Welcome back to week 3, in our previous lecture you learned about the history of life, the diversity and the magnitude of life on Earth. Now we're going to talk about the process of extinction, because extinction is, has been extremely important in the history of life. And there are really two modes of extinction. One is called Mass Extinction and that's what everybody has heard about because it's really sexy to talk about a comet coming in, an asteroid coming in and taking out 50, 60, 70% of species. But there's also background rate extinction. Extinction is going on all the time, for different reasons, and we'll talk about those. And we'll find out that extinction can actually happen very fast geologically, even if its not mass extinction. And it's just a normal part, extinction is a normal part of evolution, and it shapes evolution in, in very important ways. So here's the Fanerasoite Diversity curve again, and it starts at around 550 million years in the Cambrian and goes up to the present. And there's been rises in diversity and there's been extinctions. So there's areas here and here, here, here, here, and here, where extinction occurred at a much, much higher rate than speciation, and so diversity decreased. Now there are two big, big extinction events. The Permian Triassic extinction of and about 250, 260 million years ago, is said to have taken out about 95% of all marine invertebrate life, which this curve is all about, marine invertebrates. Then the Cretaceous Paleogene extinction, which is famous to all of you, because it is said that have, have finished off the dinosaurs that happened around 66 million years ago, and it is said to have taken out around 60% of species. These big extinctions, these very large mass extinctions are actually quite rare. And this curve on the left is what we call an Extinction Kill curve. So on the left axis, on the y axis, it's telling us what is the percent, the species extinction. So if we're talking about a million years, we can see every, if every million years an asteroid came in, for instance, it would take, it would be very small and take out a very lo small amount. But the big, big extinctions that occur over a long, long periods of time and very rare, they take, they may take out a large, large number, but those really big extinctions are rare. And we can see on the right hand figure that most extinctions are very small effect. So, all these extinctions in these different geological stages or time periods, there's, there's dozens of them, dozens of them. But they take out very, very small percent of species, and these really big events that take out, for instance, 60% or even more, are really, really rare. Now, the most famous one for everybody, of course, is the Crea, er, Cretaceous Paleogene Extinction, which took out the dinosaurs, or is said to have taken out the dinosaurs. And it hit really close to home. So it hit here on the Ukatan Penisula, and it created a big crater just on land, but also in the ocean. The asteroid itself was about 10 to 15 kilometers in diameter, very large. But it created a crater, that crater that we saw in the last slide, of about 180 kilometers in diameter. And you can see that the impact energy, so here's, here's, here's, here's the line for the asteroid. And here's the crater diameter. And here's the impact energy. So, when this asteroid hit, it generated ten to the ninth megatons of energy. And, that had a major affect on the globe. It shook the globe, and it created many, many effects. And, these effects cumulatively took out, perhaps 65% of marine life and who knows how much of terrestrial life. And it could have been done in several different ways. it loaded the the earth with dust, and just the, the sun went opaque. You could not even see. It was darkness, more, or more or less darkness. It created fires, and it, it created fires downstream on, in North America. But around the world, in many places around the world what might have happened, is all this ejecta that came out of this volcano that this crater that was, hit with the, the, this, this amount of energy, it, it sent this ejector all around in, into the atmosphere, and when that ejector came back in, it heated up the atmosphere. And in many places, this fire came from the ejecta, or it came from the, the energy that was built up in the atmosphere. But there were also shockwaves. So the shockwaves went all the way across the Atlantic Ocean, and then they came back and actually hit the East Coast of the United States, causing massive, massive earthquakes. And massive extinction, in the marine shelf environments of the eastern United States. So, there are a lot of effects of these, types of asteroids and comets. But, as snazzy as they are, the most important part of extinction is what we called Background Extinction. What goes on all the time and, and, has very good causes, so let's look at these. Here is a, here is a hypothesis of how we might look at background extinction. So, there's external forcing, that is, there's mountain building going on all the time, the Earth is, shifting on its axis, in it's path around the sun. And these cause, different, changes in the amount of heat coming into the atmosphere at different times, different cycles. And these changes, all these changes, have an effect on increasing temperature gradients, and increasing moisture gradients. That is, the gradient may get steeper, so it may get more, more hot in certain parts of the world, more, more wet, more, more cold, more, more dry. And these have effects on environmental harshness. So, if, if, the environment of the rain forest, for instance, changed to being really cold and dry, as it has long, long, long ago. Then, all those tropical organisms would, be under new regime of environmental harshness. Their physiology would be