For any one species, extinction may seem catastrophic. But over the grand sweep of life on Earth, extinction is business as usual. Extinctions occur continually, generating a "turnover" of the species living on Earth. This normal process is called background extinction. Sometimes, however, extinction rates rise suddenly for a relatively short time — an event known as a mass extinction. Mass extinctions kill off many species, but the empty niches left behind may allow other lineages to radiate into new roles, shaping the diversification of life on Earth.
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