Cretaceous Meteor Showers, the Human Ecological "Niche," and the Sixth Extinction

Niles Eldredge, AMNH

Real Audio Recording   

It's great to be part of this symposium. I think it does our institution proud, and I'm really glad everybody's here. And pushing things forward -- extinction is the millennial issue, after all.

The sixth extinction. I'm not going to spend a lot of time trying to convince you all that we're in the midst of a biodiversity crisis. I'm personally convinced of it. I'm going to more or less assume that it's true. I'm going to more or less assume that Ed Wilson is largely accurate -- how accurate I really couldn't say -- in claiming that some 30,000 species a year are becoming extinct. That's three an hour, and that it's getting worse. And the comparisons that I've been drawing over the last few years with the mass extinctions of the geological past -- and the first motive for actually turning to the geological past is to convince people that this is not a "the-sky-is-falling" kind of an argument. This is actually based on some credible evidence, some real evidence, that mass extinctions have, indeed, struck the earth many times -- six times globally, five times globally, it depends on how you count them up -- in the last 500 million years, and now we know it to be 535 million years. It says 570 million years -- that's an old chart, I apologize for that. We have just redated the base of the Cambrian.

The empirical evidence for mass extinctions actually comes just from looking at a chart of geological time. The chart of geological time was assembled in an intense spurt of effort starting in the very late, latter part of the 18th century, and going on up into the first half of the 19th century. It was largely in place before Charles Darwin wrote On the Origin of Species. So when you hear a creationist -- or anybody else, for that matter -- claim that the geological time scale is a contrivance, put there to convince the world that evolution has occurred, the answer to that was actually in outline form, in place by the time On the Origin of Species was published. And not only that, a lot of the workers who were putting it together were anti-evolutionists -- and, indeed, some of them were clergymen. So we're talking about a real empirical work here. And the divisions of the geological time scale -- we're talking sedimentary rock here -- is based on analysis of fossil faunas, for the most part, but also floras when you get up into the Mesozoic, as well. So let's take a look at some of this. . . .

The first thing you might notice on the left, for those of you who haven't taken a geological course in recent years, is the major divisions of Paleozoic, Mesozoic and Cenozoic. Paleozoic means ancient life, Mesozoic means middle life, Cenozoic means modern life. These terms were in place by the 1840s, and they refer to large chunks of time that seem to have some similarity -- particularly, again, in marine invertebrate faunas. Unifying perhaps this first chunk here is the Paleozoic and then the Mesozoic, which we otherwise know as the Age of the Dinosaurs.

I have carat marks here, indicating some of the major global mass extinction events. The one that's usually excluded -- and, instead of having six down here, there's actually five -- as most people don't view the event at the end of the Cambrian period as being coordinate with these other events. However, it was a major event in its own right. The greatest mass extinction that we have any knowledge of whatsoever, not surprisingly, falls at one of these divisions here, between the Paleozoic and the Mesozoic. This is where -- this Permo-Triassic extinction -- that Dave Raup, who just recently retired from the geology department at the University of Chicago, used the rarefaction techniques on it -- again, predominantly marine invertebrate fossil record -- trying to assess what we can know about soft-bodied faunas, and so forth. He came up with a statistic that perhaps between 70% and 95% or 96% of all the species on the face of the earth became extinct at that particular time -- what we have as a division there.

Now, the arguments -- I'm going to get into the causes of these mass extinctions in just a moment. Is this a rapid mass extinction? A sudden, catastrophic event? Or, indeed, is it an event that took a much longer time? What, really, were the causes, and how does this work in a geological period? This is still the source of a great deal of controversy. Not controversy so much as ongoing research. There is no one single model that's applicable to understanding the major mass extinctions as you go through geological time.

The Cretaceous one -- let's talk about that for just a moment. Mass extinctions were actually well-known to the early geologists. As I say, they were looking at these things, and they realized what they were at least. So did Baron Cuvier, who wrote Revolutions on the Surface of the Globe, and he had some 33 major revolutions, or extinction events, that he saw were then followed by separate creation events -- because, again, we're in a pre-evolutionary time, and he was talking about separate creation events, basically by a supernatural being. So they were realized to be actual extinction events that were taking place.

