Extinction is the loss of a species, family, or order of living things from the Earth. Species become extinct if they lose their natural habitat, are relentlessly preyed upon by a competitor, or unable to adapt to a changing environment. In the course of evolution, one species becomes extinct while another replaces it. Crises in geological history are factors in extinction. During the Earth's history, four or five episodes of mass extinction interrupted long periods of diversity. Scientists use the term punctuated equilibrium to describe extended times of balance interrupted by relatively rapid changes.

An episode mass extinction occurred 250 million years ago at the end of the Permian period. At that time, the continents were merging to form Pangea, and tremendous volcanic eruptions in Siberia were spewing lava and ash into the atmosphere. Fossil records reveal the extinction of more than 90% of the species of marine animals alive...