Extinction can be defined as the disappearance and death of a type of organism or group of organisms, particularly at the species level. Mass extinctions of numerous species have occurred throughout the history of life on Earth, and they continue to happen in the present day. On an historical scale, life on Earth has been punctuated by a number of catastrophic mass extinctions, in which diverse taxonomic groups collapse from the relatively rapid and widespread extinction of large numbers of species. (These periods of mass extinctions have historically been followed by phases during which the surviving species recover and diversify.) There have been five major mass extinctions during the past 540 million years during which species disappeared at a far higher rate than the normal (background) level. Each mass extinction was the result of one or more environmental perturbations on a global scale and led to the extinction of a significant proportion of the world’s biota in a geologically short period. By some biologists’ estimations, a sixth major mass extinction, predominantly attributed to humans’ influence on the biosphere, may now be imminent or in progress. See also: Biodiversity; Extinction (biology); Extinction of species; Paleobiodiversity; Paleoecology

The five recognized major mass extinctions (sometimes called the “Big Five”) are the end-Ordovician, late Devonian, end-Permian (Permian–Triassic), end-Triassic, and end-Cretaceous (Cretaceous–Paleogene or K–Pg, formerly known as the Cretaceous–Tertiary or K–T) mass extinctions. For example, approximately 252 million years ago, the end-Permian mass extinction was the most devastating global-scale event ever recorded, resulting in the loss of more than 96% of marine animal species and 70% of land animal species. The end-Cretaceous mass extinction, which occurred more than 65 million years ago, is particularly well known because this event led to the disappearance of the dinosaurs and the ascendance of mammals. See also: Dinosauria; Geologic time scale; Mammalia; Pelagic ecosystem recovery after end-Permian mass extinction

These major mass extinctions and the subsequent recoveries from them had an enormous impact on the course of life’s evolution. To improve current models of evolutionary processes, biologists regard it as imperative to develop fuller comprehension of mass extinctions and recoveries, including their causes. For example, substantial evidence suggests that that the K–Pg extinction event was triggered by a massive meteorite impact. It remains to be seen whether similar events might have been involved with other mass extinctions, although other factors were probably more significant in most of them. See also: Asteroid extinction; Macroevolution; Meteor; Meteorite

Overall, a firm understanding of how the Earth’s biota reacts to environmental disturbances is necessary for the management of the present-day biodiversity crisis. The geologic and fossil records are natural laboratories that can provide insights for the continuation of life on Earth, and these insights must not be dismissed. Such analyses are relevant with regard to the drastic impact of human activities on global biodiversity, and they should help in future responses to any possible environmental perturbations. See also: Extinction and the fossil record; Invasive species during the Late Devonian biodiversity crisis; Post-Paleozoic ecological complexity