But the theme of extinction started to go away from geological and evolutionary biological discourse as you go through the 19th century. So you don't see a great deal of work through the latter part of the 19th century. And, indeed, from other points of view, the sort of catastrophic nature of these sudden subdivisions ran afoul, basically, of Lyellian gradualism and uniformitarianism -- and then, later on, Darwin's picture of how the evolutionary process really ought to work. When we were -- a bunch of us, including myself -- were graduate students at Columbia University -- here at the American Museum and at Columbia, in the 1960s, Norman Newell -- who I don't know if he's in the audience right now -- was basically the only geologist and paleontologist on the planet who was convinced of the reality of mass extinctions, and that these things had something terribly important to tell us about the course of the evolutionary history of life on earth.

Basically, it wasn't until the Alvarezes came along with their hypothesis in 1979 -- I think it was first published in Science -- their whole team published the notion that bolide impacts -- whether it was a comet, whether it was an asteroid, whether it was one or more of these objects -- at the end of the Cretaceous period was the thing. They came up with this actually very good ecological theory that involves the occlusion of sunlight, the blockage of photosynthesis, that wiped out. . . . It's a good ecological theory because it deals simultaneously with plant life, and then therefore, the base of the food chain in terrestrial biomes as well as in the marine environment.

By coming up with this notion -- whatever the credibility of it is, I think it's, in its gross terms, accepted by the large majority of paleontologists and geologists right now -- reawakened interest in the whole notion of mass extinctions. And, actually the world sort of woke back up and said: Okay, we actually have mass extinctions out there, and there is evidence -- at least, in some instances -- that they were rather abrupt events. If you take the most extreme version of the Alvarez hypothesis, you'll see that it involves days and weeks -- a couple of years, basically -- a couple of years without sunlight, or sufficient sunlight to keep the food chain going, both on terrestrial and in the marine biomes.

So it sparked a tremendous amount of interest looking back, and, basically, on a very, very neglected topic. And I would like to say, also, that their whole notion, basically, of how the evolutionary process works in the geological context has changed dramatically as a result of that. And I'll speak just a little bit more about that in a moment.

But on the basis of -- the good empirical evidence here, of course, is the iridium anomaly that was first contacted, or first discovered, in Gubbio, in Italy. But it's also, it's been found, basically, on all the major continents. I think now, certainly, it's in Denmark, it's in wherever you can find an exposure of the Cretaceous-Tertiary boundary -- prompted a lot of geologists instantly to go running around and checking all of these boundaries to see if, indeed, they could find some iridium spikes in these places, as well.

And the answer is: Well, people got excited. There was an early, and it turns out to be non-corroborated, report at the Permo-Triassic boundary in China that there was, indeed, such a spike, and therefore evidence of an extraterrestrial bolide. And it looked like we had a general theory. One problem with extraterrestrial impacts, however, as a general theory of these global mass extinctions, is that all of you who have taken geology courses realize that this is just the tip of the iceberg, and there's tons and tons of subdivisions here, on a more regional and local basis, that cannot all have been caused by extraterrestrial impact. There must be something else going on that's causing faunal disruptions, faunal turnovers, episodes of extinction and biogeographic migration out and immigration in, and subsequent bursts of evolutionary change. It can't all be bolides from outer space.

So the arguments go on. There was some evidence -- and I don't know what the status of it is right now -- that there might have been a bolide impact at the end of the Triassic, which was another very major mass extinction. But the Permian data are exiguous and actually have been retracted, and there's no evidence whatsoever -- as far as I'm aware -- of bolide impact at the end of the Ordovician, or here, which is not quite at the end of the Devonian. It's the Frasnian-Famennian boundary.

Indeed, what you get here is evidence -- let me just talk just for a moment of the Ordovician and Devonian -- of another major whole area of discourse. And it fits right in with other themes we've been hearing throughout the day -- global climate change. Global climate change always seems to be involved, in some degree, in these events, even in the end of the Cretaceous. Again, complex models -- no simple one thing is usually underlying these major mass extinctions.

At the end of the Ordovician, the data indicate that the extinction came at two spasms, involving, basically, a refrigeration of the planet -- a true, genuine glacial episode -- but one that, as I say, took over a million years. And so there was one spurt of extinction, and then a million years later there was another one. And, again, there was hardly any life on the terrestrial biome. Terrestrial biomes hadn't evolved yet, except for maybe some primitive plants. So this is, strictly speaking, marine data that we're looking at, and that's marine life. But on the basis of the faunal turnover here, this is estimated, actually, as being the second-greatest mass extinction that's happened yet. And, again, no evidence of a bolide. A protracted event -- global cooling, over a million years, at least.

