Geoscience Reference
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event, leaving a minimum breeding population of no more than
0.01 percent. Carlisle does not say how he arrived at this figure, but
his claim does provoke us into contemplating the enormity of the
task of extinguishing entire genera—an almost unimaginable num-
ber of individuals must die. Although an event that kills such a high
percentage of all living creatures is indeed nearly impossible for us
to imagine, one would not want to be around to learn whether an
explosion with the energy of 7 billion bombs the size of the one
dropped on Hiroshima would do the awful job. Carl Sagan and the
other modelers of nuclear winter feared that a set of explosions
totaling only a fraction of the energy released by the Chicxulub im-
pactor might cause the extinction of the entire human race.
Surely a dying massive enough to eliminate 70 percent of all
species would leave in the fossil record clear evidence of its destruc-
tiveness. Whether it has—whether the fossil evidence corroborates
or falsifies the Alvarez theory—hinges on two critical predictions.
The first searches for evidence below the boundary; the second
searches above it.
T
HE
E
FFECTS
OF
I
MPACT ON
L
IFE
PREDICTION
1: Prior to the K-T boundary, most species were not
already going extinct for some other reason. Their extinction was
sudden and right at the boundary.
We can summarize this prediction by saying that the K-T impact
will not have been "anticipated." That is, most species that did not
survive the boundary were not already going extinct for some other
reason; instead, their extinction was delayed until the impact and
was caused by it. Critics of the theory argue that just the opposite is
true. They say that the dinosaurs, for example, were already on the
way out, and being replaced by mammals, well before the end of the
Cretaceous. In their interpretation, impact may have happened, but
if so it only finished off a few doddering stragglers. Thus meteorite
impact has no appreciable effect on life and can be ignored as a fac-
tor in evolution. To test this first prediction, geologists need to find
and to trace fossils of key species from levels well down in the Cre-
taceous right up to the boundary.
Of course, as paleontologists have known for over a century,
some species did become extinct late in the Cretaceous, but before
the boundary, as happens in any geologic period. Conversely, some
made their first appearance then. The question is whether this was
