How did the dinosaurs die out?
Chris - We're talking this week about the science of palaeontology. In other words, things that are hundreds of millions of years old, like most of the politicians in the House of Lords. Our guests this time are Alex Lu who actually studies some of the first life on Earth, Stephanie Pierce who looks at how things moved and came out of the water onto land, and sitting next to Stephanie from Imperial College is JT, Jon Tennant. Hello, Jon.
Jon - Hello.
Chris - So, you're interested in actually not where life came from, but where it went, how it got wiped out, why it disappeared.
Jon - Yeah. I look at very long and large patterns in the history of life on Earth. This is a field of study called macroevolution. So, 'macro' is Latin for big, evolution is obviously everyone's favourite subject. In the fossil record, over the last five hundred million years or so, we're punctuated by these very catastrophic periods called mass extinctions. I look at a period about 150 million years ago where depending on how you look at the fossil record and how you interpret what we've got at that particular period in time - whether or not it can achieve the status of a mass extinction or not.
So, a mass extinction is usually where something like 75% to 95% of all life on Earth just gets absolutely obliterated. Usually, many different reasons for this. Everyone loves the end of the dinosaur's meteor strike story, but I look in a lot more detail to see whether there are potentially biological factors such as what animals were eating or something which might have drove them to extinction.
Chris - How do you know that there's been a mass extinction? I mean, if I was to walk out into Cambridge now and there had been a mass extinction, it might be obvious. But how do you know from the fossil record that all to 75% of life disappeared?
Jon - So, there were periods of time where we have lots and lots of fossils and there were periods of time where we have very little. So, around 250 million years ago, there was an event which we think happened called the Permo-Triassic mass extinction where up until this point, we had quite a lot of fossils happening. We had a lot of fossils around, both on land and in the sea. But then all of a sudden, it's just gone and there's almost nothing. We think that's because the continents came together and there was this massive volcanism happening and it just made life very uncomfortable on Earth and they all just kind of died out. They were actually quite lucky to hang on. There were a few strugglers which survived on and then went on to radiate again. But then about 50 million years later, were hit again with another mass extinction, and we actually find the last 500 million years so punctuated in time by these large extinction periods.
Chris - Is the cause of the mass extinctions written into the geology? If you study rocks, can you generally find out what's happened?
Jon - Yeah, to a degree. There's a lot of debate at the moment whether there are more kind of biological factors which are recorded in the fossils themselves which drove of extinction or environmental and geological factors which are caught in the rock themselves which drove extinctions. So, if you go back to the end of the non-avian dinosaurs again, we find evidence for a meteor strike hidden in the geology at this time. There's a very thin layer of rock all around the globe and if you look at the elements and stuff which are preserved in it, we actually find an element called iridium. There's a little spike there every time and it tells us that at the same time as the dinosaurs went extinct or most of the dinosaurs went extinct. We find this spike in extra-terrestrial material, so extra-terrestrial, meaning not from Earth. If you combine that with evidence which we have done in Mexico for a huge meteor impact and perhaps even another one just off the coast of India, then we actually find very strong and compelling evidence for a meteor strike in the geological record. We also find the biological record being severely depleted in the fossils at that time.
Chris - When you say huge, how big is huge?
Jon - I think the one down in Mexico is 120 km wide. So, that's one big meteor.
Niko - My name is Niko. I'm from Longstanton. My question is, how do you know that the meteor was the only thing that led to the extinction of the dinosaurs?
Chris - Yeah, how do you know that?
Jon - We don't. This is one of the things I'm studying but at a different period in time. It's very nice to have the story of just one meteor come along and just obliterating life on Earth. But what might have happened is actually something like more of a perfect storm of events. So imagine life being pushed to the very brink. You have something like mass volcanism happening and churning up tons of gases and ash into the atmosphere. Imagine walking around Cambridge and a volcano just got off. It's going to be very difficult to breathe. We think this kind of put a lot of pressure on animals just to stay alive and it made the environment very unliveable. And then just as things were getting really, really bad, a meteor comes along and just smashes everything apart and life is just very unhappy at that time unfortunately.
