The document discusses extinction and mass extinction events throughout Earth's history. It notes that extinction is a natural part of evolution, with 99.9% of all species that ever lived now extinct. Mass extinctions have occurred periodically, where the extinction rate far exceeds the normal background rate, such as the Permian-Triassic extinction over 250 million years ago where 90-95% of marine species went extinct. Some hypotheses for the causes of mass extinctions include large meteorite impacts, as well as volcanic activity and climate changes.