What does it take to change the world? Change it, that is, on a geologic scale that can withstand the rest of deep time? Meteoric impacts have succeeded: one blasted the Moon out of the Earth. Others have caused mass extinctions. Volcanic eruptions have certainly left their mark, too, burying areas the size of California under lava flows a mile deep. In contrast, the creatures that crowd the surface of our living planet rarely have a detectable impact on the global environment. Even the dinosaurs that ruled the Earth for a hundred million years left a legacy of little more than mineralized bones and footprints in rock.

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