With respect to the apparently sudden extermination of whole families or orders ... we must remember what has already been said on the probable wide intervals of time between our consecutive formations; and in these intervals there may have been much slow extermination. Charles Darwin w HAT eXTINCTION?
Craters are physical features. Even one hidden under a kilometer of rock can be discovered using geophysical techniques, then drilled, and samples brought back to the surface and studied in the laboratory. As more and more craters have been discovered on earth using such techniques, and as the evidence that Chicxulub is the K-T impact crater has accumulated, more and more geologists have come to agree that a giant impact ended the Cretaceous. Specialists have calculated that the crash released the energy equivalent of 7 billion bombs the size of the one dropped on Hiroshima, and produced the loudest noise heard and the brightest light seen in the inner solar system in the last 600 million years. Such an event, like the passage of billions of years, is far beyond our experience and our ability to comprehend. Surely nothing could more clearly refute Hutton's maxim, "The present is the key to the past." Confined as we are to the present, it has taken geologists nearly 200 years to discover that large meteorites have struck the earth and that terrestrial craters—many of them—exist. That recognition leads to a new question: What are the consequences of a giant impact for living creatures, such as those that inhabited our planet 65 million years ago? On that point, far less agreement exists. There is consensus, however, that the answer is to be found in the fossil record.
The question of the effect of impact on life is surely important, for if the collision that left the Chicxulub crater behind did not cause the extinction of the 70 percent of species that perished at the end of the Cretaceous, the Alvarez theory would remain merely a scientific curiosity. Yes, objects from space do strike the earth now and then, but even one the size of a large mountain does little harm to life (or to geological orthodoxy). If on the other hand, impact did cause the extinction, then paleontology, geology, and biology would never be the same. Our conception of the role of chance in the cosmos, our view of life and its evolution, our understanding of our own place—each would be irrevocably altered.
The Alvarez team left no doubt that they believed that the impact caused the mass extinction. They might have called their '980 article in Science "Evidence for Impact at the Cretaceous-Tertiary Boundary" and waited until the case for impact was strongly corroborated before going on to connect it to mass extinction. Instead they gave it the provocative title, "Extraterrestrial Cause for the Cretaceous-Tertiary Mass Extinction" and set out to show both that impact had occurred and that it had caused the K-T mass extinction.
The original paper was unusually long for Science, indicating that Luis's Berkeley protege from long ago, editor Philip Abelson, understood that the new theory might be of more interest than most. However, almost all of the article's '3 pages were devoted to describing the iridium measurements and other chemical tests; the biological consequences of impact covered only half a page. This was undoubtedly because the Alvarez team, though long on scientific talent, was demonstrably short on knowledge of paleontology. Luis was a physicist; Asaro and Michel were chemists; Walter was a geologist, but not a paleontologist. In retrospect, since they did not have pale-ontological credentials, it was both proper and good strategy for the Alvarezes to introduce their theory but to leave to others the task of testing it against the facts. Walter knew that Wegener's theory of continental drift had languished for decades in part because in seeking a mechanism to explain why the continents had drifted, he strayed outside his field into the territory of the geophysicists, who immediately pronounced drift impossible, thus putting an end to the matter for half a century.
For the first few years after its appearance, paleontologists did not believe that they needed to take the Alvarez proposal seriously. Even if impact were strongly corroborated—even if the crater were found—that would not necessarily mean that the impact had caused the mass extinction. More importantly, paleontologists believed that for the all-important dinosaurs, the question had already been answered: Evidence that they had collected over more than a century, and especially in the Montana dinosaur beds in the 1970s and early 1980s, they interpreted to show that the dinosaurs had gone extinct gradually, not instantaneously. This point of view infuriated the already irascible Luis Alvarez: "I simply do not understand why some paleontologists—who are really the people that told us all about the extinctions and without whose efforts we would never have seen any dinosaurs in museums—now seem to deny that there ever was a catastrophic extinction. When we come along and say, 'Here is how we think the extinction took place,' some of them say, 'What extinction? We don't think there was any sudden extinction at all. The dinosaurs just died away for reasons unconnected with your asteroid.'"2
RETURN OF THE PTERODACTYL?
The founders of geology and biology were not much interested in extinction. Lyell thought that extinction was so impermanent that the vanished pterodactyl might return to flit through a forest once again primeval. Darwin thought that gradual change was the essence of natural selection: "Species and groups of species gradually disappear, one after another, first from one spot, then from another, and finally from the world."3 "Extinction and natural selection . . . go hand in hand."4 He did occasionally make exceptions: "In some cases . . . the extermination of whole groups of beings, as of the ammonites towards the close of the secondary period, has been wonderfully sudden."5 (Scientists of Darwin's day thought there were four main geologic periods: primary, secondary, tertiary, and quaternary.)
Extinction was simply the natural end of every species and therefore unremarkable. Biologists were much more interested in specia-tion, the process by which an evolutionary lineage divides, giving rise to two species where only one existed. Reflecting the increased interest in extinction since the Alvarez theory appeared, paleontologist David Raup of the University of Chicago and the Field Museum has written a fine book on the subject, Extinction: Bad Genes or Bad Luck?" Raup's most fundamental conclusion about mass extinction, drawn from a lifetime of study, is that, because species typically are well adapted to the normal vicissitudes of life, "for geographically widespread species, extinction is likely only if the killing stress is one so rare as to be beyond the experience of the species, and thus outside the reach of natural selection."7 His conclusion is key to understanding the role of meteorite impact in earth history.
The core concept of natural selection is that species continuously adapt to their environment. This means that organisms tend to be well suited to the normal stresses they encounter, even those that occur on a time scale of thousands or hundreds of thousands of years. (Note that Raup said that the stress has to be unfamiliar not only to individuals but to entire species.) Normal environmental changes, such as a gradual lowering or raising of sea level, or a gradual alteration in climate, cannot by themselves cause a mass extinction—they allow species time to adapt or to migrate to more favorable climes. Some species, it is true, will be unable to do either and will become extinct, but those few do not a mass extinction make. This is more than theory. Another Chicago paleontologist, David Jablonski, examined the fossil record from major mass extinctions in the geologic record to see if they correlated in any way with known changes in sea level, global climate, and mountain building; he found that they did not.8 Of course, catastrophic events such as floods, earthquakes, and volcanic eruptions can and do kill many individuals and, on rare occasions, even species (if they occur in a limited geographical area), but these are not global or even continental in their reach. For species, the opposite of the old saying, "What you don't know can't hurt you" is true: It is what a species does know that can't hurt it.
But suppose that Raup is wrong and that global cooling can cause a mass extinction. Then the evidence should be readily at hand, for the earth has just suffered a succession of ice ages, the last one ending only some 15,000 years ago. Temperatures fell so far that huge ice sheets advanced thousands of miles, covering, for example, most of the northern half of the United States. Many of the large mammal species (of perennial fascination to Homo sapiens) did become extinct during the last ice age, but the overall extinction rate was far below that of the five major mass extinctions and barely makes it onto a chart of extinction intensity (Figure 16). Admittedly, scientists
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