The dinosaurs were among the most successful creatures ever to live. They reigned for 160 million years, far longer than the few million that our genus, Homo, has so far existed. Dinosaurs came in all sizes and shapes: small and large, fast and slow. Except for today's blue whale, no creature has been larger, and surely none have been as frightening. Some, like Apatosaurus, the 80-foot, 30-ton thunder lizard whose tread shook the ground like an earthquake, were herbivores. Others, like the 45-foot-long, 20-foot-tall Tyrannosaurus rex, a favorite of film and of children, were carnivores.
But after having thrived for millions of years, suddenly, in a relative instant of geologic time, the giant reptiles vanished completely and forever, leaving in their wake the greatest scientific mystery: What killed the dinosaurs? The mystery deepens when we learn that it was not only the dinosaurs that became extinct 65 million years ago, but 70 percent of all species on the earth. The geologic boundary that marks the time of their demise is used to define the end of one great geologic era, the Mesozoic (middle life), and the beginning of the one in which we live, the Cenozoic (modern life). Curiously, the snakes, turtles, and crocodiles, which one would have thought were enough like the dinosaurs to have met a similar fate, survived. What could kill every single dinosaur but spare these other reptiles?
Since the time 150 years ago when the dinosaurs were first discovered, hundreds of scientists have struggled to solve this mystery. Glenn Jepsen of Princeton University, in a 1964 article, summed up the many solutions that had been proposed—some worthy, some feeble, some surely facetious:
Authors with varying competence have suggested that dinosaurs disappeared because the climate deteriorated (became suddenly or slowly too hot or cold or dry or wet), or that the diet did (with too much food or not enough of such substances as fern oil; from poisons in water or plants or ingested minerals; by bankruptcy of calcium or other necessary elements). Other writers have put the blame on disease, parasites, wars, anatomical or metabolic disorders (slipped vertebral discs, malfunction or imbalance of hormone and endocrine systems, dwindling brain and consequent stupidity, heat sterilization, effects of being warm-blooded in the Mesozoic world), racial old age, evolutionary drift into senescent overspecialization, changes in the pressure or composition of the atmosphere, poison gases, volcanic dust, excessive oxygen from plants, meteorites, comets, gene pool drainage by little mammalian egg-eaters, overkill capacity by predators, fluctuation of gravitational constants, development of psychotic suicidal factors, entropy, cosmic radiation, shift of Earth's poles, floods, continental drift, extraction of the moon from the Pacific Basin, drainage of swamp and lake environments, sunspots, God's will, mountain building, raids by little green hunters in flying saucers, lack of even standing room in Noah's Ark, and paleoweltschmerz.'
Such a long list of theories, so many of them downright silly and contradictory, suggests a mystery so deep as to be beyond us. Many people must have shared the attitude of humorist Will Cuppy: "The age of dinosaurs ended because it had gone on long enough and it was all a mistake in the first place."
Why, given the intense interest that dinosaurs have aroused since their discovery a century and a half ago, did it take so long to solve the riddle of their extinction? One reason is that in the historical sciences—geology, archaeology, paleoanthropology—definitive answers are particularly hard to come by. Scientists in these fields do not have the advantage of being able to design and then conduct experiments, as is done in chemistry, physics, and many areas of biology. Rather, they have to operate more as detectives. The "experiments" were conducted long ago by nature; scientists working today literally must pick up the pieces and try to interpret them. A "crime"—in this case, dinosaur extinction—is discovered, sometimes by accident. In the mystery novel, the clues accumulate and, before we realize it, the clever detective has identified the culprit. But the mysteries of the earth, like real crimes, are not always so easily solved.
Paleontology, the study of ancient life, is an especially difficult science in which to arrive at definitive answers. We can learn about prehistoric animals only from their fossilized remains, and yet many had no hard parts and therefore left no trace. Of them, we can never learn anything. Other organisms were bony; but all too often, before they could be fossilized, their bones dissolved or weathered away. The fossils that did form had a way of winding up in rocks other than those in which, millions of years later, we happen to be searching. Paleoanthropologist Meave Leakey, of the famous family whose work has transformed the study of human origins, describes her nearly futile hunt for human bone in a new field area as four years of hard work producing only three nondescript scraps.2 To take a different kind of example, in the winter after the great Yellowstone fires of 1988, thousands of elk perished from extreme cold coupled with lack of food. Late the following spring, their carcasses were strewn everywhere. Yet only a few years later, bones from the great elk kill are scarce. The odds that a single one will be preserved so that it can be found 65 million years from now approach zero. At best we can expect to find fossil evidence of only a tiny fraction of the animals that once lived. The earth's normal processes destroy or hide most of the clues.
