Animal life has been all but destroyed in sudden, planetwide exterminations five times in Earth’s history. These are the so-called Big Five mass extinctions, commonly defined as any event in which more than half of the earth’s species go extinct in fewer than a million years or so. We now know that many of these mass extinctions seem to have happened much more quickly. Thanks to fine-scale geochronology, we know that some of the most extreme die-offs in earth history lasted only a few thousand years, at the very most, and may have been much quicker.
