Stones From The

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Without help from a comet. . . I will give you a receipt for growing tree ferns at the pole, or if it suits me, pines at the equator; walruses under the line, and crocodiles in the arctic circle.' Charles Lyell

I n science as in life, timing is everything. A correct theory proposed before the time is ripe for its acceptance goes nowhere. The history of science is replete with theories ignored for years, decades, even centuries before their eventual acceptance. The most famous example is that of Aristarchus of Samos who anticipated by '8 centuries Copernicus's theory that the sun and not the earth is at the center of the solar system. In Aristarchus's day, however, Earth-centered astronomy did a good enough job of explaining the then rudimentary knowledge of the solar system, so that Aristarchus's theory was not "required." In '866, the monk Gregor Mendel published his work on the laws of genetics in the proceedings of a local society of naturalists, but no one took notice. In '900, '6 years after his death, Mendel's results were rediscovered. Continental drift had to wait half a century from Alfred Wegener's initial formulation in '9'5 to the plate tectonics revolution of the '960s and '970s.

Why do ideas that eventually prove worthy often have to wait? Typically it is because they go against the grain of the current paradigm, leaving other scientists with no way even to think about them. When first proposed, they are often little more than inspired guesses with no supporting evidence. (Mendel was an exception; he had the evidence but published it where no one saw it.] The apparatus and techniques that will eventually provide experimental support often have yet to be invented. For example, only a few years prior to '980, the Gubbio iridium anomaly could not have been detected, even if someone had been looking for it, because none of the available instruments were sensitive enough to detect iridium at the parts per trillion level.

The idea that a giant impact could cause mass extinctions, though consistently rejected by geologists, has a surprisingly long history, dating back at least to 1742, when Frenchman Pierre-Louis Moreau de Maupertuis suggested that comets have struck the earth and caused extinction by changing the atmosphere and the oceans.2 His countryman, astronomer Pierre-Simon Laplace, wrote in 1813 that a meteorite of great size striking the earth would produce a cataclysm that would wipe out entire species.3 In our own century, the distinguished paleontologist Otto Schindewolf sought an extraterrestrial cause for mass extinction. In 1970, Digby McLaren used his presidential address to the Paleontological Society to present the idea once again, leading some uniformitarians to assume that he could only have been speaking tongue-in-cheek.4 American Harold Urey, winner of the Nobel Prize in chemistry, proposed in 1973 in the widely read journal Nature that impact was responsible for mass extinctions and the periods of the geologic time scale on which they are based.5 Urey, who had published a variety of important research papers, had developed enough of a reputation in the earth and planetary sciences to be taken seriously, yet still no one paid any attention. These suggestions were catastrophist, unorthodox, and without evidence or predictions; therefore, even when made by distinguished scientists in important journals, they languished.

By 1980, when the Alvarez theory appeared, conditions had begun to improve. Iridium at the parts per trillion level was not easily measured, but it could be done at several laboratories around the world. The space age was nearly two decades old and the surfaces of other heavenly bodies were known in great detail—the map of the moon was more complete and accurate (when the ocean basins were included) than any map of the earth. It was impossible not to notice that, whatever its effect on the earth, impact had scarred every other object in the inner solar system innumerable times.

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