Eight Little Piggies (30 page)

The explosion of fruitful scientific work that this hypothesis has engendered in just a decade must match the force of 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 of the Cretaceous 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 worldview—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 guiding 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 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.

Kant told us that concepts without percepts are empty, and 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.

We should 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 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 may survive to germinate when the dust cloud dissipates. But the single-celled photosynthetic plankton only live for a few days; months of darkness might easily destroy entire populations. These photosynthetic cells form the base of oceanic food chains. If they die, then the herbivorous plankton have nothing to eat and they perish; the carnivorous plankton find no herbivores, the tiny invertebrates no plankton, the fish no invertebrates—up and up to the fragile top of the ecological pyramid.

The rates of extinction for genera in several groups of planktonic organisms are staggering (and imply an even greater percentage of species deaths, for most genera contain several species, and all must be killed to efface a genus, while surviving genera may lose most of their species): 73 percent of coccolithophorids, 85 percent of radiolaria, and a whopping 92 percent of foraminifera. But an exploration of the different rules model must focus on winners and the reasons for their success. So let us consider the primary anomaly, one cited by many authors as an argument against the dust cloud hypothesis: The diatoms, perhaps the most prominent group of photosynthetic plankton, sailed through the Cretaceous debacle with a generic loss of only 23 percent.

We might first take an experimental approach and ask if a few months of darkness can cause differential death—with some species surviving for predictable reasons and others dying because they lack the tools of success. A recent study by Kathy Griffis and David J. Chapman supports this prerequisite for explanation under the different rules model (see bibliography).

Griffis and Chapman obtained cultures of several photosynthesizing planktonic species, including some closely related to forms that survived the extinction and others belonging to groups that first appeared after the extinction and might not have fared well in the preceding darkness. In a happy example of relatively “small” science (you don’t need million-dollar grants from the National Science Foundation for all good work), they simulated the scenario of darkness by wrapping 500-milliliter Erlenmeyer flasks, each holding a population of one species, in aluminum foil. They controlled for other factors by providing constant temperatures and adequate supplies of nutrients. Three species belonging to groups that arose after the extinction died within a week. Two others, closely related to forms that pulled through the great dying, survived eight to ten weeks without light—a good estimate for the actual time of darkness in several dust cloud models.

We must then ask if a factor of success, consistent with the different rules model, can be identified. Jennifer A. Kitchell, David L. Clark, and Andrew M. Gombos, Jr., have recently made such a strong argument for the planktonic heroes of the Cretaceous debacle—the diatoms (see bibliography). Kitchell and colleagues studied a core of diatom-rich late Cretaceous sediments (before the extinction) from the High Arctic at 85.6° north latitude. They noted that a portion of the core contained finely laminated sediments. The alternating layers consisted almost entirely of diatom cells in different states: some layers made almost entirely of cells in their phase of growth (up to 96.4 percent); others of “resting spores” in a state of dormancy (up to 93.3 percent). These alternations record nothing about the extinction to come, but must represent an adaptive response to long seasonal fluctuations near the poles: With nearly six unbroken months of darkness every year, a cell that normally lives but a few weeks and subsists by photosynthesis must evolve some mechanism of dormancy—a kind of hibernation—in order to survive a long winter of worse than discontent for any organism dependent upon sunlight. Many species of diatoms have evolved such a complex life cycle; cells deprived of light or nutrients can shut down their metabolism, form a resting spore by encystment, increase in density, and sink to lower levels in the water column, awaiting the return of propitious times.

Removal of light is not the only factor that elicits transformation to a resting state. Diatoms build their cell walls of silica, which they must extract from seawater. They therefore thrive in areas of the ocean, called zones of upwelling, where deeper waters, rich in nutrients (including the vital silica), rise to the surface. But periods of upwelling are sporadic or seasonal. Diatoms must be flexible enough to take advantage of these infrequent and uncertain bounties—able both to enter a growth phase and produce a so-called diatom bloom when nutrients become available and to hunker down as resting spores when their own growth depletes the temporary building supply.

