An extinction event is a period in time when a large number of species have died out. There have been six major extinction events since the
440 million years ago, at the Ordovician-Silurian transition, a double extinction occured, probably as the result of a period of glaciation. As the seas retreated the marine habitats changed drastically, causing the first die-off, then another occured between 500 thousand and a million years later when sea levels rose rapidly.
About 365 million years ago, in the transition from the Devonian 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, 95% of all species died. This catastrophe was Earth's worst mass extinction, killing 53% of marine families, 84% of marine genera and an estimated 70% of land species such as 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.
It has been suggested that there is a cycle of extinctions, with a mass extinction occurring every 26 to 30 million years. It is difficult to date fossils accurately enough to produce a reliable result, but most studies of this hypothetic cycle suggest that another mass extinction would be due in little more than 10 million years. Some recent theories suggested that the extinction cycle may be caused by the orbit of a "death star" which disturbs the Oort cloud and sends storms of large astroids and comets towards the solar system every 26 million years.
Some people claim that we are living in the middle of a man-made Holocene extinction event right now.