Still, it comes as a shock to realize that in spite of the intense popular and scientific interest in the dinosaurs and the well-publicized efforts of generations of dinosaur hunters, only about 2,100 articulated dinosaur bones (two or more aligned in the same position as in life) have ever been found.3 Conclusions about the life and death of the dinosaurs thus rest on a small sample indeed. As a result, in spite of the fascination they hold, our knowledge of dinosaurs has progressed slowly. Until recently, as Jepsen's long list shows, the door to speculation about their demise has been wide open. Almost any notion could be proposed and avoid being refuted on the evidence.
A second reason that solving the mystery of dinosaur extinction proved so difficult is that geologists, having correctly dismissed most of the reasons on Jepsen's list, were questioning only the usual suspects: changes in sea level, geography, and climate. Throughout the history of the earth continents have grown and eroded, seafloors have divided, spread, and closed, and the earth has moved closer and farther away from the sun, all causing countless changes in sea level, geography, and climate. Perhaps one of these familiar mechanisms, reaching a rare extreme, brought down the great beasts. Yet surely creatures that had lived for 160 million years had survived many such changes and others that we can only imagine. How could a decline in sea level, even an extreme one, have caused their demise? Would it not merely have opened up more land on which they could roam? These explanations seemed contrived, a bit like accusing the staid and familiar butler of actually being the maniac who murdered the family he had served faithfully for decades. Suppose instead that the culprit was a complete stranger who appeared out of nowhere, entered violently, stayed only long enough to do the deed, and then vanished as suddenly as he had appeared. And 65 million years ago at that. Now there's a mystery to defy even the best detective.
Another problem with the usual list of geologic suspects is that although most earthly processes operate slowly, for decades scientists believed that the dinosaurs had expired suddenly. How could a gradual process cause a rapid extinction? Over the last several decades some additional dinosaur fossils have been found, but none in the few centimeters just below the Mesozoic-Cenozoic boundary; instead, they have seemed to peter out farther down, in rocks older than the boundary. The absence of dinosaur bones right at the boundary led most specialists to discard the long-standing view that the dinosaurs had expired suddenly, and to become persuaded that instead they had started to wane millions of years before the end of the Mesozoic. Contrary to public opinion, the vertebrate paleontologists thought, to paraphrase Dylan Thomas, that the terrible lizards had gone gently into the night of extinction. Not only was their disappearance not mysterious, it was inevitable, for extinction is the fate of all species. After all, the average species survives for only about 4 million years; over 95 percent of those that have ever lived are extinct. Hence, dinosaur extinction seemed to need no special explanation—it was simply the way things were and are today. The great mystery had been converted into something mundane.
A pair of scientists writing in 1979 expressed it this way: The dinosaurs "may have succumbed to a series of environmental disasters, some dying of thirst, others of hunger, and the stragglers may have perished because the reduced population density rendered the community unviable."4 What a sorry end to 160 million years of supremacy] In this view, the dinosaurs had played the game of evolution longer than most, but in the end, they too had lost, going out, in T. S. Eliot's phrase, "not with a bang but a whimper."5
Near the end of the 1970s, dinosaur specialist Dale Russell wrote a review article in which he selected and examined those theories that seemed scientifically plausible out of the scores that had been offered.6 In the end, he had to conclude that only one held up to scrutiny: The dinosaurs died because an exploding star or supernova—a literal death ray—had spread lethal radiation effects throughout our region of the galaxy. Russell ended his paper on this pessimistic note: "If a fundamental deficiency were found in the supernova model . . . the disappearance of the dinosaurs would remain an outstanding mystery of the geologic record."7
The theories that Russell examined shared three shortcomings: First, the evidence to support them had been sifted countless times and proven inconclusive. Second, new evidence was scarce. Third, the theories made few or no testable predictions. This last weakness is crucial: To be useful, a theory must make predictions that can be tested—it must lead to questions that can be examined in the field and laboratory. A theory that suggests no further action could be correct for all we know, but since it can neither be corroborated nor falsified, there is no way to find out. Theories that cannot be tested are like rocks in a stream, around which the river of scientific progress must flow.