Kitchell and colleagues discerned a common theme behind this flexibility to exploit seasonal and sporadic sources of the two necessities in a diatom’s world—silica for construction and light for growth and maintenance. Diatoms evolved the capacity to form resting spores in order to wait out predictably fluctuating seasons of inhospitable environments. They developed this key adaptation for ordinary life in normal times, not in anticipation of relative success should a comet strike the earth several million years in the future. Yet if months of darkness triggered death by extraterrestrial impact, then diatoms held a fortuitous leg up for survival (if you will pardon an inappropriate metaphor from the apex of the chain of being). Diatoms are not better than coccoliths or radiolaria; they are not fiercer competitors on an oceanic surface jammed full of wedges. They were just lucky enough to feature a physiological trick for survival, evolved for other reasons in different times. (Interestingly, Griffis and Chapman note only one feature held in common by all species that died within a week of darkness in their experiments: “None…appeared to produce resting cysts in response to the darkness.”) Kitchell and colleagues end their paper with a strong defense of the different rules model for diatom success: “These data document an incidental, but causal, dependency between a biological character, selected for in normal background times of geologic history, and evolutionary survivorship during an exceptional time of crisis in earth history.”

Life under the different rules model recalls the myth of Sisyphus, greedy king of Corinth, who is punished in Hades with an eternal task. He is compelled to roll a heavy stone up a steep hill; he groans and struggles, finally approaching the summit, but the stone always slips and rolls back down to the bottom, where Sisyphus must start all over again. Sisyphus, patiently and painfully rolling the stone up the mountain, works like life under Darwin’s metaphor of the wedge—slow and steady progress by constant struggle,
ad astra per aspera
. But this work of normal times is undone by moments of catastrophe, and nothing ever happens in a larger sense.

This comparison of life with the Sisyphus myth works up to a point, but then fails in a crucial way. The undoing of the slow and patient work of the wedge does not imply demotion all the way down to square one. Catastrophes of mass extinction do not beat life back to an earlier starting point; rather, they deflect the stone of cumulative organic change into some unexpected and unfailingly interesting side channel. They create, by their imposition of different rules, a new regime of oddly mixed survivors imbued with opportunities that would never have come their way in a world of purposeful wedging.

We may be indifferent to most of these quirky shifts that eliminated highly successful groups of the moment and passed a potential torch to unheralded creatures in the wings—groups that fortuitously held a winning ticket purchased for a different reason long ago in other circumstances. Few people lament the loss of ammonites, so long as we still have nautiloids. (In fact, I have proof that few people have ever heard of nautiloids at all, and therefore don’t give a damn in the fullest sense. Recently, the
World Weekly News
, king of the shopping-mall tabloids, published—with absolutely shameless faith in our ignorance—an unretouched photograph of a chambered nautilus labeled as a giant monster now on an earthbound path from Mars and scheduled to arrive well before the millennium.) Who cares (who even knows the names?) that all crinoids are now articulates and not inadunates, that reef corals are now scleractinians and not tabulates? Well, you may choose to disdain the details of marine invertebrate life, but you cannot be indifferent to the closest application of the different rules model—the death of dinosaurs and the resulting possibility of human evolution.

You may react to this essay by denying its claim to be conceptually troubling in the light of traditional hopes. You might say, after all, that the different rules model only validates a cliché so old and so widely appreciated that it became the motto of such straight arrow groups as the Boy Scouts and such jokers as Sancho Panza—“forewarned is forearmed; to be prepared is half the victory.” Yes, “be prepared” flexibility is a virtue. If you can keep a whole deck up your sleeve, you will surely have a useful card for any circumstance. But the cruel dilemma, the Catch-22, of evolution lies in recognizing that a species cannot consciously or actively prepare for future contingencies. A species can only evolve for current benefits and deliver its future fate to the wheel of fortune. Round and round she goes, and where she stops…. No, it’s even worse than that. For the wheel never stops, but only speeds and slows, tacks and turns, bringing life along on the grandest and most sublime of all endless chases. What the hell! Two bucks on
Homo sap
. to win—at least for a little while.