Catastrophic Events in the History of Life: Toward a New Understanding of Mass Extinctions in the Fossil Record - Part I

Author: Weinreb David B.
Institution: Molecular Biophysics and Biochemistry & Geology and Geophysics
Date: March 2002

This article is Part One of a two-part series exploring the historical changes in the theories of extinction and evolution.

Introduction: Who shall live?

The history of life, as Charles Darwin taught us over a century ago, is a struggle for survival. New organisms emerge and flourish because they are somehow better equipped for their environments than the creatures they replace. Extinction is evolution's way of weeding out those who are simply not able to compete with more specialized or more advanced organisms.

For the last 600 million years, this is how the saga of life unfolded: new forms gradually evolved and replaced more archaic, less successful forms, eventually driving them into extinction. Nearly every organism that has ever existed is now extinct.

Is extinction simply evolution's way of destroying those life forms that fail to adapt to constantly changing environments? Does a species' extinction reflect its failure to compete with the more sophisticated organisms with which it co-exists? Or, does extinction represent a random event; indiscriminately destroying lineages that otherwise could survive indefinitely? The best approach to answering such questions is to investigate episodes in the fossil record when rates of extinction have been phenomenally high: mass extinctions events.

There have been a handful of moments in the earth's history when life on the planet came terrifyingly close to being totally eradicated. At these moments, a great number of species, inhabiting diverse ecosystems across the globe, have simultaneously, or nearly simultaneously, been sentenced to extinction. Many of the condemned do not seem to deserve their fate: in studying mass extinctions, it is difficult to predict a priori which taxa shall live and which shall perish. This has prompted many paleontologists to suspect that mass extinctions do not represent a clearing of archaic, stagnant forms to make way for more evolved, more successful species. In order to fully understand why some species survive mass extinctions and others don't, we must search for the causes of mass extinctions. Unless we understand the geological events that trigger mass extinctions and why they occurred at those particular moments in the earth's history, we will not be able to comprehend the patterns of extinction and survival that accompanied these devastating events.

Paleontologists, geologists, and physicists have collaborated over the course of the last two decades to investigate the causes of mass extinction. Collectively, they have achieved a more precise understanding of why mass extinctions have occurred and offer insight into why some of those outrageously fantastic creatures that once wandered the earth are no longer with us.

Geologic Timescales and Mass Extinction Events

In the 19th century, paleontologists divided the history of life on Earth into four stages. The first era is the Paleozoic, beginning 570 million years ago (570 MYA) and lasting roughly 350 million years. Next, the Mesozoic Era (literally, "time of middle life") occurred from to 225 million to 65 million years ago. The Mesozoic is subdivided into three periods: the Triassic (225-195 MYA), which witnessed the first emergence of the dinosaurs, the Jurassic (195 to 136 MYA), when the earliest birds are documented in the fossil record, and the Cretaceous (136 to 65 MYA) when primates and flowering plants first appear. The third chapter in the history of life is the Cenozoic Era, beginning 65 million years ago, in which we are currently living. The earth's biota has witnessed five major mass extinctions in the last 600 million years. The "Big Five" include the Ordovician-Silurian event (438 MYA), the Devonian extinction (367 MYA), the Permian-Triassic event (250 MYA), the Triassic-Jurassic extinction (202 MYA), and the Cretaceous-Tertiary (K-T) event (65 MYA). Most of these events witnessed a 75% or greater drop in species diversity. The most recent of these mass extinction events, the K-T extinction, brought about the demise of the dinosaurs, signaling the end of the "Age of Reptiles" and the beginning of the "Age of Mammals." Perhaps, for this reason, paleontologists have devoted a tremendous amount of attention to the underlying mechanisms of the K-T extinction.

The K-T Extinction

The K-T extinction was the final hurrah for some of the earth's most successful organisms. The ammonites, squid-like creatures with coiled shells similar to those of the chambered Nautilus, perished during the K-T extinction. The ammonites represent one of the most evolutionarily successful groups in Earth's history, existing for more than 300 million years (by comparison, our species has existed for a brief 200,000 years, if that long). These sophisticated cephalopods diversified in oceans and seaways around the globe for hundreds of millions of years - only to perish forever at the K-T boundary. As Peter Ward, a geo-biologist from the University of Washington, comments, "The ammonites weathered everything the world had ever thrown at them, and then, BANG! .the ammonites have suffered many extinctions, but they've always come back. This was the one when they didn't come back."