really, really strained. And it would extend beyond, I mean it would, it would create a problem that they cannot physiologically handle. And so then there's, it limits on energy flows, on maintenance of re, in reproduction. And these cause individual deaths. So you can imagine that, that organisms that can't move, and they can't adapt very quickly, or can't adapt at all to these changes, potentially. individual organisms can't reproduce. They die and the population densities go down. And when population de, densities go down, populations can be extirpated, that is, they can go locally extinct. And when many of these populations of a species go extinct then the species itself goes extinct. And this idea that there are many other factors that can fit into individual deaths. But the key thing about extinction itself, it's a process of individual organisms dying, populations dying off and then the species dies off. And a, and most of this that we see in the fossil record, or a lot of it we see in the fossil record, happens because of all of these changes, over time in temperature and moisture, and they have many, many, many down steam effects. If we look at the last 66 million years, here's the asteroid, the Cretaceous Paleogene Extinction event, right there. But here's this is the global temperature curve, for the last 66 million years. And at the paleocene and eocene boundary, it was a time of very high temperature, almost 12 degrees centigrade on average, higher than it is now. And then it decreased over time, to the eocene alligascene boundary. And it got more and more arid. And then it remains stable for a while. And then it got really a sharp increase in Late Oligocene around 23, 24 million years ago. And then it had another slight dip, but remained more or less stable throughout most of the Miocene. And then around a 11 million, to 12 million there was a steep, steep, steady curve toward the present. And the northern hemisphere glaciations took started around 2.7 million years ago. Now, this very, very general curve had effect on global vegetation. So in the eocene and the oligocene, warm temperate moist forest were very very far gone. So there are 45 million year old forests, way, way up in the high Arctic, that have a lot of species diversity. And, harbored a large number of mammals, and birds, and other, other organisms. But as the climate changed from, from the eocene in through the eocene and into the oligocene, more boreal eco zones began to develop up north. And that continued with this really particular change in getting colder and colder environments. Now, these events are tied to the instigation of glaciation in Antarctica, to flows in the oceans, and so forth. But by the time we get into the miocene and pliocene, then we began to get into a world that we see today where it's cold tempered in the northern hemispheres, and tropical forests are restricted to the equatorial regions. So all of this information I've discussed about background extinction is very relevant for today. As you know, we're in a sixth extinction. humans are dumping in carbon dioxide into the air, and that's warming the globe, and it's warming the oceans. And organisms are beginning to react to that and actually decline because of it. And this is a very good example, here. This is a terrific example because Henry David Thoreau was a writer and a poet, and he lived on Walden Pond. And around those woods he kept meticulous records of the numbers of plants in those woods. And scientists have gone back to resample those woods and, and, and look at the abundances of the plants in those woods. And what were seen is major declines in the, many groups in Walden's wood. So, on this diagram, everything in pink or red is either, if it's red, it's a major decline in abundance, and if it's pink, it's a moderate decline. But the, and one of the interesting things you can see about this figure, is that it's groups of organisms that are resisting this temperature. Now, what's happening here in these woods? Well, with global warming, spring is coming earlier, so flowering is going earlier, and some, some plants cannot adjust to an earlier flowering season, and therefore, they're declining, they're not reproducing as well. And the interesting thing about it is that these are clades. So, groups of organisms that are declining significantly, are all related to one another. That is, they all have properties that don't allow them to shift their flowering season earlier in time. This has major effects because if you don't shift, if the insects come out and you don't get pollinated, then you don't reproduce, and therefore, you decline over time. And so there's many, many examples of this now. This is one of the really coolest ones that we are increasing the extinction rate, the background extinction rate, but of course, our extinction is going very rapid in geological times. So one could look at it as a mass extinction over very short periods of time, or one could look at it as this is normal background extinction, just going very, very fast. So the take home message is, from this lecture is that extinction is a part, a normal part of evolution. And it shapes many evolutionary patterns, from which groups of species survive a mass extinction for example, and, and how they recover. It also, I also tried to convey that these mass extinctions are not as important overall in terms of the number of species that go extinct, and that background extinction, mostly driven by climate change over time, is the most important,. And then finally, I tried to say that mass extinction is very fast. The current mass extinction is fast, in [INAUDIBLE] even in geological time, a little, a slower in our, in our time, but it's changing very rapidly and is going to cause many, many more extinctions in the coming years.
