Cretaceous Meteor Showers, the Human Ecological "Niche," and the Sixth Extinction
Niles Eldredge, AMNH
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
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
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
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 ...
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
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
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.