The K-T extinction also brought about the demise of the dinosaurs, which, in the words of journalist Ian Warden, are "creatures of interest to all but the veriest dullard." Several decades ago, paleontologists would have explained that dinosaurs were too oversized, too cold-blooded, and too stupid to survive. But in reality, the dinosaurs, which inhabited the earth for more than 150 million years, represent one of the greatest success stories in the history of life.

Princeton University geologist Glenn Jepsen observed in 1964 that, "Authors of varying competence have suggested that dinosaurs disappeared because the climate deteriorated. [or because of] changes in the pressure of composition of the atmosphere, poison gases from volcanic dust.continental drift, extraction of the moon from the Pacific Basin.even lack of standing room in Noah's Ark."

More recently, Peter Ward argued that the extinction of the dinosaurs truly becomes a profound enigma when we reject the view that dinosaurs "simply seemed to have died out due to uncompetitiveness in a very competitive world." Ward describes how the changing views of dinosaur paleobiology in the last three decades have made their disappearance seem even more perplexing:

"Old view: Dinosaurs were slow, clumsy, and so stupid that they needed a second brain in their pelvic region just to be able to walk; they were cold-blooded creatures with dull gray hides whose time came and went. They sort of faded away in the face of climate change, lots of volcanoes exploding everywhere, and superior competition from the warm-blooded, egg-eating, all around nasties of the Late Cretaceous period, the mammals."

"New view: Dinosaurs were fast, graceful, smart wildlife with warm blood, brilliant coloring, and excellent parenting skills whose time went by all too fast. They were so wonderful that they really ought to still exist, and the cause of their extinction is a mystery."

At the close of the Cretaceous, nearly half of all species on Earth vanish. But was the extinction sudden and catastrophic? Was it some single disastrous event that abruptly snuffed out so much of Mesozoic life? Or, perhaps, was it a less dramatic gradual event? Did the dinosaurs slowly march toward an eventual demise, going extinct, in the words of T.S. Elliot, "not with a bang but a whimper?"

Marine ecosystems were also decimated at the K-T boundary. Perhaps as many as 90% of all marine species died out at the boundary: all of the ammonites, 93% of marine reptiles, 83% of planktonic foraminifera and 65% of all sponges. More than half of all terrestrial reptiles died, including all of the dinosaurs. No terrestrial organism larger than 25 kilograms survived the K-T extinction. Interestingly, most of the higher forms of plants were only mildly affected, and mammals actually experienced an increase in diversity at the start of the Tertiary Era.

Searching for the Trigger

In the late 1970s, Walter Alvarez was studying a sequence of clay from Gubbio, Italy. The sequence of clay bracketed the K-T boundary and contains deposits from the final dusk of the Maastrichtian to the early dawn of the Danian. Below the K-T boundary section in the Gubbio clay, there is an abundance of fossil foraminifera of the genus Globotruncana. The Cretaceous deposits in this section consist of a white limestone, filled with Globotruncana. Above the Cretaceous limestone, there is a veneer of reddish clay, only about 1 centimeter in thickness and completely devoid of any fossils, above which lies the red limestone of the Danian deposits. Globotruncana is entirely absent from the red Danian limestone, although, in its place is the foraminifera Parvularugoglobigerina eugubina.

Alvarez's objective was to quantify the rate at which the faunal transition from Globotruncana to P. eugubina had occurred. This would require knowing only how long it took for the 1-centimeter thick red clay at the boundary to be deposited. Alvarez postulated that it probably took about 5,000 years for the clay to form, but he didn't know how to precisely measure the rate of deposition.

Alvarez brought a polished specimen from the Gubbio boundary clay to his father, Luis Alvarez, a Nobel Laureate and physicist at the Lawrence-Berkeley National Laboratory who also had a reputation for being a brash, arrogant man. While the range of research topics that captured his attention extended into many fields of science, he often looked down on researchers in scientific disciplines that he considered to be less rigorous than physics. He was unhesitant to criticize the credentials and character of those with whom he disagreed.

The Iridium Clock

Luis Alvarez suggested to his son that they could measure the abundance of cosmic dust in the clay. The platinum group elements (iridium, osmium, palladium, rhodium, and ruthenium) are incredibly rare in the earth's crust, although their abundance in meteorites is much higher. Broken shards of meteorites produce a cosmic rainfall, reaching the earth's surface at a steady rate. Currently, between 107 and 109 kilograms of meteoritic dust fall to Earth each year. Presuming that the sparse rain of cosmic dust and fragments has been constant and uniform over the thousands of year during which the clay was deposited, the amount of platinum group elements in the clay could provide a clock for assessing sedimentation rates.

