They stand in contrast to the background rate of extinction, which occurs even when the diversity of life is increasing.
Scientists recognize five major mass extinctions in the Earth's history.
The extinctions are measured in terms of large groups of related species, called families. The five mass extinction episodes occurred because of major changes in the prevailing ecological conditions brought about by climate change, cataclysmic volcanic eruptions, or collisions with giant meteors. The sixth mass extinction appears to be in progress now, and the main cause is environmental change brought about by human activities. No terrestrial vertebrates have evolved yet. Mammals and fishes begin to proliferate.