The earth has evolved in geologic terms, but it has also evolved in terms of the life that teems upon it. All the change that can be seen and inferred has to be explained. The argument in the eighteenth and nineteenth centuries was divided into two camps, the catastrophists and the uniformitarianists. The catastrophists explained the changes by resorting to cosmic violence, to a succession of catastrophes that bring about change suddenly. The uniformitarianists saw the earth evolving in a uniform manner over time. Both groups agreed that catastrophes do play a role in the evolution of the earth and that catastrophes still occur to reshape portions of the earth's surface or to bring about biological changes in a relatively short time. However, this is different from holding that a cosmic catastrophe has to be posited to explain change. One of the more dramatic biological changes in the history of the earth was the extinction of the dinosaurs. This was not the only such extinction to take place, but the size of the beasts involved, the length of time they ruled the earth, and the hold they have on the public imagination makes the extinction of the dinosaurs of particular note. Several theories have been offered for the extinction of the dinosaurs, among them genetic theories, theories of climatic change, and the currently accepted view that a comet or meteorite hit the earth, created conditions we might liken to nuclear winter, and killed much of the life on earth at the time.
The extinction of the dinosaurs is the extinction known as the Cretaceous Tertiary (also called the K/T) event 65 million years ago. This event has been known for some time, but the reasons for it have been difficult to determine, including the crucial issue of how long it took for the event to take place, as was said of an earlier mass extinction: "Did the old species slowly die away over millions of years, one after another, to be gradually replace...