Iridium is the easiest to measure of all the platinum group elements. The Alvarezes decided to test an "iridium clock." By comparing the levels of iridium in the clay with the levels in the two layers of limestone that sandwiched it, they could assess the relative rates of sedimentation. Iridium can be measured by neutron activation analysis, in which the clay is bombarded with neutrons, causing the iridium to become radioactive and then quickly decay by emitting gamma rays at a particular, detectable energy level.

What the Alvarezes found was nothing short of shocking. The boundary clay contained 30 times the amount of iridium as the bracketing limestone. If the rain of cosmic dust to Earth had been constant during the time of the interval of clay deposition, then the clay would have required an absurdly long time to form.

The "iridium spike" in the Gubbio boundary clay was not a localized phenomenon. The Alvarez team found an even more impressive enrichment of iridium in a contemporaneous K-T section near Copenhagen, Denmark. By 1981, an iridium enrichment had been detected in more than 100 K-T sections around the world.

In June, 1980, the Alvarez team, which now included nuclear chemist Frank Asaro and Berkeley paleontologist Helen Michel, published a groundbreaking study in the journal Science. In the introduction of their paper, they observed that nearly everything known about the mass extinctions is related to paleobiological interpretations of the fossil record. They claimed that a lack of evidence about how the earth had somehow changed at the K-T boundary had made it challenging to hypothesize about the underlying cause of the mass extinction.

The abundance of iridium in the crust is less than 0.1 parts per billion, although extraterrestrial sources have iridium concentrations greater than hundreds of parts per billion. The Alvarez team concluded from these results that the source of global iridium enrichment was an asteroid that struck the earth at the precise time of the K-T boundary and ejected debris from the impact crater. It may have taken as long as three years before the dust settled to the earth, during which time sunlight would have been blocked and photosynthesis stopped. The impact would have created a type of nuclear winter.

Impact at the Boundary

Gene Shoemaker estimated that, given that there are presently 700 Earth-orbit-crossing asteroids with diameters greater than 1 kilometer, one object with a diameter greater than 10 kilometers will strike the earth on average every 100 million years. Thus, it is certainly not impossible that one such object, with a diameter of about 6 kilometers, could have struck the planet 65 million years ago.

The 1980 Alvarez team paper was truly a bold step. They not only provided evidence for a major impact at the end of the Cretaceous, they took the theory a giant leap further when they proposed that this impact single-handedly created the environmental havoc leading to a mass extinction.

In the year after the Alvarez impact theory was proposed, paleontologists were reluctant to accept the notion of a catastrophic impact. They believed the extinction at the boundary was not instantaneous and sudden, as the Alvarez team proposed, but rather a gradual turnover. Over a period of at least several million years, the majority of paleontologists believed, most species slowly died off. They had been arguing for decades that the K-T event was a gradual transition. The dinosaurs, the ammonites, and their contemporaries had been dwindling in diversity for millions of years; for these paleobiologists, the K-T boundary didn't represent a sudden sweep of devastation, but rather a last whimper for taxa that were long destined for extinction. Could a meteorite impact have been devastating enough to initiate a global catastrophe, one in which 90% of all marine species perished?

Imagine a meteorite slightly larger than Mount Everest, racing toward Earth at a speed 100 times faster than a commercial airliner. The air between the racing meteorite and the surface is compressed, resulting in perhaps the loudest sonic boom ever heard. The compressed air reaches a temperature five times hotter than the surface of the sun, creating a blinding flash of light. The impact releases a burst of energy equivalent to 100 million megatons of TNT - 1,000 times more powerful than a simultaneous detonation of the entire world's nuclear arsenal. When it crashes into the crust of the earth, the force of the impact sends debris weighing 60 times the meteorite's weight into the atmosphere. A cloud of debris remains suspended in the atmosphere for at least two years, blocking sunlight and plunging the biosphere into a long, dark winter. In his 1997 book, T. rex and the Crater of Doom, Walter Alvarez imagines the moment of this devastating impact:

