No one was sure. Since nobody had ever bothered to study extinction in an organized way, there was little information about rates of extinction in other geological eras. So scientists began to look closely at extinction in the past, hoping to answer anxieties about the present.

The second development concerned new knowledge about the death of the dinosaurs. It had long been known that all dinosaur species had become extinct in a relatively short time at the end of the Cretaceous era, approximately sixty-five million years ago. Exactly how quickly those extinctions occurred was a subject of long-standing debate: some paleontologists believed they had been catastrophically swift, others felt the dinosaurs had died out more gradually, over a period of ten thousand to ten million years - hardly a rapid event.

Then, in 1980, physicist Luis Alvarez and three coworkers discovered high concentrations of the element iridium in rocks from the end of the Cretaceous and the start of the Tertiary - the so-called K-T boundary. (The Cretaceous was shorthanded as "K" to avoid confusion with the Cambrian and other geological periods.) Iridium is rare on earth, but abundant in meteors. Alvarez's team argued that the presence of so much iridium in rocks at the K-T boundary suggested that a giant meteorite, many miles in diameter, had collided with the earth at that time. They theorized that the resulting dust and debris had darkened the skies, inhibited photosynthesis, killed plants and animals, and ended the reign of the dinosaurs.

This dramatic theory captured the media and public imagination. It began a controversy which continued for many years. Where was the crater from this meteor? Various candidates were proposed. There were five major periods of extinction in the past-had meteors caused them all? Was there a twenty-six-million-year cycle of catastrophe? Was the planet even now awaiting another devastating impact?