All the plausible theories of dinosaur extinction were based on the assumption that the earth has always worked as it does today, a reasonable supposition that geologists have employed almost from the beginning of their science. In this large-scale view, change derives not from sudden catastrophe but from deliberate and inexorable processes—erosion, deposition, the long-term rise and fall of the sea, the uplifting and downwearing of continents—whose full effects can only be seen after the passage of hundreds of thousands, even millions, of years.
But what if one were to set aside the assumption of slow change and ask whether an event as rare as dinosaur extinction might not have had an equally rare cause—a catastrophe that would appear only on the same time scale, say, every 50 million to 100 million years? If that were the case, the geologic sleuths needed to detect an event that no human being had ever seen, the evidence for which might be buried 65 million years in the past. To make the task even more difficult, the cause turned out to be one in which, at the start, almost no geologists believed.
Success at solving the mystery of dinosaur extinction required a rare concatenation of circumstances and a healthy dose of good luck. Though, as often happens in science, more than one person was on the right trail; a remarkable pair got there first. One of the pair was a Nobel Prize-winning physicist with no professional experience in geology, a problem solver unable to resist going after the biggest scientific mystery and who, late in his career, could afford to do so with impunity. Luis Alvarez was a man of unusual energy and curiosity. Without these qualities, he would have had neither the interest nor the willingness to tackle a problem so far outside his discipline. A scientist at an earlier career stage likely could not have taken the risk of working in a field so far outside his or her own, where the chances of failure or success were harder to predict.
The other person was an experienced geologist who understood the complexities of the dinosaur puzzle but, having worked in other branches of earth science, had no previously announced position on dinosaur extinction to defend. A meticulous scientist, he helped to build the case among geologists themselves that something far outside their experience had caused the extinction. His presence kept opponents from dismissing the new theory as merely the work of an arrogant, geologically challenged, physicist.
Success also depended on a third and special factor—the relationship between the two scientists. Luis and Walter Alvarez were father and son. Their kinship kept at bay the kinds of issues that so often retard progress—mistrust, rivalry, priority, arguments over who should be the senior author on a research paper. Walter's gentlemanly style helped to offset the wrath generated among geologists by his pugnacious father.
A fourth critical element was that often essential ingredient in scientific discovery: serendipity. Luis and Walter Alvarez were lucky. But although the initial discovery that eventually led to the solution of the riddle was accidental, the minds of Luis and Walter Alvarez, in combination, were well prepared to grasp the opportunity that serendipity presented. And, the timing was right. As both Jepsen's amusing list and Russell's scientific analysis demonstrated, geologists really had little to show for 150 years of trying to find out what had killed the dinosaurs. No one really knew, and therefore no strong rival theory existed. Furthermore, by the 1970s, the exploration of space, upon which so much of the solution would hinge, was in full bloom; those who had been paying attention knew that impact catastrophes were rife in the history of the solar system—just look at the craters on the moon. Finally, by 1980, geologists had developed tools to recognize the much more obscure and rare craters that exist on the earth.
As Russell was writing his article, the Alvarezes and their co-workers at Berkeley were pursuing one of the theories that Russell had mentioned in passing but for which no evidence had existed: the idea that at the end of the Mesozoic era, a large meteorite had struck the earth and thrown up such a vast cloud of dust that it darkened the sky, lowered world temperatures by many degrees, halted photosynthesis, disrupted the food chain, and thus gave rise to a great mass extinction.
Shakespeare's Caesar tells us "The fault, dear Brutus, is not in our stars, But in ourselves." The Alvarez theory held that the fault was not in the dinosaurs themselves but, almost, literally, in their stars! They died through no deficiency of their own, but simply as a result of being in the wrong place at the wrong time. In contrast to almost all the possibilities on Jepsen's list, the Alvarez theory made predictions (Luis counted 15) and therefore it could be tested. The most striking prediction was that somewhere on the surface of the earth there may lie hidden a crater exactly 65 million years old.
