Today, we refer to such events as “mass extinctions,” incidents in which many species of animals and plants died out in a geological instant. They are so profound and have such global reach that geological time itself is sliced up into periods—Permian, Triassic, Cretaceous—that are often defined by these mass extinctions.
Debate over what caused these factory resets of life has raged ever since Cuvier’s time. He considered them to be caused by environmental catastrophes that rearranged the oceans and continents. Since then, a host of explanations have been proposed, including diseases, galactic gamma rays, dark matter, and even methane from microbes. But since the 1970s, most scientists have considered the likely root cause to be either asteroid impacts, massive volcanic eruptions, or a combination of both.
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