So, it's not just one thing. It could be several different things contributing towards ecological breakdown, environmental breakdown. It's also very important to look at that because when you consider what humans are doing today to the planet, we're pumping carbon into the atmosphere. If you kind of think that's like almost an analogue for mass volcanism happening, we can look at these extinction events, see how animals responded, see that they suffered greatly. We can almost kind of predict exactly what's going to happen in the future.
Greyden - Hello. My name is Greyden and do you know how the meteor killed all of the dinosaurs even if they're really far away?
Jon - Even if they're really far away.
Chris - I mean, can we slightly adjust that also to add to the point that, why did some things not disappear because crocodiles for example, their ancestors have been around for 300 plus million years and they're still here, aren't they? So, that's an excellent question. Why did some things succumb and not others?
Jon - Some things are just better designed for surviving. So, if we take the typical dinosaur analogue again, dinosaurs at this time when the meteor hit were very big and they are very specialised. Generally, the bigger you are and the more pressure that's put on an ecosystem which you're living in, the more likely you are to die because you need more resources to survive. So, if you have things like small birds and crocodiles which are living in the oceans which might not have been as affected, then there's different probabilities that they might have survived or died.
Malcolm - My name is Malcolm and I'm from Longstanton. My question is, how did the meteorite affect flying animals, apart from the ones that were right under the meteor?
Jon - So, there were two different groups of flying animals at this time. There are the pterosaurs and there were the birds. They're actually in direct competition in the kind of duel for the skies. Imagine the battle of Britain. You have giant pterosaurs. Some of them were humongous. It's like 10 or 15 meters in wingspan. Birds generally got a lot smaller. So, it might've been again that pterosaurs which is too big, there just wasn't enough food or just places for them to live. But perhaps the ability to fly as well meant that they can migrate away from areas which were being worst affected by the meteor so maybe if you could fly down to the poles. But again, this is very much an ongoing point of research. People are just beginning to be able to assemble the data sets which enable them to ask these massive questions about extinction.
Chris - Kate...
Kate - Bill Pope who's @underbundle on Twitter asks, "What might the world look like today if the Permian great extinction hadn't happened?"
Jon - Probably a lot better because there probably aren't be any humans around to destroy the planet. So, life would probably actually be doing quite well.
Chris - Can you just tell us what the Permian...?
Jon - Sorry, the Permian extinction. The end-Permian extinction was an event, 252 million years ago where all of the continents that we know today crashed together into a giant super continent called Pangea. There are combination of factors relating to this which led to about the wiping out of about 95% of life on Earth. So, if you have reduced coastlines because you've got all the continents coming together, there's less coastal environments for animals to live in the ocean. If you got less animals going on then there's, you know, ecosystem degradation, things begin to - like food chains and things breakdown and there's less suitable places for animals to live. If you like living in a shallow sea and then all of a sudden, your shallow sea is replaced by a really big deep sea, you're not going to be too happy there and you probably are going to go extinct or radiate, or evolve into something different.
Kate - Kevin Nagel asks, I think this is sort of a fight question - which were more vicious, pre-historic land animals like T. rex and raptors or crocodiles and sharks?
Jon - Well, if somebody wants to pit a T-Rex against a shark. I'm pretty sure a shark will win because the T-Rex will drown. It's a very interesting question. We don't actually know what and raptors were like. We have Jurassic Park to go on, but these are obviously exaggerated because directors aren't scientists and they just want what is going to draw in an audience. But we don't really know. I mean, if you look at a chicken, they're actually really friendly. Have you ever picked up a chicken? It's not going to come and try and bite your head off. They're generally quite friendly. Most birds are. Apart from occasionally pooping on your car, there's not that much which they do which is really life threatening. So, we really don't know that much about the actual behaviour of dinosaurs to guess. Let your imagination run riot.
