An extinction event (also extinction-level event, ELE) is a period in time when a large number of species die 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 in the Phanerozoic Eon as currently recognized. Extinction events without doubt also occurred during the Proterozoic and Archaean Eons as well. These earlier events are, however, less well documented.
- 488 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.
- 444 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.
- 360 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. See Late Devonian extinction.
- 251 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.)
- 200 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 non-avian dinosaurs.
- Current event. Most biologists believe that humans are currently causing another extinction event, the Holocene extinction event. E.O. Wilson of Harvard University, in his book The Future of Life estimates that at current rates of human destruction of the biosphere one-half of all species of life will be extinct in 100 years. A survey by the American Museum of Natural History in 1998 found that the vast majority of biologists agreed with Wilson's assessment, and numerous confirmatory studies in the years since then-- led by the IUCN's annual "Red List" of threatened species-- have now produced a scientific consensus on the subject. (http://www.well.com/user/davidu/extinction.html)
Extinction event refers to extinction of species, not all life. Although many life forms may become extinct, the usual connotation is that the "event" is at most a transition in dominant life forms. For example, the Cretaceous-Tertiary extinction event promoted the domination of spores and swamp life for a period almost directly after the event. A complete extinction of all known life forms may be possible, but no such event has ever been discovered.
Postulated extinction cycles
It has been suggested by Raup and Sepkoski 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 hypothetical cycle suggest that another mass extinction would be due in little less than 10 million years. A recent theory, for which no real evidence exists, suggested that the extinction cycle is caused by the orbit of a hypothetical companion star dubbed Nemesis that 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.
The collision of a large asteroid, or other impact event, with the earth is one of several hypothetical scenarios put forward in recent years that scientists believe may cause or trigger an extinction event (another is global nuclear warfare). The 1998 motion picture Deep Impact referenced the term ELE extensively in relation to a predicted comet collision with earth.
- Leakey, Richard and Roger Lewin, 1996, The Sixth Extinction : Patterns of Life and the Future of Humankind, Anchor, ISBN 0385468091
- Wilson, E.O., 2002, The Future of Life, Vintage (pb), ISBN 0679768114
- The Current Mass Extinction Event (http://www.well.com/user/davidu/extinction.html)
- Nemesis - Raup and Sepkoski (http://muller.lbl.gov/pages/lbl-nem.htm)
- Richard Muller, 1988, Nemesis, Weidenfeld & Nicolson, ISBN 1555841732
- BBC Extinction Files: Mass Extinctions: http://www.bbc.co.uk/education/darwin/exfiles/massintro.htm
- Calculate the effects of an Impact: http://www.lpl.arizona.edu/impacteffects/