(or at least to dilute it to insipidity)
with his favorite argument about "the imperfection of the geological record."
A hopelessly inadequate record can compress millions of years of missing data
into an apparent "event." If we could recover the full flow of time,
the individual items of a "mass extinction" would spread out on both
sides of illusory suddenness, indicating a much longer period of successive disappearances
at a rate little, if at all, accelerated beyond the usual tempo of wedging in
ordinary times.
A more popular argument (the ghostbuster of accommodation) admits the
unusual character of mass extinctions, acknowledging a substantial acceleration
in the tempo of death, while arguing that the environmental stresses of
these parlous times do not introduce a new regime in the causality of
death, but rather, only accentuate the power of the wedge. In these worst
of times, the motor of biological competition runs faster as increased
stress drives up the intensity of struggle. Mass extinction only "turns
up the gain" on business as usual. If the "ancient and beaten"
make their exit in ordinary times as superior wedges push themselves into
the bustling economy of nature, then the rate of departure can only accelerate
in the tough moments of mass extinction when dogs eat dogs and men must
be men.
This honing of the wedge provides the traditional context, usually not
well explained in general writing on mass extinction, that has made all
the recent news about truly catastrophic causes based on extraterrestrial
impact so controversial and so threatening. In 1980, the father and son,
physicist and geologist team of Luis and Walter Alvarez, along with Frank
Asaro and Helen Michel, first published their evidence that a large extraterrestrial
object struck the earth some 65 million years ago and triggered the great
Cretaceous extinction.
The explosion of fruitful scientific
work that this hypothesis has engendered in just nine short years must match the
force or the impact itself. The Alvarezes' hypothesis has opened the small and
arcane paleontological field of mass extinction into a grand arena of interdisciplinary
cooperation. In October 1988, I spent three wonderful days at Snowbird, Utah,
listening to several hundred excited scientists, from geochemists to planetary
physicists, paleontologists to climatic modelers, ponder the causes of mass extinction.
Debate on the Cretaceous event still swirls about two crucial issues: timing (one
impact or many, one moment or an extended period of multiple bombardments) and
cause (massive volcanic effusions have been proposed as an alternative, but impact
now holds the upper hand according to most workers in this field). Moreover, the
case for impact has not been established (or precluded) for other mass extinctions,
so we do not know whether the Alvarezes' persuasive account or the episode ranks
as the explanation for a single event or a general theory of mass extinction.Nonetheless,
I think we now know enough to summarize the new views in a statement that must
suggest a radically revised perspective on the evolutionary meaning of mass extinction.
These events, with their catastrophic causes, are more frequent, more sudden,
more profound in their extent, and more different (from normal times) in their
results than we had imagined. Mass extinction does not just turn up the gain on
competition, so that wedging can proceed more ruthlessly and more efficiently;
mass extinction entrains new causes that impart a distinctive stamp to evolutionary
results. And if the history of life owes its shape more to the differential success
of groups in surviving mass extinction than to accumulated victories by wedging
in normal times, then a major component of Darwin's world view--and the only
sensible argument that he could supply for our deepest, culturally bound hope
of progress--has been compromised or even overturned.
I can envision two models of causality in mass extinction that challenge
wedging and consequent progress as a prominent vector of life. In the
random model, species live or die by the roll of the dice and the
luck of the draw; success reduces to little more than being in the right
place at the right time when the comets hit, the fires roar, the earth
darkens, and the oceans are poisoned. In the different rules model,
species live or die for definite and specifiable reasons. But the causes
of success are quirky and fortuitous with respect to initial reasons for
evolving the features that secure survival. The wedge operates in normal
times between mass extinctions. Organisms evolve features to enhance success
in continuous ecological struggle. The cause of mass extinction then hits
in all its sudden fury. Certain features are the passkeys to survival--tolerance
of extreme climatic stress, for example. But these features must have
evolved during normal times dominated by wedging. And they must, in principle,
have arisen for reasons unrelated to their later (and lucky) use in seeing
their possessors through the unanticipated debacle of mass extinction.
(I say "in principle" because, unless our basic views on causality
are seriously awry and the future can control the present, organisms cannot
evolve a feature for its potential utility several million years later
when the comet hits.)
I see a role for the random model especially in the most severe events.
If we accept David M. Raup's estimate of 96 percent species extinction
in the Permian debacle, then entire groups may have been lost by something
akin to unalloyed bad luck. Yet I remain committed enough to a more conventional
view of causality to think that, of my two proposals for radical reform,
the different rules model must apply more often. We can specify causes
for differential survival in mass extinction, but the features that secure
success must have evolved for unrelated reasons, usually in the regime
of wedging during normal times.
As Kant told us that concepts without percepts
are empty, and as Harry Truman said, "Show me, I'm from Missouri," the
different rules model, however interesting or elegant in the abstract, will have
no power without empirical documentation from the fossil record. I have written
a peculiar essay with a prologue more than twice as long as the main point--the
proposed example of the different rules model to follow. Still, what comes next
is the heart of this essay, not its epilogue.
If we wish to illustrate the different rules model, we might begin with
the most prominent group to die and the most widely cited cause of extinction.
Most people think first of dinosaurs when they ponder the Cretaceous extinction.
But our clearest and most extensive evidence comes from the opposite end
of the discredited chain of being--the single-celled oceanic plankton.
Extinctions are so prominent in these creatures that paleontologists speak
of a "plankton line" marking the rapid and simultaneous termination
of numerous lineages. As for "killing scenarios" in extraterrestrial
impact, the most widely cited reason (though causes must have been complex,
interacting, numerous, and varied) invokes a device of Moses against Pharaoh:
a thick darkness over all the land, even darkness which might be felt.
An impacting comet or asteroid, the argument runs, would excavate a massive
crater and send aloft a thick cloud of particles that would envelop the
earth in sufficient darkness to shut down all photosynthesis for several
months.
A tie
between the scenario of darkness and the death of plankton seems easy to formulate.
A few months of darkness might not faze a tree (especially if the impact occurred
in winter); a plant that lives for decades might shut down its factories for a
few months. And even if plants die, seeds can often survive to germinate when
the dust cloud dissipates. But the
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