Same thing in the Devonian. Although there's some implication that there might be some anoxic turnover involved here -- these events where carbon dioxide and other gases that are trapped below the surface in the ocean, or in lakes and so forth, all of sudden bubble up and create havoc in the ecosystems. There was a recent paper in Science claiming that the Permo-Triassic extinction might very easily have involved a major anoxic event, as well as a climatic change, as well.

So no single simple model explains these patterns. But they are real, they're empirical, they're based on the fossil record. And, as I say, we're not analyzing these phenomena so much as just simply documenting them -- had been begun and had been in place before Darwin even came along.

How does this affect our views on the evolutionary process? This is where I really start getting excited. The Permo-Triassic event -- there was this huge turnover in these Paleozoic groups -- the brachiopods, the various kinds of corals that where there. It's not as dramatic as the mammal and dinosaur story, which I'll mention in just a moment. But there was a group of corals, the tetracorals, that went all the way through from the Ordovician -- from our first coral reefs -- up, sporadically appearing and evolving, and being cut back, the usual kind of a thing, and finally disappearing at the end of the Permian.

Now, corals, along with everything else -- the first stage of the Triassic is about 6 or 7 million years long, and it's ecologically very strange looking. There's no normal marine faunas to be found, or very few. It takes about 6 or 7 million years after a major mass extinction event for evolution basically to spring back into place and to reinvent, as I like to think about it, the ecological wheel. And so the more devastating, the more encompassing, the more global the event is, the greater the change and the higher taxonomically the effect is. And, consequently, when you get evolution replacing, as I say, ecologically replacing the creatures that have gone extinct during the episode, they are higher-ranked; consequently, much more different phylogenetically. This greater phylogenetic space. . . . I'm not putting that very well -- between what went extinct and what evolved. And the corals are no exception.

The modern scleractinian corals have a sixfold symmetry; the Paleozoic ones had a fourfold symmetry. And when I was a student -- and they also have different crystallography and mineralogy of the coralite. Down here you have calcite. Up here -- all the way up through and including the modern corals, it's made out of aragonite. In both instances calcium carbonate. And we were made to speculate how this would -- in fact, I think I even had a question on an exam about this -- how could you get, how could you transform, how could you evolve a tetracoral into sixfold symmetry coral and change the mineralogy? What must have gone on there?

Well, what actually happened was, this group became extinct, the hexacorals, modern-day corals -- the sister group of sea anemones, which are naked corals, basically -- and they're the sister group of each other. There are some corals known from the Cambrian with sixfold symmetry, and, presumably ... there were anemones throughout the Paleozoic. They were well-evolved. And it was after, presumably -- and this is the scenario that I prefer -- after this extinction, after the final loss of the tabulate corals, then you get a calcification and one lineage of the naked sixfold corals, and you basically reinvent corals. And that seems to be the way evolution is. And the dinosaurs both appear basically where the crossbar is in the Triassic. This was known back in Lyell's time. And the data have hardly changed much at all for either group -- they've been extended down just very slightly.

For reasons unknown to anyone why, it was the reptiles -- particularly the dinosaurs, but also some of their collateral kin -- which did the radiation and got all of the big jobs, if you will -- all the varied jobs, ecologically speaking, in the ecosystems of the terrestrial biomes. And mammals were at a high level differentiated, but, ecologically speaking, there were no large herbivorous mammals, or things like that. And, in fact, as we all know, all the dinosaurs have to do is build a fourth floor and see what was going on in the Mesozoic. They were cut back tremendously here, for reasons which are not completely understood. Dinosaurs had tremendous vicissitudes in their history. But when, finally, whatever it was -- that meteorite, or some people think it was a huge volcanic event, actually, that produced the iridium anomaly -- occurred 65 million years ago, that was finally it for the dinosaurs. I think they really were on the wane, anyway. When they finally went off, after a lag, then you get, for the first time, a differentiation and ecological way of placental mammals, in particular, and you get, all of a sudden, and rather rapidly, these large herbivorous mammals, and so forth.

So the old model, which always reminded me of Godzilla versus King Kong -- I just read in the Times, I just realized the creator of Godzilla just died -- a Japanese man. And I didn't realize, until I read his obituary, that, in the Japanese version of the Godzilla versus King Kong, King Kong dies, not Godzilla -- which I thought was really interesting. Because in the version that I saw, which also involved an American Museum of Natural History vertebrate paleontologist as one of the foils in it, King Kong finally wins. And King Kong wins by virtue of obviously not his brawn, but by his smarts.