"In the zone where bedrock was melted or vaporized, no living thing could have survived. Even out to a few hundred kilometers from ground zero, the destruction of life must have been nearly total. Sterilized by the intense light from shock-compressed air and from the fireball of rock vapor, crushed when pores and cracks in rock were slammed shut by the passing shock wave, and bombarded by the falling debris of the ejecta blanket, little or nothing was left alive in this central area. Soon the earth's surface itself became an enormous broiler - cooking, charring, igniting, immolating all trees and animals, which were not sheltered under rocks or in holes. Entire forests were ignited, and continent-sized wildfires swept across the lands."
Vast wildfires sucked oxygen out of the soot-filled atmosphere. Earthquakes registering magnitude 13 on the Richter scale caused landslides as well as tsunamis -- some over a kilometer in height - that flooded coastal regions. Any creature fortunate enough to survive the initial fireball would have been threatened by showers of debris, acid rain, and years of impenetrable darkness. Alvarez tells us, "We wonder how anything could survive this environmental apocalypse. Yet there were survivors, and their descendents populate the world today."

When the impact ejecta settled to Earth and light returned, the earth became oppressively hot. The release of carbon dioxide from incinerated limestone at the impact site turned the planet into a gigantic greenhouse. Alvarez estimates that it may have been thousands of years before enough carbon dioxide was removed from the atmosphere to bring temperatures back to normal ranges. The meteorite may have impacted a subterranean deposit of gypsum (calcium sulfate). Once ejected into the atmosphere, the sulfate would have combined with water to create acid rain. Acidic shallow water environments would have become uninhabitable.

Debating the Theory

The bold suggestion that an impact brought about the K-T extinction was not a new idea. In 1973, University of Chicago Nobel Laureate Harold Urey hypothesized that the Cretaceous ended with a collision with a comet. But his idea was widely ignored and, eight years later, when the Alvarez team actually announced evidence for such an extraordinary event, their theory still met considerable resistance.

Paleontologists offered as a major criticism of the Impact Theory the fact that the fossil record provided no hard evidence for a sudden catastrophic extinction. Furthermore, paleobotanists such as Leo Hickey of Yale University commented that the patterns of differential survival and extinction were inconsistent with the notion of a catastrophic extinction. For instance, tropical plants, which would be expected to suffer the most during several years of global darkness, actually were only moderately affected by the extinction. In contrast, temperate plants were devastated, in spite of the fact that temperate plants can suffer through periods of reduced sunlight better than tropical floras.

At the 1985 meeting of the Society of Vertebrate Paleontologists, a survey conducted by The New York Times revealed that a meager 4% of paleontologists were convinced that a meteorite impact had brought about the mass extinction, although 90% were receptive to the theory that an impact had indeed occurred at the K-T boundary. In short, paleontologists were convinced by the Alvarez report that an impact had occurred, but they doubted the link between the meteorite crash and the global extinction.

Robert Bakker, a dinosaur expert with the University of Colorado Museum, attacked the Alvarez theory, exclaiming,

"The arrogance of these people is simply unbelievable. They know next to nothing about how real animals evolve, live, and become extinct. But, despite their ignorance, the geochemists feel that all you have to do is crank up some fancy machine and you've revolutionized science. The real reasons for the dinosaur extinctions have to do with temperature and sea level changes, the spread of diseases by migration and other complex events. In effect, they're saying this: we high-tech people have all the answers, and you paleontologists are just primitive rockhounds."

Bakker and his colleagues felt strongly that the fossil record told a story that was terribly inconsistent with the scenario implicit in the Alvarez model. But Bakker expressed a sentiment held by many in the scientific community: that paleontology had remained a low-tech endeavor. Paleontologists were still "digging in the dirt," while researchers in other disciplines had embraced novel technologies and become more eager to collaborate with scientists outside their own fields.

These sentiments were summed up harshly by Luis Alvarez, who, frustrated by the resistance he encountered, lamented, "I don't like to say bad things about paleontologists, but they're really not very good scientists. They're more like stamp collectors. I can say these things about some of our opponents because this is my last hurrah, and I have to tell the truth. I don't want to hold these guys up to too much scorn. But, they deserve some scorn because they're publishing scientific nonsense."

Within the few years that followed the emergence of the impact theory, the Alvarez team faced two types of opponents. First, there were the paleontologists, who, for the most part, were willing to accept that an impact had occurred, but were not convinced that the event, no matter how catastrophic, could explain the patterns of extinction at the eleventh hour of the Cretaceous. While they did not doubt the environmental repercussions of an impact would have been devastating enough to trigger a mass extinction, they felt the list of survivors after the K-T event was irreconcilable with the climatological and environmental consequences of a meteorite impact.

