Origin Story: A Big History of Everything

by David Christian

The greatest of all mass extinctions came at the end of the Permian period, 248 million years ago. This time, more than 80 percent of all genera vanished, including the last of the trilobites. The precise causes of this mass extinction remain uncertain. It might have been due to rising magmas that broke through the crust in massive volcanic eruptions that sent enough ash into the air to block photosynthesis. We find modern evidence of this in a large volcanic region of Siberia known as the Siberian Traps. The eruptions pumped huge amounts of carbon dioxide into the atmosphere, so when the dust settled, carbon dioxide levels spiked, oxygen levels fell, and the oceans warmed. When Earth burped, the biosphere shuddered. By some estimates, oceans may have been as warm as thirty-eight degrees Celsius, hot enough to kill most marine organisms and stop nearly all photosynthesis in the seas. Warmer oceans could hold less oxygen and support less life, and deep beneath their surface, thawing balls of frozen methane known as clathrates may have released huge bubbles of methane. This was a greenhouse mass extinction; it killed by heating rather than freezing.10 In an extreme greenhouse world, large organisms survived only in the cooler polar environments in the far north and south of the vast supercontinent of Pangaea.