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...


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