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Could these overheated spots have been the places where life on earth got started? This "hot world" hypothesis has won many converts. Norman Pace, a microbiologist at Indiana University, speculates that the thin crust of primitive earth, as prone to cracking as an eggshell, would have made hydrothermal vents far more common than they are today. Geochemist Everett Shock of Washington University calculates that at high temperatures organisms can get extra energy from nutrients. "The hotter it is," says Shock, "the easier life is." (Up to a point. No one has yet found a microbe living in conditions hotter than 235 degreesF.)
Still, the question remains: Did life originate in the vents, or just migrate there? The vents may not have been a cradle but an air-raid shelter for organisms that originated near the ocean surface, then drifted to the bottom. There, protected by thousands of feet of water, these lucky refugees might have survived a series of extraterrestrial impacts that killed off their relatives near the sunlit surface.
THE INGREDIENTS
Stanley Miller's glass-jar experiment 40 years ago suggested that the components of life were easily manufactured from gases in the atmosphere. The conditions he re-created in his laboratory faithfully reflected the prevailing wisdom of the time, which held that the earth was formed by a gradual, almost gentle convergence of rock and flecks of dust under the influence of gravity. According to this model, the earth started out cold. Its deepest layers did not catch fire until much later, after the decay of radioactive elements slowly turned up the thermostat in the core. Thus, heavy elements such as iron did not immediately melt and sink to the core, but remained close to the surface for hundreds of millions of years.
Why is this important? Because iron soaks up oxygen and prevents it from combining with carbon to form carbon dioxide. Instead, the carbon, and also the nitrogen, spewed into the atmosphere by ancient volcanoes would have been available to interact with hydrogen. The serendipitous result: formation of methane and ammonia, the gases that made the Miller experiment go.
It was, says Chyba, "a beautiful picture." Unfortunately, he adds, it is probably wrong. For the violent collisions now believed to have attended earth's birth would have melted the iron and sent it plummeting to the depths. As a result, the early atmosphere would have been composed largely of carbon dioxide -- and organic compounds cannot be so easily generated in the presence of CO2.
Where, then, did the building blocks of life come from? Quite possibly, many scientists think, organic compounds were transported to earth by the very comets, asteroids and meteorites that were making life so difficult. At the University of California at Davis, zoologist David Deamer has extracted from meteorites organic material that forms cell-like membranes. He has also isolated pale yellow pigments capable of absorbing energy from light -- a precursor, Deamer believes, of chlorophyll, the green pigment used by modern plants.