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formations around the world and was long known to mark a great extinction of the dinosaurs and many other forms of life. There are at least two other mass extinctions, one at the end of the Triassic, about 205 million years ago, and another one, the most significant known so far, at the end of the Permian, 250 million years ago. Ninety-five percent of all species in existence then died out.
    Only in recent years have paleontologists and evolutionary biologists come to recognize the importance of mass extinctions in evolution. These events brought chaos and destruction to the planet, and opportunity. New forms of life evolved rapidly to occupy niches in the environment left open by the disaster.
    But explanations for these extinctions have been hard to come by. There has been no end of argument about the extinction of the dinosaurs. So the discovery of this highly unusual concentration of iridium exactly at the time of a mass extinction was intriguing. One of the events that would produce such a spike would be the impact of an asteroid hitting the earth. Iridium is common in asteroids but not in the earth’s crust and an asteroid of sufficient size—about ten kilometers in diameter, Alvarez estimated—would do the trick. He worked with his father, Luis Alvarez, a Nobel Prize- winning physicist, and two geochemists, Helen Michel and Frank Asaro, all at the University of California at Berkeley. Michel and Asaro had found iridium spikes at two other sites marking the end of the Cretaceous—in Denmark and New Zealand.
    A great scientific debate continued as many more iridium spikes in a clay layer of the same age were found, including in some parts of the Hell Creek Formation. But there were, and are, many puzzles about why some animals went extinct and others did not. And a question remained, for a time, about where the evidence of such a collision was. This was the kind of impact that would leave a mark.
    It wasn’t until 1990 that seafloor cores drilled in the Gulf of Mexico showed quartz that had been transformed by an impact of the sort an asteroid would cause. And it lay underwater in an area near the town of Chicxulub that had been spotted a decade earlier as a potential impact crater. That first claim did not attract scientific attention, but the “shocked quartz” did. Other evidence accumulated—glass deposits in Haiti and sand in Montana, blown from the crater.
    What exactly the asteroid did to the global environment is not known, and explanations still abound for the mechanisms of extinction, but what is clear is that there was a massive and immediate global extinction after an asteroid impact of literally unimaginable proportions. And the best place to see the fossil killing field, or iridium layer, and the terrestrial life before and after it is the Hell Creek Formation.
    A year and a half ago astronomers identified what may have been the source of the Chicxulub meteorite, a collision in the asteroid belt that occurred 160 million years ago in a group of asteroids known as the Baptistina family. As was pointed out several times in the news of the discovery, this would mean that the Chicxulub asteroid was set on its course about 100 million years before it hit, in the middle of the Jurassic age.
    Any modern human with even a hint of pessimism about the future of the human race has to have some sympathy for the creatures of the Hell Creek ecosystem, completely unaware that many of them were about to disappear forever. Of course, some of them were about to get their big chance—mammals, for instance. They were small, perhaps able to survive on a variety of foods, including leftovers from the extinctions, and not dependent only on photosynthetic plants, which seem to have had a hard time.
    Evolution driven by catastrophe is not what Darwin had envisioned. He, and many of those who came after him, saw natural selection acting gradually, preserving, or selecting for, the traits of animals that left more offspring.