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A Big Fizz, Not a Big Flood

With 'catastrophism' back, a new mass-extinction theory arises

August 04, 1996

In the 19th century, scientists such as Charles Lyell and Charles Darwin swept aside what they saw as the superstitions of centuries past to construct an elegant and logical picture of species evolution on Earth. The belief that humans and animals survived the Great Flood on Noah's Ark was replaced by the scientific notion of "orthodox gradualism," in which gradual changes in the genetic pool enabled some organisms to thrive while others perished.

Scientific discoveries in the last decade, however, have suggested that Darwin, in endorsing the concept of natural selection, took some leaps of faith of his own. Assuming that the numbers of a given species normally remain more or less constant, Darwinian science taught us that dinosaurs gradually died out because they were dull creatures unable to compete with savvier species such as our mammalian ancestors.

But recent studies revealing that dinosaurs were in fact brighter than we had thought have cast doubt on those theories. Growing evidence from the sediments on which the dinosaurs walked 65 million years ago has added to this doubt by suggesting that the animals became extinct because of calamities like asteroid impacts, not less adaptive genetic traits.

While this theory remains controversial, it has marked the return of "catastrophism," the geological doctrine that holds that at intervals in Earth's history most living things have been destroyed by cataclysms and replaced by a new population.

A study published in last week's issue of Science magazine, however, suggests that "catastrophes" may be too pejorative a term, for these shifts actually spur on the kind of dramatic genetic change on which evolution depends.

The Science study may help explain the greatest of all mass extinctions on Earth--the eradication of 90% of its animal species at the end of the Permian period (about 250 million years ago). Its authors argue that a huge swelling from the oceans of carbonated water--not unlike the small bubbles that rise from a glass of club soda--poisoned the atmosphere with lethal levels of carbon dioxide. This air killed off a broad range of life, not only the primitive plants we associate with the era but saber-toothed tortoises and reptiles with giant, fan-shaped scales.

In drawing together once-segregated disciplines like paleontology and chemistry, the Science study exemplifies the kind of collaborative, interdisciplinary thinking needed to fathom a biological history that is becoming, as Harvard paleontologist Stephen Jay Gould puts it, "enormously more quirky than we could ever have imagined."

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