They're regularly spaced in time. David Raup
During the 1970s, when the Alvarezes were developing their theory, a young paleontologist named John Sepkoski was at work at the University of Rochester, compiling the ranges of geologic ages during which each family of fossil organisms lived. (Recall that biologists subdivide organisms into kingdom, phylum, class, order, family, genus, and species. We belong, in the same order, to the animals, chordates, mammals, primates, hominids, genus Homo, and species Homo sapiens). Sepkoski was not going to all this trouble in order to study mass extinction, but rather to learn more about how biologic diversity has changed over geologic time. Meteorite impact was the furthest thing from his mind.
Sepkoski scoured the world literature of paleontology, searching out even the most obscure journals in the most unfamiliar languages, slowly adding information to his database. The data he entered for each family were simple: name, geologic age of the oldest and youngest recorded occurrences of species belonging to the family, and the literature references. Sepkoski was fortunate to have had the encouragement of his senior colleague at Rochester, David Raup, who happened to be predisposed toward the statistical approaches to which a large database lends itself. By 1978, both scientists had moved to the University of Chicago, further strengthening a department of paleontological powerhouses. There, Sepkoski continued to upgrade and polish his compendium, until it contained 3,500 families and 30,000 genera.2 One day a senior colleague, the
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