course there are. So we said as an animal gets larger the surface does not increase as fast as the volume. But that also means that something like an elephant has quite a small external surface for its weight. And that means, because an elephant is a warm-blooded animal, that it doesn't lose heat very rapidly to the environment.

And that's an advantage that can make you very efficient. And that means, for example, that an elephant only has to eat its own body weight in food every 30 days. Okay, it's still got to eat a lot of food, but it can focus on eating low-quality food, of which there's plenty on the planet. It can just browse on branches and leaves. The mouse, in contrast, has a large surface for its volume, and that means it's got to eat its own body mass in food every six days.

And it can't just eat low-quality food. It's got to focus on finding really energy-dense food to keep that fast metabolism going. Now that's one advantage then of being large, but there's a second one as well, especially if you are a herbivore. If you become very, very large, then you're going to become very difficult for a predator to kill. And you can see that even lions can be kept off by elephants.

They are large and intimidating, and we believe that even in the past, those huge sauropods were pretty much immune from the most fearsome predator of all time, Tyrannosaurus rex.

So we've seen that there are many advantages to large body size, and we see in the fossil record that often groups of vertebrates do evolve large body size. But in those first steps that the vertebrates took when they came out of the water, what tempted them to come onto land? Well, there obviously must have been opportunities there and food there, and those opportunities were only there because before the vertebrates came out onto the land, the plants did the same.

And that's what we're going to look at in the next episode.

Well, I really hope you enjoyed that video, and if you did, please do share the link with colleagues and friends. If you'd like to get your own copy of the book of course, there's also a link below. There's a lot more in chapter eight about vertebrates, including more about those scaling relationships and exactly how they work, and also a section on animal development. How do animals go from just being a single cell to being a complex body full of organs and tissues?

Otherwise, look out for the next episode, which is all about plants. (birds chirping)