Geoscience Reference
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The database can also be used to help answer a question over
which paleontologists have puzzled: Are the giant extinctions fun-
damentally different from the background extinctions, or do they
merely represent the extreme end of a continuum? Figure 16 helps
us decide. This chart was constructed by dividing the 600 million
years since life began to flourish, at the beginning of the Cambrian
period, into intervals of 1-million-year duration, and computing for
each the number of species still alive at the end as a percentage of
those alive at the beginning. The mean is a 25 percent extinction
rate per million years (on the average, of 100 species alive at the
beginning of a 1-million-year period, 75 were still alive at the end).
The Big Five lie off on the right tail, but there is no break between
them and the lesser extinctions—the distribution appears continu-
ous. Background and mass extinctions therefore do not seem to be
qualitatively different, but rather to grade imperceptibly into each
other. If all extinctions had a common cause, but one that operated
at different intensities at different times, this is the pattern we
would expect. We cannot say, however, that some combination of
extinctions with different causes might not give the same result.
The great extinctions reached diverse organisms in almost every
ecological niche. The K-T extinction wiped out animals as unlike as
microscopic foraminifera, intricately coiled ammonites, land plants,
and dinosaurs—from the tiniest creatures of the sea to the largest
denizens of the mountain slopes. Obviously, such different organ-
isms, in such completely different environmental settings, did not
compete in Darwin's sense. Most species that died appear to have
been as successful as those that survived. Before their fall, there
would have been no reason to predict that they would be the ones
to go, yet go they did. The converse is also true. In most cases it is
impossible to say why the species that survived did so; certainly it
was not because they were more "fit." Thus, Raup concludes, evolu-
tion and survival may be more matters of chance than fitness, of
good luck than good genes. In his view and that of the Alvarezes, the
dinosaurs, and the others that joined them in disappearing at the
end of the Cretaceous, were more than anything unlucky enough to
be in the wrong place at the wrong time.
T HE F OSSIL R ECORD
When the Alvarez theory broke upon the world, most paleontolo-
gists were quite confident that it could immediately be judged by
the weight of more than a century's worth of fossil evidence and
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