Geoscience Reference
In-Depth Information
destroyed Hiroshima; according to Figure 24, it would result in the
death of about 20 percent of species. A loss of 20 percent of spe-
cies every 7 million years is equivalent to a 100 percent turnover in
35 million years, which is only about 6 percent of the time that has
elapsed since the Cambrian began. Thus it is more than enough to
account for the record of extinction observed in the rocks.
To answer Raup's question as he answered it: Yes, in theory, im-
pact could have caused all extinctions. To turn the question around,
since it is inescapable that the earth has been bombarded by mete-
orites of a range of sizes since life began over 3.5 billion years ago,
and since even a modest-sized impact releases huge amounts of
energy, how are we to escape the conclusion that not just in theory,
but in practice, impact has caused many extinctions?
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There may be an additional way to shed light on the role of impact:
by focusing not on the mass extinctions themselves, but on the nor-
mal intervals of background extinction in between. If impact drives
mass extinctions, then in the times between impact events, few
extinctions would be expected. Can we tell whether the tempo of
extinction and evolution in between the large extinctions is consis-
tent with a history of impact? After Darwin, evolutionists came to
have the view that natural selection operates steadily, all the time.
Organisms continually undergo small changes that, when summed
together, produce large effects. As long as environments are stable,
natural selection operates to adapt organisms ever more perfectly.
When environments change, natural selection allows them to adapt
just enough to keep pace. This model conforms exactly to the uni-
formitarian view of earth behavior: Change is gradual, but cumula-
tive, and in time can be prodigious.
If this gradualistic view is correct, the fossil record should reflect
it. As natural selection works its way, plants and animals should
evolve steadily, little by little, leaving much of the work of evolution
to be done in between mass extinctions. But, as Darwin's contem-
poraries knew, this is not really what the fossil record reveals.
Instead, most evolutionary change occurs in a burst right after a new
species diverges from its ancestor. After the initial spurt, species
change little, sometimes remaining static for millions of years. For
example, the lampshell brachiopod genus
Lingula
found today ap-
pears just like its 450-million-year-old fossil ancestor. Niles Eldredge
