Science: Did Comets Kill the Dinosaurs?

A bold new theory about mass extinctions

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In the end, however, most of the scholars responded to the lure of Darwin, insisting that creatures die out because they are no longer fit to survive and must give way to the supremacy of the new. That argument seemed to apply particularly well to the dinosaurs, which were denigrated as being too big, too slow, too pea-brained and too cold-blooded for their own good.

As geology became more precise, scientists determined from the fossil record that at least five great dyings--and numerous smaller events--have occurred in the past 600 million years. Among the more significant: the Cambrian disasters some 500 million years ago, when many species of segmented creatures called trilobites disappeared from the seas they once dominated; the biggest of the extinctions, the Permian cataclysm of 248 million years ago, when up to 90% of all marine species died; and the late-Cretaceous event 65 million years ago, which saw the destruction of the dinosaurs and many other groups of species, including the microscopic organisms responsible for creation of the white cliffs of Dover. The effects on evolution were profound. "In wiping the slate clean," says Muller, "these catastrophes opened up ecological niches and prevented stagnation."

Yet for all the evidence of destruction, scientists could not figure out where the eraser was hidden. Geologists naturally looked to the earth for explanations, citing changes in climate or sea level. By the mid-1960s, scientists had concluded that the planet's tectonic plates are continually on the move, bumping and grinding against each other to produce earthquakes and mountains, or separating to tug apart land masses and rearrange the oceans. When sea levels change, they reasoned, animal habitats in low-lying areas may be destroyed, or the climate farther inland may grow more extreme.

What is more, the restless tectonic plates spawn volcanic eruptions, which spew carbon dioxide into the air. The increase in atmospheric carbon dioxide results in a greenhouse effect, which traps the sun's energy, causing temperatures on land and in the sea to rise markedly. Conversely, crustal movement may allow frigid ocean currents from the poles to invade tropical waters, leading to a worldwide drop in temperatures. Those species that cannot adapt to the earth's erratic behavior simply succumb. To many paleontologists, as well as geologists, it seemed to make sense.

Serious questions remained, however. Why, for example, were extinctions so selective, devastating some species while leaving others virtually unscathed? Try as they might, scientists could not devise a single elegant theory to tie the loose ends together. They were about to get what they asked for, but like the British author W.W. Jacob's infamous monkey's paw (which granted wishes --at a price), it would not be an unsullied blessing.

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