The second group of opponents was a faction of geochemists who argued that the iridium spike in the Italian and Danish sections could be explained by an alternative hypothesis. These researchers suggested that massive volcanic activity responsible for the Deccan lava flows in India during the late Cretaceous may have ejected trillions of tons of greenhouse gases (as well as high concentrations of platinum metals, including iridium) into the atmosphere. The environmental consequences of volcanic activity would have been quite similar to the effects of an impact. Perhaps, those geochemists argued, the effects of such a vast outpouring of lava would have been slightly less catastrophic and abrupt than a collision with an asteroid the size of San Francisco.

Deccan Flood Traps

In the mid-1980s, a team of Indian geochemists, supervised by N. Bhandari of the Physical Research Laboratory in Ahmedabad, discovered an iridium spike in the lava beds of the Deccan Traps, corresponding to exactly the time of the K-T boundary. At roughly the same time, French geologist Vincent Courtillot used the magnetic reversal timescale to conclude that the Deccan volcanic eruptions started at least one million years before the K-T boundary and were sustained for about 1 million years after it. If Deccan volcanism was the chief cause of the extinction, then it is unlikely that the extinction was as rapid as the Alvarez team suggested. However, dinosaur fossils are recovered from within the sediment layers that sandwich the Deccan lava flows, providing strong evidence that dinosaurs endured at least part of the volcanic catastrophe.

Furthermore, the iridium levels detected by the Bhandari team was detected in a layer of sedimentary rock deposited after the major period of volcanism had ended. In simple terms, the iridium spike did not occur at the same time as the volcanism that these researchers claimed was responsible for the mass extinction. Iridium is not detected in the majority of the sedimentary layers of the Deccan Traps, and wherever it is detected, its concentration is relatively low compared to what one would expect in sediments so close to the site of volcanic activity.

The advocates of the Deccan Traps volcanism theory simply asked the Alvarez group: Where is the smoking gun? A 10-kilometer meteorite, they argued, could not hit the earth and wipe out half of all species on the planet without leaving some sort of evidence of its existence. Show us, they demanded, the crater.

Smoking Gun?

The Alvarez team anticipated that they would have to address this issue when they published their first paper in Science in 1980. They end the paper with these words:

"There is a 2/3 probability that the object fell in the ocean. Since the probable diameter of the object, 10 km, is twice the typical oceanic depth, a crater would be produced on the ocean bottom and pulverized rock could be ejected. However, in this event we are unlikely to find the crater, since bathymetric information is not sufficiently detailed and since a substantial portion of the pre-Tertiary ocean has been subducted."

How does one search for a crater that may not even exist any more? In 1989, Alan Hildebrand, then with the Geological Survey of Canada, found a rather peculiar 50-centimeter-thick layer of sediments in Massi de la Selle, Haiti. The sediments contained both shocked quartz grains (minerals that have been deformed under conditions of extreme pressure) and tektites (terrestrial rocks believed to have been ejected into the upper atmosphere after an impact and, after orbiting the earth, settled back to the surface). The thickness of this sedimentary layer suggested that "ground zero" was probably quite close to Haiti.

Geophysicists employed in the petroleum industry had been studying the basement rocks of the Caribbean for decades. These researchers measure variations in the strength of the earth's gravitational field in different parts of the earth's crust. Gravity anomalies are zones where the local gravitational field may be slightly stronger than in surrounding regions. The presence of these anomalies often suggests a broad area of more dense rock because the strength of a local gravitational field is proportional to its mass. Geophysicists often search for gravity anomalies in order to pinpoint the location of oil-bearing rocks.

Hildebrand discovered an early report by geophysicists Glen Penfield and Antonio Camargo that had been largely ignored at the time of its publication in 1981. Penfield and Camargo detected circular gravity anomalies in the northern Yucatán Peninsula centered near the town of Chicxulub. Their findings suggested the existence of a belt of dense, compacted rocks buried beneath a kilometer of sediments. Penfield and Camargo concluded their 1981 presentation at the meeting of the Society of Exploration Geophysicists with these words: "We would like to note the proximity of this feature in time to the hypothetical Cretaceous-Tertiary boundary event responsible for the emplacement of iridium-enriched clays on a global scale and invite investigation of this feature in the light of the meteorite impact-climatic alteration hypothesis for the late Cretaceous extinctions."

