Extinction event
An extinction event 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.
Encyclopedia
An
extinction event 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. These extinction events generally consist of participation by most major taxonomic classes and thus yield
extinct birds, mammals,
fishes, invertebrates and other lower life forms.
Extinction events
The classical "Big Five" mass extinctions identified by Raup and Sepkoski 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:
- 488 million years ago — a series of mass extinctions at the Cambrian-Ordovician transition eliminated many brachiopods and conodont
...
s and severely reduced the number of
trilobite species.
- 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.
- 360 million years ago — near the Devonian-Carboniferous transition 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.
- 251 million years ago — at the Permian-Triassic transition about 96% 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 Researchers from Ohio State University and NASA have presented findings suggesting that a meteorite, some 50km in diameter, collided with Earth approximately 250 million years ago, south of Australia in what is now Wilkes Land
Wilkes Land is a large district of land in eastern Antarctica [i], formally claimed by Australia [i] ...
, eastern
Antarctica, called
Wilkes Land crater. The findings were presented as part of a poster session at the American Geophysical Union's 2006 Joint Assembly meeting held over May 23-26 at the Baltimore Convention Center. The crater, some 480 km wide was found by observing radar images that showed a circular ridge 1.6 kilometres below the ice sheet. In addition differences in density, detected through gravity measurements taken with NASA's GRACE satellites, revealed a mascon , an upwelling of material from the earth's mantle. The mascon was centred within the crater walls. The impact is postulated to have been the cause of the
P-T extinction event and possibly to have initiated the break-up of the
Gondwana supercontinent, creating the tectonic rift that began Australia's migration northward, away from Antarctica.
- 200 million years ago — at the Triassic-Jurassic transition about 20% of all marine families as well as most non-dinosaurian archosaurs, most therapsids, and the last of the large amphibians were eliminated.
- 65 million years ago — at the Cretaceous-Paleogene transition about 50% of all species became extinct . This extinction is widely believed to have resulted from an asteroid or comet impact event.
- 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.
- 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.
- 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.
- 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.
- Gamma ray burst - A nearby gamma ray burst 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. However, a recent study by leading GRB researchers say that GRBs are not possible in metal rich galaxies like our own.
- 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 . More recently, Rohde and Muller have suggested that biodiversity fluctuates primarily on 62 ± 3 million year 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 , though the time scale now available 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.
There is however no one single theory that can account for all of the specific extinctions. Although one theory may explain the mass extinctions on land it may not account for all of the extinctions in marine conditions. The only theory that accounts for most of the extictions is the gamma ray theory. This would explain the selective extinctions and also would explain mass speciation which follows mass extinction. The radiation causes manipulations in DNA and RNA which lead to the sudden development of new species and also the sudden disappearance of previous species.
Controversy
In 2005, Andrew Smith and Alistair McGowan of the
Natural History Museum suggested that the apparent variations in marine biodiversity may actually be caused by changes in the quantity of rock available for sampling from different time periods. The diversity of the marine life appears to be proportional to the amount of rock available for study. Based on statistical studies, roughly 50% of the apparent diversity modification can be attributed to this effect.
ELE in movies
See also
References
- Richard Leakey and Roger Lewin, 1996, The Sixth Extinction : Patterns of Life and the Future of Humankind, Anchor, ISBN 0-385-46809-1. Excerpt from this book:
- Wilson, E.O., 2002, The Future of Life, Vintage , ISBN 0-679-76811-4**
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- Richard A. Muller, 1988, Nemesis, Weidenfeld & Nicolson, ISBN 1-55584-173-2
External links
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- , part of The Extinction Website.
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- - LiveScience.com
