The great event that made all of human history possible, including this magazine. occurred some sixty-five million years ago. Its primary evidence lies in America.

Few people know that mammals evolved at the same time as dinosaurs, more than two hundred million years ago. They did not arise later and drive dinosaurs to extinction by their superiority. They lived, rather, for one hundred million years as small, rat-sized creatures in the interstices of an ecological world ruled by dinosaurs. In no way did they challenge or displace dinosaurs. Then, some sixty-five million years ago, dinosaurs were wiped out along with many other forms of life in one of the great episodes of mass extinction that have punctuated the history’ of life. The small mammals survived and took over a world emptied of its former rulers. We evolved much later as a result of this good fortune ‘we are a cosmic accident. not the result of a predictable process1. If the extinction had not occurred, dinosaurs would probably still dominate the earth, and conscious creatures would not have evolved.

But what wiped out the dinosaurs’ We now have good evidence that a large extraterrestrial body—an asteroid or comet —struck the earth at this time, perhaps sending aloft a cloud of dust making the earth too cold and dark for large reptiles the “nuclear winter” scenario is based on the same argument. I would like to have seen the explosion and its aftermath; I only hope I would have known when to duck.