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Maynard Smith, John
producing a 200,000 square mile (over 500,000 square km)
lava flow called the Deccan Traps. Several interacting causes,
the relative importances of which are still debated, brought
about the Cretaceous extinction.
The Cretaceous extinction was not the biggest extinction
event in Earth history. The mass extinction that
brought the Permian period, and the whole Paleozoic era, to
an end 250 million years ago was, in the words of paleontologist
Douglas Erwin, the “mother of all extinctions.” At
this time, at least half of all families of organisms became
extinct. Because some families contain many species, it has
been estimated that up to 95 percent of all species became
extinct at that time! Because this event was further back
in time, it is more difficult to study: There has been more
time for evidence to be lost, and it is more difficult to calculate
precise dates for the events that occurred at that time.
Despite this, evidence has been found of an asteroid impact
at the end of the Permian period. Nevertheless, it is difficult
for scientists to determine to what extent these events associated
with the Permian extinction occurred simultaneously.
Perhaps the biggest unsolved mystery regarding mass
extinction events is what appears to be their recurring pattern.
Paleontologists David Raup and J. J. Sepkoski have calculated
a 26-million-year cycle of recurring mass extinctions.
The pattern is not perfect: Mass extinctions have not occurred
every 26 million years, nor have they occurred at precisely this
interval. However, their results are statistically significant. Scientists
have been unable to explain normal geological events
that might cause such a recurring synchronous pattern. Earth
scientists Marc Davis, Piet Hut, and Robert Muller have suggested
that there is a small companion star to the Sun, which
sweeps a mass of comets and asteroids along with it. The orbit
of this star, they speculate, brings it close enough to the earth
every 26 million years to bombard the Earth with comets and
asteroids. Even those that suggest the existence of this “Nemesis
star” admit that it has not been seen or otherwise detected.
Solar flares, which would flood the Earth with intense
radiation, have been suggested as a possible cause of mass
extinctions. Scientists know little about the timing of these
solar flares through the history of the solar system, and these
flares would leave no evidence of having struck the Earth,
other than the mass extinction itself. Because they cannot
think of a way to investigate this possibility, most scientists
dismiss solar flares as a cause of extinction events.
Following each mass extinction event, the diversity of
species has not only recuperated but increased. The best fossil
record that is available is that of marine invertebrates (see
fossils and fossilization). This record indicates a steady
increase in worldwide species diversity, especially after mass
extinction events (see biodiversity). One example of evolution
following a mass extinction is the adaptive radiation
of mammal species after the extinction of the dinosaurs (see
adaptive radiation). Many scientists fear that species diversity
may not recover from the mass extinction that is now
occurring as a result of human activity, because unlike past
mass extinctions, the cause of extinction (human activity) is
continuing unabated.
Some extinction events, although not considered mass
extinctions, have still had an important effect on the evolutionary
history of life. The Hemphillian extinction event
about five million years ago produced relatively few extinctions,
but among them were many grazing animals. A diversity
of horse species was reduced to just one.
Terrestrial plants may not respond to mass extinction
events as rapidly or as markedly as marine and terrestrial animals.
Most plant extinctions have been caused by long-term
climatic changes. While many plants became extinct during
the five mass extinction events, their response was delayed
until several million years after each of the extinction events.
Life has recuperated from mass extinctions largely
because asteroid collisions and other planetary disasters have
been relatively rare for the past one or two billion years.
According to earth scientists Peter Ward and David Brownlee,
humans should, so to speak, thank their lucky star for
this: The Sun is a relatively stable star, compared to most in
the universe, and the planet Jupiter (which can be considered
a star that never ignited) has swept part of the solar system
free of most asteroids except those in the asteroid belt. Craters
on the moon appear to be mostly three billion to four
billion years old, indicating that during this time period comets
and asteroids were very common in the solar system. Even
though bacterial life appeared on Earth soon after its formation,
frequent bombardment of the Earth during that period
may have delayed the appearance of complex cells until about
a billion and a half years ago (see origin of life). The evolution
of complex life-forms would have been impossible on a
planet subjected to frequent mass extinctions.
Further Reading
Courtillot, Vincent. Evolutionary Catastrophes: The Science of Mass
Extinction. Trans. Joe McClinton. New York: Cambridge University
Press, 2002.
Davis, M., P. Hut, and R. Muller. “Extinction of species by periodic
comet showers.” Nature 308 (1984): 715–717.
Hallam, Anthony. Catastrophes and Lesser Calamities: The Causes
of Mass Extinctions. New York: Oxford University Press, 2004.
———, and P. B. Wignall. Mass Extinctions and Their Aftermath.
New York: Oxford University Press, 1997.
Raup, David M. Extinction: Bad Genes or Bad Luck? New York:
Norton, 1991.
Villier, Loïc, and Dieter Korn. “Morphological disparity of ammonoids
and the mark of Permian mass extinctions.” Science 306
(2004): 264–266.
Ward, Peter, and David Brownlee. Rare Earth. New York: Copernicus
Books, 2000.
———, and Alexis Rockman. Future Evolution. New York: Freeman,
2001.
Maynard Smith, John (1920–2004) British Evolutionary
biologist John Maynard Smith was one of the leading
evolutionary theorists of the 20th century. He contributed
greatly to an understanding of how the process of evolution
works. Maynard Smith’s contributions to evolutionary science
included: