An extinction event (also extinction-level event, ELE) occurs when a large number of species die out in a relatively short period of time. Based on the fossil record, the background rate of extinctions on Earth is about two to five taxonomic families of marine invertebrates and vertebrates every million years.

Since life began on Earth, a number of major mass extinctions have greatly exceeded the background extinction rate present at other times. Though there were undoubtedly mass extinctions in the Archean and Proterozoic, it is only during the Phanerozoic Eon that the emergence of bones and shells in the evolutionary tree has provided a sufficient fossil record from which to make a systematic study of extinction patterns. The number of major mass extinctions attributed to this most recent 540 million years varies from source to source, with some authorities arguing for as few as 5 or more than 20. These differences stem primarily from the threshold chosen for describing an extinction event as "major", and what set of data one chooses to believe is the best measure of past diversity.

Extinction events

The classical "Big Five" mass extinctions identified by Raup and Sepkoski (1982) are widely agreed upon as some of the most significant: End Ordovician, Late Devonian, End Permian, End Triassic, and End Cretaceous.

These and a selection of other extinction events are highlighted below:

1. 488 million years ago — a series of mass extinctions at the Cambrian-Ordovician transition (the Cambrian-Ordovician extinction events) eliminated many brachiopods and conodonts and severely reduced the number of trilobite species.
2. 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, and then another occurred between 500 thousand to a million years later when sea levels rose rapidly. It has been suggested that a gamma ray burst may have triggered this extinction.
3. 360 million years ago — near the Devonian-Carboniferous transition (the Late Devonian extinction) a prolonged series of extinctions led to the elimination of about 70% of all species. This was not a sudden event, with the period of decline lasting perhaps as long as 20 million years. However, there is evidence for a series of extinction pulses within this period.
4. 251 million years ago — at the Permian-Triassic transition (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.)
5. 200 million years ago — at the Triassic-Jurassic transition (the Triassic-Jurassic extinction event) about 20% of all marine families as well as most non-dinosaurian archosaurs, most therapsids, and the last of the large amphibians were eliminated.
6. 65 million years ago — at the Cretaceous-Paleogene transition (the Cretaceous-Tertiary extinction event) about 50% of all species became extinct (including all non-avian dinosaurs). This extinction is widely believed to have resulted from an asteroid or comet impact event.
7. Present day — the Holocene extinction event. A 1998 survey by the American Museum of Natural History found that 70% of biologists view the present era as part of a mass extinction event ,the fastest to have ever occurred. Some, such as E. O. Wilson of Harvard University, predict that man's destruction of the biosphere could cause the extinction of one-half of all species in the next 100 years. Research and conservation efforts, such as the IUCN's annual "Red List" of threatened species, all point to an ongoing period of enhanced extinction, though some offer much lower rates and hence longer time scales before the onset of catastrophic damage. The extinction of many megafauna near the end of the most recent ice age is also sometimes considered a part of the Holocene extinction event.

Causes for mass extinction

With the exception of the Cretaceous-Tertiary mass extinction, which is widely attributed to an impact event, and modern day extinctions associated with the proliferation of human civilization, it is not well known what has caused other mass extinctions. Some of the hypotheses are discussed below.

1. Impact events - The impact of a sufficiently large asteroid or comet could create Megatsunamis, global forest fires, and simulate nuclear winter from the dust it puts in the atmosphere. Taken together, it is not surprising that these and other related effects might be sufficiently severe as to disrupt the global ecosystem and cause extinctions. Only for the End Cretaceous extinctions is there strong evidence of such an impact. Circumstantial evidence of such events is also given for the End Permian, End Ordovician, End Jurassic and End Eocene extinctions.
2. Climate change - Rapid transitions in climate may be capable of stressing the environment to the point of extinction. However, it is worth observing the recent cycles of ice ages are only believed to have had very mild impacts on biodiversity. Extinctions suggested to have this cause include: End Ordovician, End Permian, Late Devonian, and others.
3. Volcanism - The formation of large igneous provinces, which can involve the outflow of millions of cubic kilometers of lava in a short duration, are suggested to poison the atmosphere and oceans in a way that may cause extinctions. This cause has been proposed for the End Cretaceous, End Permian, End Triassic, and End Jurassic extinctions.
4. Gamma ray burst - A nearby gamma ray burst (less than 6000 light years distance) could sufficiently irradiate the surface of the Earth to kill organisms living there and destroy the ozone layer in the process. From statistical arguments, approximately 1 gamma ray burst would be expected to occur in close proximity to Earth in the last 540 million years. This has been suggested as an explanation for the End Ordovician extinction event.
5. Plate tectonics - It has been suggested that the opening and closing of seaways and land bridges may play a role in extinction events as previously isolated populations are brought into contact and new dynamics are established in the ecosystem. This is most frequently discussed in relation to the End Permian mass extinction.

Other hypotheses, such as the spread of a new disease or simple out-competition following an especially successful biological innovation are also considered; however, it is often thought that the major mass extinctions in Earth's history are too sudden and too extensive to have resulted solely from biological events.

Postulated extinction cycles

It has been suggested by several sources that biodiversity and/or extinction events may be influenced by cyclic processes. The best-known of these claims is the 26 to 30 million year viral cycle in extinctions proposed by Raup and Sepkoski (1986). More recently, Rohde and Muller (2005) have suggested that biodiversity fluctuates primarily on 62 ± 3 million cycles.

It is difficult to evaluate the validity of these claims except through reduction to statistical arguments regarding how plausible or implausible it is for the observed data to exhibit a particular pattern, as the causes of most extinction events are still too uncertain to attribute to them any specific cause let alone a recurring one. Much early work in this area also suffered from poor knowledge of the geological time scale (errors > 10 million years at times), though the time scale now available (uncertainties all < 4 million years) should be adequate for studying these processes.

While the statistics alone have been judged as sufficiently compelling to warrant publication, it is important to consider processes that might be responsible for a cyclic pattern of extinctions and future work may focus on trying to find evidence of such processes.

One theory, for which no real evidence exists, suggests that the extinction cycle could be 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. Another similar theory suggests that the Solar System's oscillations through the plane of the galaxy results in periods of comet showers. Other theories suggest geological instabilities that might allow heat to periodically build up deep in the Earth, which is then released through mantle plumes, periods of major volcanism and active plate tectonics.

If any of these theories are correct, then it is worth noting that both Raup and Sepkoski and Rohde & Muller predict another naturally caused mass extinction event within the next 10 million years.