Mass extinctions can’t be planned for. The triggers for these catastrophic events – the impact of an asteroid, rapid changes in the makeup of the earth’s atmosphere, and the like – are contingent events in the history of life on earth that no species can forsee. (Not even our own.) The devastating global extinction which occurred about 65.5 million years ago, for example, may have been the result of a really bad day in which asteroid impact, intense volcanic activity, and other causes played a part. The rapid ecological changes which resulted rapidly winnowed down the diversity of life on earth.

Of course, life was not entirely extinguished by the end-Cretaceous extinction. Many species survived, and, although the reason why certain