What killed the dinosaurs? For 150 years, that question has stumped even the best scientists. But no longer. At last the great mystery has been solved. The story of the solution is fascinating in its own right, and the answer helps us to understand our place in the universe: It raises the revolutionary possibility that the history of the earth, and of life upon it, has been altered repeatedly by a nearly invisible, and previously unrecognized, cosmic process. It forces us to ponder the role of chance in the solar system, even in the evolution of our species. And by helping geologists to cast off outmoded dogma and to acknowledge that our planet is subject to the same processes as other bodies in the solar system, it has transformed the science.

My interest in the story of dinosaur extinction developed over many years as a geologist, professor, and museum director. It peaked one day in December 1993, when the latest issue of Nature landed on my desk.1 Staring up at me from the cover was a photograph of a zircon crystal, scarred with the crisscrossing fractures that I knew indicated extreme shock pressure. "Fingerprinting impact debris," the caption read. The accompanying article revealed how analysis of the shattered zircon helped to corroborate the radical theory that scientists from the University of California at Berkeley had proposed in 1980 to explain the disappearance of the dinosaurs. Fascinated by the detective work involved, I conceived of writing a book that would tell the story of how the great mystery was solved, and explore the extraordinary implications of the new theory.

The story turned out to be richer than I had ever imagined. It provides the best lesson in this century of how scientists challenge and overthrow orthodoxy, and how science really works, not in the mythical ivory tower but down in the trenches. It shows how the suggestion that the dinosaurs died in a catastrophe brought on by the collision of an extraterrestrial object violated geologic dogma not once but twice, and incurred the wrath of many. The cast of characters who took part in the bitter debates that followed shows all too clearly that scientists are passionate, and sometimes flawed, human beings.

My exploration of the claim for an extraterrestrial cause drew on many different fields of science: vertebrate paleontology, micro-paleontology, evolutionary biology, rare-metal chemistry, astronomy, magnetism, statistics, geologic age dating, and the physics of nuclear explosions. Reviewing the evidence from these many disciplines and the writings of geologist colleagues gave me a new appreciation of just how much my field has changed.

Natural history, and geology in particular, has always fascinated me. The founders of my hometown, the small college community of Berea, Kentucky, wisely decided to site it "where the mountains kiss the bluegrass," at the foot of the Cumberland Mountains. To a boy interested in nature, the setting was perfect. Heading out of town on my bicycle, I soon found myself among an abundance of birds, rocks, fossils, butterflies, and Native American artifacts. Like all children, I was fascinated by dinosaurs, which seemed more like creatures of fantasy than ones that had actually lived.

Having been raised in a small academic community, it was perhaps also natural that I would earn a doctorate in my chosen field, geology, and then go on to another college town, Oberlin, Ohio, to teach. One thing led to another and eventually I found myself a college president and then a museum director. My move to the Natural History Museum of Los Angeles County has been especially gratifying because here I am surrounded by birds, rocks, fossils, butterflies, and artifacts, like the ones that got me started so many years ago. But now I preside over 35 million of them!

Many colleagues have helped me in writing this book, though I alone am responsible for any errors of fact or interpretation. The following read the manuscript and made helpful suggestions: Alfred Fischer, Peter Griffin, John Harris, Brian Huber, Adriana Ocampo, Kevin Pope, Donald Reich, Robin Simpson, and Peter Ward. My assistants Pat Reynolds and the late Patricia Barron aided me in many ways, as did Museum Librarians Donald McNamee and Mark Herbert. Former W. H. Freeman and Company Editor Elizabeth Knoll, now at Harvard University Press, had faith that a book on the Alvarez theory would serve a useful purpose. Jonathan Cobb, formerly Senior Editor at W. H. Freeman, was a joy to work with and contributed significantly to the quality of the book. My wonderful agent, Barrie Van Dyck, never gave up. Finally, my wife, Joan, and our daughter, Joanna, were patient beyond any reasonable expectation during the lengthy process of writing this book while I was holding down a full-time job directing a museum. To all of them I am grateful.

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