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 seven major
extinction events.
500 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.
440 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.
365 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.
252 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.)
195 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 dinosaurs.
20 thousand
years ago through today, humans are causing another
extinction event. Hunting and overfishing have already caused
extinctions and population collapses of many large land animals and
fish species. Industrial development is causing habit destruction and
climate changes which are bringing about the extinction of many animals
and plants throughout the world.
How often do
Extinction Level Events occur?
It has been suggested 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 more than 10 million years. There is abundant
evidence that we are currently living in the middle of a man-made
Holocene extinction event.
A recent theory, which has been largely discredited, 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.
An even more recent theory, which is still being evaluated, is that
periodic large scale vulcanism along
continental rifts may include
eruption events named verneshots which launch gigatonnes of rock into
sub-orbital trajectories. The consequent impacts are expected to have
very similar effects to asteroid impacts.
This theory explains the
periodicity of extinction events as well as the apparently coincidental
occurrence of large-scale impacts and vulcanism for at least three of
the extinction events without relying on coincidence in the way that
the asteroid impact theory does.
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.