Chris - So do you think they're actually quite friendly then, some of them? You don't think they were violent?
Jon - I probably wouldn't try and hug a T-Rex still, but maybe. You don't know.
Chris - What about these giant birds that lived in South America? The Terror birds, weren't they? I mean, these had a beak a meter long, some of them, even bigger. I don't think they were notoriously friendly. I think they were peckish.
Jon - Probably. I don't think humans are around really to record that much, but if you consider things like ostriches, again, today, ostriches are very primitive birds, probably some of the closest ancestors we have to dinosaurs. They're generally quite friendly. If you start poking them or trying to annoy them then they're going to get a little bit angry. But generally, they're quite placid.
Chris - Let's hear it for dinosaurs. What's your question?
Lisa - My name is Lisa, I'm from Cambridge. Are you able to detect disease?
Jon - Disease. Yes, we are actually. I think again, if I use T-Rex as an example, if you look at the jawbones in T-Rex, you can actually see evidence of like bacterial infection and things. I think there might be some evidence of cancerous growths in the bones of dinosaurs as well. You can actually maybe suggest this as a cause of death. They probably didn't have chemotherapy or anything, back then to give them really much of a fair shot. But yeah, you'd certainly do see traces of disease and often these are just left in the remains of bones. It can be quite interesting. This is evidence of what was happening to an animal when it was alive. When you find a bone of a dinosaur, that's evidence of just it being dead, but if you find these marks and things on a fossil - disease, even things like bite wounds and things, you can actually see what was going on during life or what killed the actual animal it's really quite cool.
Chris - Kate, anything on the Twitter or...?
Kate - full circle man on Twitter is trying to give us all nightmares. He says, what fossils do we have on spiders? Were they larger and more venomous?
Jon - Mister invertebrate, would you like to take that one?
Chris - Alex, do spiders fossilise? They do, don't they? I mean, we've got amber spiders, haven't we?
Alex - Yes, so spiders do fossilise. I think there's one in the Cambridge Museum actually in the Sedgwick which is called Mega arachni and it's 300 million years old, and about 60 centimetres in diameter - very, very large spider fossil. I think the fossil record of spiders goes back to around 400 million years ago, but the interesting thing as well is you don't just get the spiders. There's also evidence that their webs have been preserved and particularly in amber. I know there's an example of cretaceous amber from Sussex actually where if you look with a microscope, actually, inside the amber, you can see these little coiled up bits of spider web. So, not only do we know that the spiders are present, but we also see that they're behaving in exactly the same way as modern spiders are.
Corrine - My name is Corrine and I'm from Cambridge. So, you're talking about amber and that takes us to that famous moment in the Jurassic Park movie. Is there ever a chance that we could bring dinosaurs back?
Jon - So, there are some very hopeful scientists out there. They're generally considered to be a little bit crazy in the paleontological community. There's research going on at the moment over in Japan where some guy is trying to almost reverse engineer a dinosaur from a chicken. So, what he want us to do is mess with a chicken embryo and mess with its genes a bit so that you superficially create something that looked a bit like a velociraptor.
Even in your wildest dreams, you won't ever get an actual dinosaur. You probably have something that you know lays eggs, clucks, and goes around making chicken sounds and looks like a dinosaur. But it won't ever actually be one. They're gone forever. Until we create some kind of zombification process, it's not going to happen.
But as well as that, I think they mentioned earlier that something about proteins and molecules had been found. This is by a team of scientists led by Mary Schweitzer over in the US and they believe that they've actually found like almost the structural residue of DNA and they've used various techniques borrowed from biotechnology and biochemistry to actually demonstrate that we have this extremely fine level preservation which even 5 or 10 years ago, we wouldn't have been able to dream of finding. At the end of one her papers, she said, "I think about 5 or 10 years. We'll be able to have about 20% of a genome of a T-Rex" and there's a reason why that paper never really made it, particularly much in the media and it's because she was dismissed as a bit of a wacko after that. But if you kind of open your imagination, we could be able to do things in the future. I mean, there are incredible developments in genomics and being able to sequence DNA, happening. It's just a case of what the fossil record lets us do.
