The First Mass Extinction Was The Ordovician-Silurian Extinction
The Ordovician-Silurian Extinction occurred about 439 million years ago and caused 86% of life on earth to die off. This mass extinction was caused by precisely the opposite of what we are experiencing today: global cooling, as opposed to global warming.
Scientists believe that about 500 million years ago, there were too many plants on earth pulling CO2 out of the atmosphere. As a result, the temperature of the planet cooled off and caused glaciers to grow. Sea levels dropped off, and entire ecosystems were destroyed.