Climbing out of the oceans
Chris - Why is that a challenge for things coming out of the water?
Stephanie - The challenge is, is that in the water, we usually find things like fish and fish have fins and they can float around in a very buoyant way. But on land, that just doesn't work. You can't move around on land with fins and the air doesn't keep you buoyant. So, in order to come out of the water onto land, you have to reconstruct your whole skeleton, all your muscles, your physiology, your sensory systems, how you deal with dehydration, all sorts of things, in order to be able to survive out of water and under the forces of gravity.
Chris - So, it's not trivially just crawling out of the water one day and going onto land. There's a lot of changes that have to happen to an animal to make it capable of surviving on land for any period of time.
Stephanie - Yes, there's a whole suite of morphological and biological changes that need to occur in order for this major transition to happen.
Chris - Do we know what order they must've happened in? Did they all at once? Did they happen sort of slowly, everything at the same time? What do we know about this?
Stephanie - So, it happened in a period of time known as the Devonian and that's about 400 million years ago. It happened basically over about a 50-million year period. At the beginning of this period, we find fish and at the end of this period, we find land animals. And these land animals are known as tetrapods. Tetrapods are all animals that have four limbs with digits - so fingers and toes.
Chris - What is striking to me is that you're talking about a period which is not that long after the period that Alex was talking about. He's got some of the most primitive life or at least impressions of it in front of him, very, very basic things that look like leaves. Within a hundred million years, you've got animals crawling out onto land.
Stephanie - Yeah, well for a long time, life was fairly simple. We had very unique organisms as Alex was mentioning before where we don't really understand their morphology and biology. But when we get to about 530 million years ago, we came upon a Cambrian explosion. The Cambrian explosion is very important because this is where we see an explosion in the diversity of life, all sorts of different animals. Not just simple animals, but animals that are plant-like, all the way up into giant predators. It's after this period of time that organisms start to focus and start to evolve in different directions, into animals that we have around today. In the Cambrian explosion, we actually have the precursors to vertebrate animals. So, these are animals that have backbones and these are the animals that eventually turn into tetrapods.
Chris - What would the land that these animals were beginning to come up onto have look like? What was there already if anything?
Stephanie - Well, at the beginning of the Devonian and up until sort of the precursors before tetrapods were starting to evolve, there wasn't really anything on land. Almost everything lived in the water. So, if you actually got in a time machine and you went back to the Devonian, and you stood on the land, it would've been really, really quiet. There wouldn't have been any birds flying around and chirping, there wouldn't have been any bees humming around.
Chris - I bet, if you lived in Britain, it would've been raining.
Stephanie - There might have been a bit of rain. But it would've been a very, very quiet place. The waters were a different story. They were very, very active with marine life. From invertebrates, so things like shelled animals and corals and stuff like that, all the way through to a massive diversity of fish. And so, it's during this time that the fish start to change. Certain groups of fish start to change. These are known as the lobefin fishes. They're called lobefin fishes because their fins are actually much different than other fish fins. They start to develop bones in their fins that look much like our bones. So, they start to develop things like a humerus and a radius and ulna. And it's from within these lobefin fishes that we see the evolution of tetrapods.
Chris - So, if it was such a boring place on land, what was the lure for these animals to come out of the water and want to go onto the land? There were plants there presumably.
Stephanie - Yes. So, by the time that these animals were starting to want to come out onto the land, we have the beginning of plants actually forming communities on land. So, we start to see an environment which could protect animals. The waters were incredibly busy - big predators, huge fish, a few meters long. They were really fierce predators. So, if you could survive that of the water, just for a short period of time during the day, you probably had a very good shot of surviving that day. But there's also a food source because we now have some invertebrates living on the land. Another factor is oxygen. So, we're starting to see the development of plant communities on the land and plants actually burrow their roots into soils and this starts to erode the soils. That actually gets deposited into water. We also have leaf litter and stuff like that and when that goes into the water as well, we start to reduce the amount of oxygen in the water because things are decomposing. Now, these early fish tetrapod animals were actually living right at the water's edge. This water might have become depleted in oxygen. So, if you could find a way to get your head out of the water and breath oxygen from the air, you might have also had a better shot of living.
Chris - What's your name?
Meluka - Meluka and I'm from Cambridge. My question is, when you said that to go out onto the land, the animals had to change all their muscles and their bone structure. How much time would it take?
Stephanie - That's a very good question. I would have to say - I mean, as I said, the period of time where we go from animals that are very much fish into animals that are tetrapods - so animals with limbs, with digits - is about 50 million years. So, it took about 50 million years to get to the body plan that we see today in a modern tetrapod.