Alex - I was just going to add to that. There's another team in America. We may not be able to get dinosaurs back, but there's a team in California looking to try and get mammoths and also the passenger pigeon. That's their first effort because it didn't go extinct too long ago, I think 60 years or so. But from genetic material for the passenger pigeon, that's from a museum specimen and the mammoth, they've found frozen mammoths about 10,000 years old in Siberia. They can actually extract DNA from there. So although amber is not involved, you can still get the DNA and it's not perfect, but they are trying to find host animals like elephants for the mammoth that they might be able to bring back those sort of animals.
Chris - We'll just interject a second because our third experiment needs a little bit of time. So Kate and Dave, what have you got in mind?
Kate - So, for some prehistoric creatures, they're really funny looking and this is my favourite. What's this?
Boy - Dimetrodon.
Kate - See, everybody knows it but me. So, the dimetrodon as we can see has a big sail on its back. Now, that doesn't look to me like it's for fighting. Dave, what's it for?
Dave - So again, it's a fossil record so noone knows for certain, but one of the theories is it's all to do with heat and how well it can take advantage of the temperature in the outside world. Now, to get an idea why having some kind of big sail on your back might be useful in the outside world, what I'm going to do is look at how water changes temperature in two different ways. I've got a pot of water here and I've got a thermometer. At the moment, it's sitting at about 56 degrees centigrade and I have two identical glasses and I'm going to fill each one of them with the same amount of water.
Okay, so you can imagine this is an animal which is producing lots of heat and it's nice and warm. Now, I'm going to consider two different shapes of animal. One of them is a nice, kind of compact animal like a glass of water for example. The other one is a really kind of spread-out animal. So, I'm going to pour the water into a tray. So, it's only about half centimetre deep.
Kate - So, this water in the tray is like the blood going through the dimetrodon's sail I suppose.
Dave - So, if the body is hot in the environment outside and it's pumping around the sail then that would be very similar, yes. Or if you're thinking about other modern animals, it's very like the blood going through the ears of elephant.
Kate - So, we'll wait and see what happens to the water after some more questions.
Chris - Okay, any more questions while we wait for the water to do its thing. There is one over here.
Meluka - Hi. My name is Meluka and I'm from Cambridge. Going back to the subject about the fossils of the enormous spider, if you found one of those alive and it bit you and it was like venomous, what would happen to you? Would you like die or...?
Chris - They did have venom, did they, these spiders, do we think?
Alex - Yes, we're not entirely sure whether this particular spider had venom. I mean, there are spiders around today which don't and obviously, spiders that do and it's really the strength of the venom that determines what happens to you, whether you'll die from the bite or whether you'll just get a little bit ill, or whether actually, nothing will happen at all. But with something that big, I'd imagine it would leave some quite large puncture holes.
Arusha - Arusha from Cambridge. Just following on from the spider questions, a spider that's 60 centimetres in length, what might it have fed on?
Alex - Jon has just suggested humans, that's not the case because humans weren't around. But there's quite a lot of large spiders around today. So, in the tropics of Malaysia, there are bird-eating spiders that can reach 30 or 40 centimetres in diameter and they do catch birds. So, if you have something that big, presumably, it was feeding off something, or could feed off things that were a lot charger than any of the other spiders around at that time could.
