Friday, November 10, 2017
Our planet Earth has extinguished large portions of its inhabitants several times since the dawn of animals. And if science tells us anything, it will surely try to kill us all again. Working in the 19th century, paleontology pioneer Georges Cuvier saw dramatic turnovers of life in the fossil record and likened them to the French Revolution, then still fresh in his memory.
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.
Those asteroid (or comet) impacts have captured the public imagination ever since 1980, when Luis and Walter Alvarez found global traces of iridium, which they inferred to be extraterrestrial, at the geological boundary that marked the disappearance of the dinosaurs. The identification of the Chicxulub impact crater in Mexico soon after sealed the deal. Impacts have been proposed to explain other mass extinctions, but there's very little actual evidence to support those links. In the words of researchers David Bond and Stephen Grasby, who reviewed the evidence in 2016: "Despite much searching, there remains only one confirmed example of a bolide impact coinciding with an extinction event."
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