And so that was the old model, coming right out of why the mammals finally succeeded the dinosaurs. Because it was inevitable -- superior form of reproduction, superior form of basically smarter and superior form of animal -- tetrapod, in general. So it was inevitable. And now we know that -- we can see fairly clearly -- that that wasn't the case; that it took extinction to actually drive the dinosaurs out. They were doing just fine. And if this meteorite, or whatever it was, hadn't hit, we'd probably still have dinosaurs here right now. So I think this resonance between extinction and evolution is extremely important, and basically has changed our perspective on things enormously over the last 20 or 30 years.

Now, the question is -- because I do want to link this up with humans -- what do Cretaceous meteorites have to do with human behavior right now on the planet? Which is to say habitat destruction. I mean, with overharvesting, introducing alien species, perhaps diseases -- these are all important vectors, basically, in the sixth extinction that's going on right now. But I can't help but harp on habitat destruction. Just get in an airplane and fly anywhere, and you'll see habitat destruction.

This is a cartoon from Elizabeth Vrba's work from the New York Times. I was just debating a creationist -- that's why I was on an airplane. His cartoons were all Gary Larson cartoons. At least this is based on some real data. Elizabeth Vrba, at Yale, spent her whole career looking at the Plio-Pleistocene of eastern and southern Africa, particularly antelopes. And this, again -- if you read Y as 5 million years ago and X as 2.5 million years ago, these are based on real basic data. And one thing I really love about this particular diagram -- it looks exactly like diagrams we've been producing recently for the Paleozoic and the Appalachian basins. There are 13 faunas that last, on average, 5 or 6 million years. They remain intact -- very little going on, very little extinction going on, very little evolutionary change going on amongst the components. The seashore lines are shifting in their distribution, so these Paleo environments are marching all over the place. The communities stayed pretty much intact; the species stayed pretty much intact. And then something happens -- usually climate change, or some event. Very tough to tell back in the Paleozoic.

Here we are 2.5 million years ago, and what happened -- there's very good evidence for it. Again, this is not my research, but there's good evidence for a global cooling over a period of about 300,000 years that had an enormous effect on the biota of eastern Africa. And I don't want to go into it in any great detail, but you'll see this kind of pattern -- where species are dropping out, and other species, on the other side of this line, are coming in. And the dropping out and coming in is twofold. It's ecological, first and foremost -- because what was going on, in terms of the vegetation, is that grasslands were spreading at the expense of woodlands fairly rapidly. She has sort of a threshold-effect model that she talks about there.

So this turnover came. And so if you're looking at the mammals there, the mammals that are adapted to the woodlands are going to track that habitat -- they're not going to be there anymore. There are going to be some generalists, like impalas, who are going to go right through -- and, in fact, they do go right through. And there are going to be some other mammals that are already adapted to this kind of more open savanna grassland kind of environment that are going to come in. So that's just ecological replacement, and a lot of the fossil record is only that. And it looks like extinction and evolution, but it's not.

Some of it, however, is real extinction. It's not always possible to tell what is what here -- and some of it -- and this is her turnover pulse notion, where she claims that if you have a rapid fragmentation of habitats, it's prima facie -- well, it's just the right kind of situation to expect to get speciation -- fragmentation of species -- and rapid evolutionary change. And the real reason why I've got this up here, though -- because I now have to hook humans into this pretty rapidly -- is that right at 2.5 million years ago is roughly when Australopithecus africanus became extinct. And you get two lines -- Paranthropus, the famous obligate herbivore robust lineage . . . stenotoic, very rapid evolution and quick extinction . . . and then Homo habilis, or some people call it rudolfensis at this point, and the first stone tools show up right now, too.

So the point there is that human beings were just like the rest of the fauna and subject to the vicissitudes. And our evolutionary patterns of these hominids is exactly the same as it is with the other components of the biota -- no change there.

Two other points, before I get to my last characterization, which I hope will convince you that you can link humans up convincingly with the bolide impacts at the end of the Cretaceous: 1.65 million years is the onset of the first glaciation, and that's just about the date of at least the oldest-known specimens of Homo erectus, or the only ones now Ian Tattersall calls Homo ergaster. Larger-brained species, it's got a more sophisticated tool kit, and so forth. The significance here is -- well, and also they had fire. This is the last time, I think, you can point with confidence to a global climate, or any other kind of environmental event, and say: This had a direct effect on human evolution, whether it was an extinction event or an evolution event.