But the interesting thing about the carboniferous, appeared that these giant spiders and the giant scorpions were living is that pretty much, every type of insect we see at the time could reach giant sizes. There were dragonflies with wingspans of 70 centimetres. Millipedes that were several meters, well. Not several meters. Maybe a couple of meters long. It is the one period in time in Earth history when things could get very, very large compared to modern organisms. The idea for why that might be is that there's evidence that oxygen levels on the planet were a lot higher at that particular point in time. So at current levels we're at 21% of our atmosphere is oxygen. The estimates are that in the carboniferous, 35% of the atmosphere was oxygen. If you're an insect, the way you breath is almost like diffusion. So, you're limited in your size by the amount of oxygen that's around you because the oxygen can only diffuse a certain distance before you've used it all up into your body. So, if there's higher oxygen levels, any organism that survives by diffusing in the oxygen can get to bigger sizes. We think that's what was driving this massive evolution, this gigantism at that time.
Chris - Kate...
Kate - Jeremy on Facebook wants to know what gap in our knowledge of pre-history are you guys most vexed by? What's missing from our knowledge?
Chris - Jon?
Jon - Well, one of the things which we always hear is how biased the fossil record is. People who study modern animals and DNA, they love throwing that one at us "Oh, you can't do anything with the fossil record. It's so biased." By this, they mean that only in certain periods of time and in certain places do we find certain types of fossils. But, palaeontologists, we don't see that as a kind of end to all things. We actually have now many different techniques where we can overcome these biases. Although our data may never be as good as going out into a forest and being able to sample every animal out there. We're certainly able to overcome some of these biases which other scientists actually think makes us almost like, not able to be useful in any way. But yeah, I'd say, just what the fossil record is able to offer can sometimes be incredibly frustrating. I mean, we can have periods of 5 or 10 million years where there's just nothing there.
Chris - Stephanie...
Stephanie - Gosh! Well I'd love to go out there and find an amazing early tetrapod fossil. That would be really great! Well for me, it would be amazing to see the point in time where we actually see the evolution of modern locomotion behaviours. So, as I said with some of the early tetrapods that I've worked on, they sort of weren't really using their limbs like modern animals do. They were sort of hauling themselves out of the water onto mudflats. I'm really interested in, at what point in time did animals put four limbs on the ground, lift their body weight off the ground and start moving one limb at a time. Because after that happened, we start to see an explosion in a way and diversification in the way that animals move. We not only have animals moving one leg at a time. We start seeing animals walking, running, animals that are moving their limbs under their body, animals that are going from four limbs to two limbs like we see in tyrannosaurus rex, and everything in between. They can gallop, they can run fast like a cheetah, and then eventually, of course, they can fly in the air. So, this point where animals start to move like a modern tetrapod is really, really important.
Chris - Kate and Dave, how's your water coming along?
Kate - How's water coming on Dave? You've been measuring it as we'be been going along.
Dave - So, if we measure the temperature of the one which has been sitting in a nice compact glass...
Kate - So, what kind of animal is this representing?
Dave - So, this is something kind of spherical. So, if you imagine a kind of really big fat animal. Polar bears are really good example of this kind of really big fat, and almost round. That water is now at about 51 degrees centigrade.
Kate - So, hasn't come down by that much.
Dave - It's only come down by maybe 5 or 6 degrees centigrade whereas the ones which have been spread-out - so this is an animal with great big, long kind of spindly limbs or kind of big flat animal, the temperature has come all the way down to about 36 degrees centigrade. This is because basically, things can only lose heat at their surface. So, be it animals or lumps of water, if you've got a small surface, you're a really compact thing then you can't lose or gain heat very well. If you're spread-out, you've got much more surface so you can gain or lose heat, and a lot more places at the same time.
Kate - So, our dimetrodon on the war here. Was it trying to cool itself down or warm itself up?
Dave - Well, the theories go that it was probably trying to warm itself up because it was a predator. It was trying to catch things. If it could hang around in the sun and have this great big sail to warm up its blood then all the chemical reactions go in faster which means it can run faster so it can catch the other animals that are kind of still a little bit cold and a bit kind of sleepy in the morning. You can kind of charge after them and grab them and get its breakfast very efficiently.
Kate - So, the early dinosaur catches the dinosaur I suppose.