Ironically, the Penfield-Camargo lecture occurred the same exact week when most researchers studying the K-T boundary had convened in Snowbird, Utah to debate the Alvarezes. Thus, on almost the same day the most vehement critics of the Alvarezes argued that there was no crater to substantiate the theory of a meteorite impact, Penfield and Camargo presented the first evidence for a crater.

Amazingly, only several months after the Alvarez team announced the elevated iridium in the Gubbio clay, Penfield and Camargo had discovered the smoking gun: a crater of appropriate age, large enough to have been produced by impact with a 10-kilometer wide object. And, more amazingly, nobody seemed to have noticed.

It turned out that petroleum geologists working for the Mexican oil giant Petroleos Mexicanos had discovered the circular Chicxulub (a Mayan word meaning "red devil") structure in the 1950s. In search of oil reservoirs, Petroleos Mexicanos researchers drilled into the structure and collected rock cores. In 1980, immediately after the publication of the Alvarez paper in Science, Penfield wrote to Walter Alvarez to inform him of the existence of this structure. He never received a response.

Despite the evidence, the debate is far from over. Did the impact at Chicxulub cause a global mass extinction? Or were the horrific environmental consequences localized in North America, the region that logically would have been most directly affected by the impact debris, tsunamis, seismic waves, and acid rain in the wake of the impact?

Trusting the Fossil Record

The key issue in the debate between the proponents of the Impact Theory and paleontologists is whether the extinction was sudden or gradual. Many paleontologists are confident the extinction was gradual, and feel that even if the impact had occurred and did have globally catastrophic effects, it was not the principal factor responsible for the decline in species diversity in the late Cretaceous.

How accurately can we tell whether an extinction in the fossil record is gradual or sudden? At the 1981 Snowbird I conference, Phil Signor and Jere Lipps of the University of California at Berkeley suggested data on species diversity extracted from the fossil record might be much harder to accurately interpret than most paleontologists had previously assumed.

Signor and Lipps demonstrated that the number of unique ammonite genera known from the fossil record declines sharply at the end of the Cretaceous. However, the area of sedimentary rocks across the globe was also decreasing. Thus, there are fewer sedimentary rocks of Late Cretaceous origin in which ammonites could have been deposited and preserved. These authors demonstrated that ammonite genus diversity throughout the entire Mesozoic correlates with the number of square kilometers of sedimentary rock available to be sampled for each geological period.

Next, consider the dilemma of a paleontologist who is searching for the fossils of a rare dinosaur. The question this researcher must address is whether this species went extinct suddenly at the final moments of the Mesozoic or whether it died out gradually in a slow demise that lasted throughout the Maastrichtian. If this particular species was widespread during the Cretaceous, then it should theoretically be abundant in the fossil record; we would expect to find it at virtually every stratigraphic horizon. If the fossil persisted until the bitter end of the Cretaceous, then we would expect to find it in the sediments just below the Impact Event boundary. However, if the species was very rare (as are most dinosaur fossils), then the probability of finding it in the youngest Maastrichtian sediments is rather low, even if it did live up until the very end. Simply stated, geologists are unlikely to determine the true time at which a species went extinct unless the species is very common in the record. Signor and Lipps demonstrated that a sudden extinction, like that predicted by the Alvarez team, might indeed appear gradual to a paleontologist sifting through Late Cretaceous deposits.

On the closing page of his 1998 book, Night Comes to the Cretaceous, James Lawrence Powell writes,

"Today we have gone about as far as science can go in corroborating the notion that the impact of a meteorite caused the extinction of the dinosaurs. But as always, answering one set of questions raises others, and we are left pondering the true role of impact. As even its bitterest opponents have to admit, the Alvarez theory has brought geology not only a new set of questions, but also a greatly improved set of sampling techniques and analytical methods for answering them. Paleontologists collect much larger samples and subject them to statistical tests. Today geologists know how to find and identify terrestrial craters. These are the hallmarks of a fertile theory."

Indeed, the Alvarez theory, which once seemed so far-fetched and contrived as to provoke ridicule and scorn, now has become universally praised. And, beyond a doubt, the debate that followed the first discovery of the iridium spike revolutionized the way paleontologists study the history of life.

In Part II of this series (to be published in April 2002), we explore further theories and new evidence for possible mechanisms that bring us closer to understanding the causes of mass extinctions.