Is another mass-extinction imminent?
Humans often get the blame for wiping out species, and indeed our track record isn't good, but species have been going extinct on en masse for millions of years - long before we were around. We've been hearing about mass-extinctions, including the Permian mass-extinction, the biggest extinction of all time when life nearly died out entirely, 250 million years ago. What role might humans be playing in making history repeat itself?
Bristol University palaeontologist Mike Benton joins Chris Smith...
Chris - First of all, can we just look at one aspect which is an intriguing component of these die-offs which is that every time we've had a mass extinction, the answer is that nearly all life disappears, but not all life. There's always something left and what is it about the life that's left that means it doesn't disappear?
Mike - Well of course, after an extinction event, the survivors, as Pauls said, may be in some cases especially adapted to living in conditions of poor oxygen or high temperatures. In other cases, they're just lucky. And then it depends on what the physical conditions on the Earth are in the time after the extinction event. Often, people have assumed that you have the stress, the impact of the meteorite which are dead dinosaurs or the massive volcanic eruptions and all of the consequences of that. And then the Earth kind of settles down rather quickly.
Well, we now know is in some cases, at least after this big and permanent mass extinction that Paul was talking about, the Earth did not settle down. For five million years, there were a number of repeated and rather similar crisis. So, as life picked up and got going again, certain groups started to speciate. Wham! They were hit again and set back. So, it took about 10 million years in that case.
Chris - It really was survival of the fittest. What would've been the transition or the sorts of life that emerged as successes to what was wiped out during that Permian mass extinct?
Mike - In the early part, there were what we call sometimes disaster species. These are the one that can take advantage of the rather stressful conditions in the immediate aftermath as Paul was saying. Species that can survive in conditions of high temperature or low oxygen. But then as the Earth settled down and returned to equilibrium in some way then you get the founders of the major new groups. And so now, we can look back at the terrible extinction 252 million years ago and think, "Well, it actually punctuated the history of life and reset evolution by wiping out all of the previously dominant groups. So, we can be glad. We're here, the dinosaurs came after that, various other groups were highly successful. But it did take a long time to recover.
Chris - I was going to say, is it actually this that helped to lay the groundwork for us coming along because I know that obviously, mammals didn't really begin to dominate until after the dinosaurs had exited, but they had to come from somewhere? Where the grounds were all sort of set out then with this mass extinction, enabling mammals to begin to appear?
Mike - It seems that may be the case because as Paul said, there were a whole lot of different reptiles and so on at the end of the Permian which were quite sprawling awkward pictures not very fast moving. After they had been wiped out, there was quite a change in pace in the following period of time called the Triassic. And the ancestors of mammals and indeed the ancestors of dinosaurs and various other groups got going. They were no longer sprawling animals with the arms and legs out to the side. They kind of stood upright as mammals do, birds do, and could move faster and adapt to other things ultimately including flights and various other modes of life.
Chris - So, we effectively selected for animals that were much fitter, reproductively fitter, they were more adaptable, and if it hadn't been for these mass extinction events then we wouldn't have species that are so resilient today perhaps?
Mike - That might be. I think certainly, there were stresses there. It would be very wrong to think that there was kind of inevitability or a kind of enhancement in a sort of mystical way. But in terms of evolution and the pace on land at least seem to hot up. And maybe in the sea because I think there were major steps in evolution of life in the sea, faster moving fishes which were the ancestors of the modern fishes that are very familiar to us in the oceans, like cod. All those types that they track back to then and they were preyed upon by giant marine reptiles which David Norman mentioned earlier.
Chris - One thing that's always baffled me is if you look at a crocodile that you've got there something which pretty much is reminiscent of what dinosaurs would've been sort of like, haven't you? They survived because the ancestors of crocodiles go back hundreds of millions of years. They weren't wiped out. So, why did they persist but these other stressors that did for the dinosaurs and things that came before them, why did crocodiles make it?
Mike - Very hard to say in any particular case because you know, whatever I say is untestable, so one has to be careful. Crocodiles and their ancestors have always been probably relatively low metabolic rate creatures - cold blooded we would say. There aren't very many species of them today. They were much more successful during the age of the dinosaurs. There were even bipedal, two-legged crocodiles that were even plant-eating crocodiles. So, they were more adaptable then it seems than they are today.
Whether it was something to do with that low metabolic rate because we know pretty well that dinosaurs in one way or another were warm blooded - a bit like birds, a bit like mammals today. Whether that meant that they larger food supplies, that's certainly something you would expect. Many of the dinosaurs of course were positively huge and being huge is not a great thing when the Earth is undergoing stress.
Chris - Because of course, getting rid of that heat becomes a problem doesn't it? Lastly Mike, can you just tell us whether or not we might see history repeating itself. Do you think that there are other things coming now that means that it was the dinosaur 60 million years ago. Next is humans?
Mike - I don't think we can specifically say that because the earth has changed. And so, responses to different kinds of environmental stress is maybe different. But there's no question as we heard earlier that humans are now driving an extinction at quite a fast rate and maybe a hundred times the rate that it ought to be because extinction is normal as I think you said. Before, species don't exist forever. But currently, the rate of extinction is at least 100 times what it ought to be.
Chris - Ouch! Well have to watch our behaviour, weren't we? Thank you very much. That's Mike Benton from Bristol