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The ongoing lower level of species extinction intensity occurring between episodes of mass extinctions. Extinction is a normal part of the history of life. Out of every 100 species that have ever lived on Earth, 99 are no longer in existence. However, occurrences of species extinction can be split into two categories: mass extinctions, in which substantial percentages of the global diversity of life disappear within a geologically brief interval, and background extinctions, which represent a much lower rate of species loss that occurs routinely over geologically long intervals between mass extinctions. Background extinctions are ongoing consequences of normal environmental changes, local catastrophes, or interspecies competition. On the basis of the fossil record, evolutionary paleontologists have generally estimated that the baseline level of background extinction is approximately 1 extinction per million species per year (recent analyses, however, indicate that the rate might be as low as a tenth of that figure, so further investigations are necessary). Background extinction is mostly a local phenomenon: It befalls only one or a few species at any time, usually within a particular area. In contrast, hundreds or thousands of species can be affected worldwide during mass extinctions. Five mass extinctions have occurred during the past 550 million years on Earth, and investigators believe that the Earth is currently amid a sixth mass extinction, termed the Anthropocene extinction, caused predominantly by environmentally destructive human activities. See also: Anthropocene extinction; Biodiversity; Extinction (biology); Extinction and the fossil record; Extinction of species; Fossil; Geologic time scale; Macroevolution; Mass extinctions; Speciation; Species concept
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