The dinosaurs, and many other creatures such as the pterosaurs, mosasaurs, and ammonoids, went extinct at the end of the Cretaceous. There is considerable evidence that dinosaurs, after reaching a peak of diversity about seventy-four million years ago, began a gradual decline. Unquestionably, though, something delivered the coup de grâce. In Chapter 8 I tell the story of the bitter, vicious controversy that raged over the reasons for the mass extinction that ended the Mesozoic. Many regard the dinosaurs’ demise with sadness. Many a kid has fantasized that somehow, somewhere, there is a lost forest where T. rex continues to prowl. But if the dinosaurs had not gone extinct, it would be most unlikely that we would be here. During the Mesozoic, our mammalian ancestors were small, nondescript, probably nocturnal creatures. While dinosaurs were dominant, there simply was no free ecological space for large mammals to occupy. Only after the extinction of the dinosaurs did the remarkable evolutionary radiation of mammals occur.
The dinosaurs were the lords of the Mesozoic. Their demise offers a deep lesson to us: The earth does not grant tenure. On the contrary, 99 percent of all species that have ever lived are now extinct. Even more sobering is the realization that several different hominid species have lived on Earth. Only one survives—Homo sapiens. We tend to think that the “sapiens” part of that name, our intelligence, gives us an edge that will allow us to escape the fate of other species. Well, we certainly have been successful in the short term—there are six billion of us on the planet, making us by far the most numerous species of large animal. By contrast, only about two thousand tigers survive in the wild. But the deepest lesson of the dinosaurs is the awful, incomprehensible depth of time. Sixty-five million years from now, will any of our descendants be alive? If so, will they be in any sense human? Will they remember any of our literature, our art, our science, our religions? After all, geological time shows that the only thing permanent is the fact of change. Evolution on Earth began with the first self-replicating molecules four billion years ago, and it will continue until the sun dies.