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Extinction Event
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An extinction (level) event (also known as a mass extinction or biotic
crisis) is a widespread and rapid decrease in the amount of life on
Earth. Such an event is identified by a sharp change in the diversity
and abundance of macroscopic life. It occurs when the rate of
extinction increases with respect to the rate of speciation. Because the
majority of diversity and biomass on Earth is microbial, and thus
difficult to measure, recorded extinction events affect the easily
observed, biologically complex component of the biosphere rather
than the total diversity and abundance of life.
The age of the Earth is about 4.54 billion years old. The earliest
undisputed evidence of life on Earth dates at least from 3.5 billion
years ago, during the Eoarchean Era after a geological crust started to
solidify following the earlier molten Hadean Eon. There are microbial
mat fossils found in 3.48 billion-year-old sandstone discovered in
Western Australia. Other early physical evidence of a biogenic
substance is graphite in 3.7 billion-year-old metasedimentary rocks
discovered in Western Greenland. More than 99 percent of all species,
amounting to over five billion species, that ever lived on Earth are
estimated to be extinct. Estimates on the number of Earth's current
species range from 10 million to 14 million, of which about 1.2
million have been documented and over 90 percent await description.
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Extinction occurs at an uneven rate. 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. Marine fossils are mostly used to measure extinction
rates because of their superior fossil record and stratigraphic range
compared to land organisms.
Since life began on Earth, several major mass extinctions have
significantly exceeded the background extinction rate. The most
recent, the Cretaceous–Paleogene extinction event, which occurred
approximately 66 million years ago (Ma), was a large-scale mass
extinction of animal and plant species in a geologically short period of
time. In the past 540 million years there have been five major events
when over 50% of animal species died. Mass extinctions seem to be a
Phanerozoic phenomenon, with extinction rates low before large
complex organisms arose.
Estimates of the number of major mass extinctions in the last 540
million years range from as few as five to more than twenty. These
differences stem from the threshold chosen for describing an
extinction event as "major", and the data chosen to measure past
diversity.
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