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4.2.4 Mass extinctions
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Siyavula
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1 What does this mean?
When all the members of a species die out the species has become extinct. A species is deduced to be extinct
if it stops appearing in apparently later (more recent) rock. There seem to have been times in the earth's
history when many species have become extinct at about the same time. Such an event is called a mass
extinction.
2 Five major mass extinctions
Evolutionists think that there were ve large mass extinctions, with the two largest being:
• The rst major mass extinction is said to have occurred at the end of the Permian period. This is
thought to have been about 225 million years ago. At that time 90% of marine life seems to have
become extinct.
• The second is thought to have occurred about 65 million years ago, at the end of the Cretaceous Period.
This led to the disappearance of the dinosaurs.
Each time a mass extinction seems to have happened, many new kinds of fossils appear in later (more recent)
rocks. These are considered new because their fossils were absent from older-dated rocks. Transitional
evolutionary ancestors of these new organisms are also not found in older rock. This sudden appearance of
new species is explained as follows. After a mass extinction a large part of the environment is cleared of the
life forms which had previously dominated (controlled) the earth. This makes way for new forms to evolve
and ll the many new vacant ecological niches (open spaces in the ecosystem).
2.1 Possible causes of mass extinctions
Mass extinctions can be caused by various factors. These may be extra-terrestrial, due to climate change,
caused by humans, genetic, or due to continental drift:
• Extra-terrestrial events are caused by something outside of the earth, e.g. by a meteorite. At least two
mass extinctions are thought to have been caused by huge meteorites striking the earth. A meteorite is
s chunk of rock from outer space, which enters the earth's atmosphere. These meteorites are thought
to have been about 10 km in diameter. Some scientists believe that such impacts happened many
millions of years ago. Other data suggests that a meteor of diameter 200km hit the earth only a few
thousand years ago. This does not t in with the geological time scale and is usually ignored. (This
is based on observations of the changing tilt of the earth's axis, made over a period covering most of
recorded history.
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• Climate change can cause extinction.
• Link to simulation - glaciers http://phet.colorado.edu/en/simulation/glaciers1
• Human activity can cause extinction. Humans are destroying the habitat of many types of plants
and animals. This is done by chopping down forests and draining swamps to grow crops and to build
houses.
• Continental drift might have caused extinction. According to the theory of continental drift, all land
used to be joined in a supercontinent called Pangaea. This later split into Laurasia in the north and
Gondwanaland in the south. There is much evidence for this. As the continents moved apart, some
populations were split and moved apart. This could have caused extinction as habitats and climates
changed as a result of the continental drift.
See http://www.exploratorium.edu/origins/antarctica/ideas/gondwana2.html2
What killed the dinosaurs: http://www.pbs.org/wgbh/evolution/extinction/dinosaurs/3
Biodiversity: the 6th great wave: http://news.bbc.co.uk/2/hi/science/nature/3667300.stm4
Vertebrate extinction crisis (videos): http://www.arkive.org/newsletter/?u=722a3ad7755a24108f2cd402e&id=db50e9fd5d
IUCN_Press_Release10_27_2010&utm_medium=email5
2.1.1 Activity: Mass extinctions
1. How do extinction events seem to help the process of evolution?
2. Name any four possible causes of mass extinctions.
3. Complete this table:
Geological time period when extinction is thought to have occurred.
Years before present when evolutionists think this happened
Main feature of extinction event
End of the Permian period.
Dinosaurs became extinct.
Table 1
Activity: Answers
1. How do extinction events seem to help the process of evolution? They possibly cleared the earth of
previously dominant species, allowing new species to evolve into the ecological niches this opened up.
2. Name any four possible causes of mass extinctions. Meteorites, volcanoes, climactic changes (e.g. ice age
/ global warming), man-caused (e.g. over-hunting), due to genetic deterioration
3. Complete this table:
Geological time period when extinction is thought to have occurred.
Years before present when evolutionists think this happened
Main feature of extinction event
continued on next page
http://phet.colorado.edu/en/simulation/glaciers
http://www.exploratorium.edu/origins/antarctica/ideas/gondwana2.html
http://www.pbs.org/wgbh/evolution/extinction/dinosaurs/
http://news.bbc.co.uk/2/hi/science/nature/3667300.stm
http://www.arkive.org/newsletter/?u=722a3ad7755a24108f2cd402e&id=db50e9fd5d&e=&utm_source=ARKive&utm_campaign=db50e9fd5d
IUCN_Press_Release10_27_2010&utm_medium=email
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End of the Permian period.
225 million years ago
90% of marine life became extinct
End of Cretaceous era
65 million years ago
Dinosaurs became extinct.
Table 2
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