The new theory ran into trouble immediately. To accept that the Alvarezes were right, the vertebrate paleontologists would have had to admit that they were wrong: The dinosaurs had gone extinct suddenly after all. Even more seriously, the theory required all geologists to accept that one of the greatest events in earth history was explained by a random catastrophe. The trouble was, the notion of catastrophism as the cause of geologic events had been cast off '50 years previously; in fact, abandoning it had been central to the birth of geology as a modern science. In the decades since, geologists had been eminently successful in explaining earth history by appealing only to slow, noncatastrophic processes. To fall back now on a catastrophe to explain dinosaur extinction would seem to be a return to the dark ages of their science.
A group of scientists led by Professor Charles Officer, now retired from Dartmouth College, not only believed that the Alvarez theory was wrong, as did many, but actively set out to refute it. They published hundreds of papers presenting evidence that they believed contradicted the theory. But in Luis Alvarez, the critics found a brilliant scientific opponent who loved a good fight. The debate descended to one of the all-time lows of scientific discourse. Insults were thrown with abandon and careers were damaged. A Berkeley paleontologist labeled the theory "codswallop," Luis responded that paleontologists were nothing more than "stamp collectors"—to a physicist, the ultimate insult. Critics reached back to '954 to dredge up Luis's controversial role in the notorious hearings that led to the dismissal of J. Robert Oppenheimer, father of the atom bomb, as a security risk. The debate turned ugly indeed, with ample blame to go around. The making of science is often not the pretty sight that textbooks and scientific papers written after the fact would have us believe.
Geologists also resisted because, in another area of their discipline, they had just weathered a scientific revolution. The late Thomas Kuhn, and other students of the history of science, have shown how almost all scientists work within a common set of understandings of their discipline—within the confines of a particular ruling paradigm.8 Sooner or later, new evidence is found that the current paradigm cannot explain, and the paradigm needs to be replaced. The more senior scientists tend to cling to the fading paradigm, on which they have built their careers and reputations, some going to their graves obstinate in their belief. The young turks, and others who are more flexible or iconoclastic, rally to exploit the new paradigm. But all good things must come to an end, and eventually the former young turks, now scientific senior citizens, are themselves displaced by a new group of scientific provocateurs. All of this has great human cost, as scientists are no more inclined than anyone else to admit they were wrong.
In the 1960s and 1970s, geologists had made a revolutionary paradigm shift: They had finally come to accept a modern version of the theory that the continents are not fixed on the surface of the earth, but drift, even colliding with each other to throw up mountain ranges such as the Appalachians and the Urals. The theory of continental drift, originally proposed in 1915, had been largely forgotten by mid-century. Then, suddenly, a new kind of evidence became available from studying the magnetism of the ocean floors. It showed that continents do not actually drift through the earth's mantle (the second zone of the layer-cake structure of crust, mantle, and core); rather, the surface of the earth is divided into giant, moving plates that carry continents along on them, as though they were riding piggyback on a huge, ponderous conveyor belt. Now, with only a decade to adapt to this revolution, geologists were being asked (and by a physicist!) to accept another: that Mother Earth does not always evolve as a result of imperceptible change conducted over deep geologic time, but sometimes, perhaps even often, as the result of sudden and apparently random catastrophes.
The meteorite impact theory proved even more difficult for geologists to swallow than continental drift, for it appeals to a deus ex machina, exactly the opposite of the way they normally work and think. Impact theory asks geologists to look, not down at the familiar terra firma that has always drawn their gaze, but up at the sky—for them, an unfamiliar and uncomfortable posture. It requires that they abandon a strict interpretation of uniformitarianism—the view that all past changes can be understood by studying only processes that can be seen going on today—for well over a century the guiding paradigm of geology. But the impact theory has implications not only for geologists. It requires that biologists consider the possibility that evolution may be driven by survival not of the fittest but of the luckiest. And it requires all of us to take more seriously the role of chance in the history of our planet, our species, and ourselves.
As this book was being written, Eugene Shoemaker, the scientist whose lifetime of work helped to create and to validate this second revolution, tragically died while mapping his beloved impact craters in the Australian outback. Years of attempting to persuade his fellow scientists of the importance of meteorite impact in earth history had led Gene to observe that "most geologists just don't like to think of stones as big as hills or small mountains falling out of the sky."' Physicists, however, have had no such compunction.
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