Steps Toward Life

Four billion years ago, when the young earth was still enveloped in a deadly atmosphere of ammonia and methane, the first forerunners of life emerged. How those complex molecules were formed remains a profound mystery. But scientists believe that some of the earth's primordial atmospheric molecules were broken up into their constituent atoms; regrouping into new molecules, these atoms formed organic compounds called amino acids, which are the building blocks of protein--and of life.

Exactly what caused that chemical concatenation has long been the subject of lively scientific debate. Was the crucial reaction powered by intense ultraviolet radiation from the sun? By bolts of lightning in the primeval skies? Or by the searing heat of volcanic eruptions on the surface of the young planet? Researchers have found that amino acids can be produced in laboratory simulations of each of those conditions. Now a team of investigators at Cornell University has proposed that another natural phenomenon might have played a role. The catalyst of genesis, they say, could have been the shock waves of thunderclaps or even of meteors plunging into the atmosphere.

The Cornell researchers, a young Israeli chemist named Akiba Bar-Nun and his biochemist wife Nurit, tested the theory in a relatively simple experiment. They filled one end of a brass-and-Pyrex tube with a mixture of ammonia, methane, ethane and water vapor--all probable ingredients of the earth's early atmosphere. A thin plastic membrane separated the gases from the other end of the tube, which contained chemically inert helium. The Bar-Nuns increased the helium pressure until the membrane broke. This produced a shock wave that swept into the gaseous mixture at high speed, momentarily creating temperatures of several thousand degrees. In seven separate experiments, at least four amino acids were left behind.

The process was remarkably efficient. Carl Sagan, director of Cornell's Planetary Studies Laboratory, calculates that as much as 36% of the ammonia was converted into amino acids--a far better yield than that obtained in tests using ultraviolet radiation. Reason: the temperature rises resulting from the shock waves were too brief to break up any of the newly formed molecules. Indeed, the shock-tube process worked so well that Sagan has suggested a highly practical application: a cheap method of making amino acids for protein food supplements to fight malnutrition.

Did the laboratory experiment really simulate conditions that existed in the earth's primitive atmosphere and suggest how the precursors of life were created? To help answer such questions, scientists may soon try a much larger proving ground. In 1972, the first of several unmanned space probes is scheduled to pass close to Jupiter, which apparently has a methane and ammonia atmosphere much like the one that one shrouded the earth. Data from the missions may confirm that the processes now under way in Jupiter's atmosphere parallel those that occurred on earth bil lions of years ago.

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