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Extinction is, quite literally, the end of a particular evolutionary line, the end of a species, a family, or a larger group of organisms. While it may be bad news for the victims, it's a 'natural' event in the history of life on earth. Extinctions, mostly at the level of species, have been occurring constantly at a low 'background rate', usually matched by the rate at which new species appear - with the result that biodiversity is constantly increasing. But there have been periods in the Earth's history when biodiversity crashes. Indeed this has been a powerful force in evolution, wiping the slate clean of up to 96% of all species, and providing the survivors with a world full of opportunities into which they can diversify. These are the mass extinctions, when more than 50% of the Earth's species vanish in the geological instant of a few million years. The cause for the Permian and cretaceous mass extinctions is still unknown by scientists, below is a list of possible causes. A graph showing past mass extinctions over millions of years Speculated Causes of the Permian Extinction Although the cause of the Permian mass extinction remains a debate, numerous theories have been formulated to explain the events of the extinction. One of the most current theories for the mass extinction of the Permian is an agent that has been also held responsible for the Ordovician and Devonian crises, glaciation on Gondwana. A similar glaciation event in the Permian would likely produce mass extinction in the same manner as previous, that is, by a global widespread cooling and/or worldwide lowering of sea level.
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