An extinction event (also extinction-level event, ELE) is a period in time when a large number of species died out. The normal background rate of extinctions is about two to five families of marine invertebrates and vertebrates every million years. Since life began on Earth, this background extinction rate has been punctuated by six major extinction events.
- 500 million years ago a series of mass extinctions at the Cambrian-Ordovician boundary (the Cambrian-Ordovician extinction events) eliminated many brachiopods and conodonts and severely reduced the number of trilobite species.
- 440 million years ago at the Ordovician-Silurian transition two Ordovician-Silurian extinction events occurred, probably as the result of a period of glaciation. Marine habitats changed drastically as sea levels decreased, causing the first die-off, then another occurred between 500 thousand and a million years later when sea levels rose rapidly.
- 365 million years ago in the transition from the Devonian period to the Carboniferous period about 70% of all species were eliminated. This was not a sudden event; evidence suggests that the extinctions took place over a period of some three million years.
- 252 million years ago, in the Permian-Triassic extinction event, about 95% of all marine species went extinct. This catastrophe was Earth's worst mass extinction, killing 53% of marine families, 84% of marine genera, and an estimated 70% of land species (including plants, insects, and vertebrate animals.)
- 195 million years ago, the Triassic-Jurassic extinction event eliminated about 20% of all marine families as well as most non-dinosaurian archosaurs, most therapsids, and the last of the large amphibians.
- 65 million years ago, the Cretaceous-Tertiary extinction event killed about 50% of all species, including the dinosaurs.
A recent theory, which has been largely discredited, suggested that the extinction cycle is caused by the orbit of a companion star which periodically disturbs the Oort cloud, sending storms of large asteroids and comets towards the Solar System every 26 million years. Another, similar theory suggests that the Solar System's oscillations through the plane of the galaxy results in periods of comet showers.