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event, leaving a minimum breeding population of no more than 0.0' 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 number 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.

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