Throughout its history, Earth has experienced five mass extinction events. See, for example, ‘Timeline Of Mass Extinction Events On Earth’. It is now experiencing the sixth.
1. The Ordovician-Silurian Extinction, which occurred about 439 million years ago, wiped out 86% of life on Earth at the time. Most scientists believe that this mass extinction was precipitated by glaciation and falling sea levels (possibly a result of the Appalachian mountain range forming), catastrophically impacting animal life which lived largely in the ocean at the time.
2. The Late Devonian Extinction happened about 364 million years ago and destroyed 75% of species on Earth. Possibly spread over hundreds of thousands of years, a sequence of events that depleted the oceans of oxygen and volcanic ash that cooled the Earth’s surface are believed to have driven the extinctions. It was to be 10 million years before vertebrates again appeared on land. ‘If the late Devonian extinction had not occurred, humans might not exist today.’