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That impression began to change in 1983, when Sepkoski and a colleague, Paleontologist David Raup of the University of Chicago, presented a paper in Flagstaff, Ariz., at a symposium on mass extinctions. The pair had examined data on marine organisms that had become extinct over the past 250 million years, a list of about 3,500 families that included an estimated quarter of a million different species.
Through intricate statistical gymnastics, the two scientists found that most of the dyings fell into as many as a dozen events of varying severity, some of which had previously gone unrecognized as being mass extinctions at all. Then the kicker: the catastrophes were separated by periods of 26 million years. Though many scientists were skeptical, the Chicagoans did receive crucial support from one expert. Erle Kauffman, a paleobiologist at the University of Colorado in Boulder, announced that he had used advanced radiometric techniques to date fossils from a handful of midsize extinctions in the Cretaceous period and found them to be, yes, 26 million years apart.
The implications were compelling. No earthly phenomena, no Vesuvian < eruptions, no swelling seas, no ice ages could explain the regularity. Eyes turned heavenward. As Raup and Sepkoski declared in their paper on the subject, "We favor extraterrestrial causes." It did not take long for skygazers, ever an imaginative group, to begin coming up with candidates. Because researchers could not invent a convincing celestial event that would periodically disturb the nearby belt of asteroids that circle the sun between the orbits of Mars and Jupiter, attention turned to swifter and icier astronomical travelers, the comets.
Among the first cyclic mechanisms considered was one already familiar to astronomers: the sun's undulating route around the crowded Milky Way galaxy, an island of some 100 billion stars. The Milky Way is shaped something like a sunny-side-up egg, 100,000 light-years in diameter, with a bulge (the yolk) in the middle and three flat, dusty arms (the egg white) forming a circle around it. Like all its fellow stars, the sun revolves around the galactic center, taking about 250 million years to complete a round trip. As it moves, it bobs up and down through the central plane of the galaxy, where most of the stars and dust clouds are concentrated. Says Michael Rampino, a geologist with the Goddard Institute for Space Studies in New York City: "It's like a horse on a merry-go-round." Significantly, the sun and its accompanying planets hit the dustiest sections once every 33 million years or so, a figure that does not quite jibe with the apparent extinction cycle of 26 million years but is at least within the margins of astronomical and geological error.