Niko - I am Niko and I am from Longstanton. My question is, what's the amphibian up there?
|A life reconstruction of the early tetrapod Ichthyostega from the Late Devonian of East Greenland - ulia Molnar, Stephanie E Pierce, The Royal Veterinary College.|
Chris - There's very large animal on the screen behind you. What is the animal?
Stephanie - The animal. Well, this is an animal that I worked on a lot. Its name is Ichthyostega and it is an early tetrapod. It's actually one of the first early tetrapods to evolve. So, this animal isn't a fish anymore. It actually has limbs with digits. So, it has fingers and toes, but it still mainly lived in the water.
Chris - How big was that?
Stephanie - That animal could've got up to about a meter and a half. So, it's probably the size of a giant dog.
Chris - It looks like a massive lizard. It's a sort of big lizard-like thing with very big teeth.
Stephanie - Yeah. It wouldn't have lizard-like skin. It would be more like salamander-like skin or fish-like skin, but it was a fearsome predator in the water. It had a huge head with massive teeth. It had huge big arms and it had a long tail. If you look at the very back, what you'll notice is that it has its hind limbs sticking out and they very much look like paddles. They actually look like seal flippers. And so, this animal is really, really good at swimming. But because it had this big four limbs, it actually could haul - what we think - that it could haul itself out of the water, using its four limbs and probably bask in the sun on mudflats and perhaps even feed on some of the animals that were on the shoreline.
Chris - How do you know it was pulling itself up because you've got sort of fossilised tracks left by these creatures in the mud?
Stephanie - Well, we actually performed a study on these animals. So, part of the research that I'm involved in is actually reconstructing the 3D skeletons of these very early tetrapods. To do these, we use x-rays and specifically, we use micro CT scans. What that allows us to do is force x-rays through the fossils and then we can resolve the fossils that are hidden inside the rock. We can use intricate 3D modelling software to go through, colour in each of the different bones. And eventually, you can come up with a whole 3-dimensional reconstruction of the skeleton. From this, we can start to move the limbs around, how did they move, how much mobility was there, and what can that tell us about how the animal was moving.
Chris - Kate...
Kate - This picture on the wall is green, like a lot of the dinosaurs that I've seen, but (Chris Barrow) has asked us on Facebook, how do we know what colour dinosaurs were?
Stephanie - If we talk about what colour early tetrapods were, that is a very hard question indeed because we actually don't have the material in order to test that. If we want to talk about the colour of dinosaurs, well, a lot of work has been done recently, looking at the chemical signatures of feathers using various new technologies. Some of the techniques that they've used in order to examine these feathers have shown that different dinosaurs, well, the non-avian dinosaurs might have been for instance, white or black or red in colour.
John - My name is John from Steeple Bumpstead. Is there any evidence that any animals went back into the sea?
Stephanie - Lots of animals went back into the sea. So, one question is, it's very hard to determine if a very early tetrapod had an ancestor that was land based and really quickly went back into the sea because the timeline isn't that great. Our fossils, we get one at a time, but whales are mammals that went back into the sea. We have in the Jurassic and Triassic a huge amount of reptiles that went back into the sea. These are called plesiosaurus and ichthyosaurus. Ichthyosaurs actually looked like dolphins, but they were indeed reptiles. So, there's many instances of tetrapods going back into the see and it's probably for a very similar reason for why tetrapods came out - to get away from predators, to find food sources that weren't being exploited.
Arushon - My name is Arusha from Cambridge and my question is, is there a particular place on the planet where most of this coming out to the sea onto the land happened?
Stephanie - Yes. One of the areas that produces a lot of early tetrapod fossils is in Greenland, just like we heard about the beginning of life. So, they actually have rocks which are of an ideal age - the Devonian period - in order to try and trace this evolutionary transition. Some really important fossils also come from arctic Canada. Recently, there are some stuff in the UK even. So, up in Scotland or some really important specimens that can really give us some insight into this important event.
Liz - Hello. It's Liz from Longstanton again. When you say that you find these fossils of the animals coming out of the water in these places, would it be like it is now cold or was the climate completely different when they did it?
Stephanie - The continents as you know them today weren't like that in the past. So for instance, Greenland wouldn't be way up north. It actually would've been much closer to the equator than it is today. So, they would've been more in temperate climates and there are some ideas that perhaps during this time, there might have even been some arid conditions some time and then even some really wet conditions. So, it depends on what specific formation that you're looking in, in terms of what the exact environment was like, but the animals where we find them, that's not the environment that they were living in back 400 million years ago.