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Galvanized by this radical proposal, researchers are hunting for an agent that could explain the apparent clockwork regularity of the celestial barrages. Some suggest that a companion star to the sun periodically comes close enough to nudge comets gravitationally out of their natural habitat--a cloud of comets that circles the sun far beyond the orbit of Pluto--sending them hurtling toward earth. Others assign that role to Planet X, while some insist that the slow, bobbing ride of the sun and its planets around the Milky Way galaxy is responsible. Whatever the details, declares Paleontologist J. John Sepkoski Jr. of the University of Chicago, the evidence for periodic mass extinctions "very strongly implicates an extraterrestrial mechanism."
If the theory is correct, the next catastrophe will not occur for at least 13 million years. But the effect of the new concept on science and scientists has been much more immediate. One measure is the ferocity of debate it has generated in the past few months among eminent researchers, at conferences and in the letters-to-the-editor columns of major newspapers and staid journals alike. Supporters of the Milky Way proposal dismiss the Nemesis notion as "unlikely" and "ad hoc," and death star advocates are scornful of the galactic concept. Many consider all the newfangled extraterrestrial scenarios to be half-baked takeoffs of H.G. Wells. Says an indignant Dewey McLean, a paleontologist at Virginia Polytechnic Institute: "It's science gone absolutely bonkers."
The controversy has engulfed not only astronomers, geologists, paleontologists and astrophysicists but even evolutionary biologists. If the cyclic theory is true, the biologists argue, many assumptions about the course of evolution on earth--and even the likelihood of finding complex life forms on other planets--will be overturned. Says Whitmire: "Just the possibility that life here has been controlled by an astronomical event is very far reaching."
In one sense, the current fights over extinction theories are merely the latest variation of a venerable tradition that dates back to the early 19th century, when a growing corps of paleontologists and geologists had determined that the world is not the static Eden-like meadow of legend. At least intermittently, they concluded, it is an unstable, dangerous place, where vast numbers of species, like the giant mastodons, mysteriously disappear. Eventually, after analyzing the bones long thought to be the remains of dragons, they pieced together the almost more fantastical story of the dinosaurs and their inexplicable demise. They zealously dug up fossils of mysterious plants, insects, small animals and marine organisms, compared them with living ones, and pondered their disappearance from the earth. Had it been sudden and catastrophic, or gradual?
Some Victorian scientists viewed the discoveries as evidence of a primordial flood, possibly Noah's. A few linked extinctions to the fury of volcanoes; their conjecture was based on the extraordinary explosion in 1883 of Krakatoa, a volcano between Java and Sumatra, which darkened the skies and triggered a giant tidal wave that drowned 36,000 people.