Throughout its 4.5 billion-year history, Earth has endured numerous mass extinctions, each of which has wiped out more than three-quarters of the planet's species and greatly reduced its biodiversity. These contractions in Earth's overall biodiversity can be triggered by both nonliving processes, such as volcanic eruptions or asteroid collisions, and living processes, such as organisms' altering of Earth's atmospheric chemistry.
Yet life often bounces back. In fact, in the long term, life on Earth has trended toward increasingly complex forms and ecological organization in spite of these somewhat regular setbacks.