Then,.9 million years ago, the onset of the second glacial: Humans, we now see, had gotten out of Africa, into Eurasia, prior to that, but not in any great numbers, apparently. At .9 million years ago, the onset of the second glacial, human beings actually left Africa and went north, and tools are all over the place. Presumably, they were hunting the megafauna that were there in abundance in Eurasia.

I think, if these patterns hold up, and if I'm characterizing them correctly, that's dramatic and compelling evidence that we had expanded our niche. And I can't think of any other as dramatic example in the fossil record -- or any other source of data -- about niche expansion like that. And I think you have to think that it's culturally mediated.

Now, let me just get to -- because I am just about out of time. I'll skip over, because we've been talking about the diaspora, and it's been fascinating to me. That we came out of Africa, and that not only the African megafauna, but the tropical Asian megafauna survive pretty much intact. I think that's good evidence that human beings were directly involved with these extinctions, and I've been fascinated to hear the pros and cons of this debate.

I was going to show you -- very quickly I will. . . . very abstract. I got into some very abstract stuff in the 1980s -- this is hierarchy theory. It says that organisms do two kinds of things: they are matter-energy transfer machines, and then they reproduce. Reproduction is the only physiological activity of organisms that is not essential for the survival of the individual organism itself. As a consequence of obtaining energy just to grow, differentiate, develop, and maintain the soma, on the one hand, and the consequence of reproduction are enormous. You form communities of conspecifics in the case of reproduction, and that's the breeding population. That's just ongoing reproduction that's doing that. Whereas the matter-energy transfer game is taking place in the context of local ecosystems. So you have what John Damuth calls "avatars," which are conspecifics, but you are interacting with them in an energetic sence -- so you're competing with them, you're cooperating with them -- whatever you might be doing. But that's that kind of matter-energy ecological sense of local conspecifics, and that's a. . . . They are the form, the components, of the local ecosystems.

Species, on the other hand, are collectivities of deems -- the widest group of deems that share common reproductive adaptations. The point -- we could go into this at greater length perhaps later -- is that species are not economic machines. Now, let me just give you the two points about human beings. Human beings are amazing -- we're amazing in many ways -- in terms that we're aware that we're alive, and all the sort of adaptations that people talk about, the fact that we have culture and speech, and so forth. Ecologically, I don't think we've looked at ourselves carefully enough.

Ten thousand years ago, when we invented agriculture -- starting in the Mideast, but in many other places, as well, or several other places as well, perhaps slightly later -- we did not, as ecologists are prone to say, expand our ecological niche. What we did was to actually abandon it. We actually abandoned local ecosystems. We stepped outside of local ecosystems; we declared, basically, independence from them, by taking control of our own food supply. The effect of that has been to remove the Malthusian cap on our population sizes. Because population sizes are limited by the carrying capacity of the environment, which means the local ecosystem's sum total of the number of species is the sum total of the number of organisms that can be carried by the local environments that they're integrated into. We changed the rules of that. And the best evidence for that is, there were between 1 and 5 or perhaps 10 million -- that's the outside estimate of number of humans on the planet 10,000 years ago. We got 5.8 billion now.

Point two about this is that, starting with this crude way of the diaspora, but then going -- well, you can even see it with Neanderthal trade routes -- people have been integrating themselves economically, unlike any other species, again, harking back to this particular kind of abstract diagram. We're a very different kind of an animal. Right now we are exchanging $1 trillion worth of goods and services amongst ourselves a day. We're integrated globally in an economic sense. And it turns out that the system that we are in. . . . So that's the first species. We are the first species in the history of the planet not only to abandon local ecosystems, but to emerge in 10,000 short years as a global -- as an integrated, economic entity that is part of the system. And the system that it's in is the summation of all the local ecosystems on earth, which is sometimes called "Gaia," the global system. So we didn't leave nature after all; we redefined our position in it. It's because, though, that we've stepped out of local ecosystems that our population took off. It's population size plus the unequal distribution of wealth on the planet among human beings that I think is really at the root of our sixth extinction, our biodiversity crisis, that we're faced with right now.

I'm out of time -- sorry to have to rush at the end here -- but there's the analogy. We are basically acting, in terms of destroying the environment, just like those Cretaceous meteorites and those other kinds of models for those mass extinctions in the past have done. We've got here because we've been successful at reinventing ourselves ecologically. I only wonder if we can reinvent ourselves still more and come to a concept of enough, and stabilize our own selves, before we kill off everything on the planet.