With our short lives, humans tend to think of past events as having taken place at precise times. American independence was declared July 4, 1776, and so on. To be sure, historical eras are a bit more vague. No one woke up one morning in, say, 1500, looked out the window, and said, “Oh look, honey, the Middle Ages are over. We’re living in modern times now.” (Indeed, the very phrase “Middle Ages” was first used only in 1722.)
But on the vast scale of geologic time—4.4 billion years since the formation of planet Earth—things are vague indeed by human standards. Most sources will tell you, for instance, that the Mesozoic era, the age of the dinosaurs, lasted from 252 million years ago to 66 million years ago. The transition between the preceding Paleozoic era and the Mesozoic, however, did not happen overnight. Paleontologists think it took a least a quarter of a million years, perhaps twice that, as vast volcanic eruptions in what is now Siberia outgassed billions of tons of carbon dioxide, raising the planet’s average temperature by at least eighteen degrees Fahrenheit, greatly altering the climate. By the time the transition was over, 90 percent of all living species had gone extinct.
But if the timing of the beginning of the Mesozoic is a bit nebulous, the end of it is anything but. Indeed, the Mesozoic era ended in a single day and the Cenozoic era, in which we still live, began. It was,