Mass extinction

A mass extinction is a geologically brief episode during which extinction rates rise far above the normal background rate and a large fraction of species disappears worldwide. The fossil record shows five such events in roughly the last half-billion years, often called the Big Five. They fall near the ends of the Ordovician, Devonian, Permian, Triassic and Cretaceous periods, and each is estimated to have removed around three-quarters or more of the species alive at the time.

The largest was the end-Permian extinction, about 252 million years ago, sometimes called the Great Dying, which killed most marine species and many on land. Huge volcanic eruptions, and the warming and loss of oxygen in the oceans that followed, are the leading explanation. The best known is the end-Cretaceous event, about 66 million years ago, when an asteroid struck near Mexico's Yucatan Peninsula and the non-avian dinosaurs died out.

Mass extinctions reshape life. By emptying ecological niches, they let surviving lineages spread into new roles, as mammals did after the dinosaurs vanished. Many scientists warn that present-day species losses caused by humans could become a sixth mass extinction.