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80 million years. 4 He found that until 65 million years ago, mammal
diversity was low, and that in the aftermath of the Chicxulub impact
it fell even lower. The number of species then rose sharply to reach
a plateau about 50 million years ago, where it has remained since.
Almost no correlation exists, however, between climate and the
appearance and extinction of mammals. Instead, the appearance of
new species was largely controlled by the number already present:
When mammals were few to start with, more new species appeared.
The driver of mammalian diversity thus seems to be not climate but
the number of vacant ecological niches. Alroy's study does not rule
out the possibility that some mass extinctions have been caused by
rare extremes of climate. But taking the view that it is what a species
does know that cannot hurt it, Alroy noted that cyclical changes in
the position and shape of the earth's orbit relative to the sun pro-
duce changes in climate every 20,000, 40,000, and 100,000 years.
(Most geologists believe these are the causes of the repeating ice
ages.) Species, which live on the average for a few million years, have
of necessity survived scores of changes in climate and sea level.
In summary, it seems fair to say that, nearly two centuries after
Hutton, there is precious little positive evidence that changes in cli-
mate and sea level cause mass extinctions. It is up to the proponents
of the claim that they produce such evidence.
C HANCE IN E ARTH H ISTORY
AND IN S CIENTIFIC D ISCOVERY
Not only have the advances since the Alvarez theory appeared
brought about a transformation of geology, they have greatly illumi-
nated the role of chance in our solar system. We see that not only the
death of the dinosaurs, but our presence on the earth, is contingent
on the particular way in which the solar system originated and
evolved. The K-T impact was set in motion nearly 4.5 billion years
ago with the birth of the solar system. From that primordial chaos
arose a comet or an asteroid that through the subsequent eons was
intermittently pounded by impact and continually nudged by gravity.
Had one collision been just a bit more or a bit less energetic, had
gravity tugged a little more here or a little less there, the impactor
would have had a different size and a different orbit. The dinosaur
killer would have struck at some other time in the earth's history, or
missed our planet entirely, and dinosaurs would not have become
extinct when they did. Who knows, perhaps their 160-million-year
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