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Classic but Questionable

Stanley Miller’s experiment in 1953 is often cited as evidence that spontaneous generation could have happened in the past. The validity of his explanation, however, rests on the presumption that the earth’s primordial atmosphere was “reducing.” That means it contained only the smallest amount of free (chemically uncombined) oxygen. Why?

The Mystery of Life’s Origin: Reassessing Current Theories points out that if much free oxygen was present, ‘none of the amino acids could even be formed, and if by some chance they were, they would decompose quickly.’a How solid was Miller’s presumption about the so-called primitive atmosphere?

In a classic paper published two years after his experiment, Miller wrote: “These ideas are of course speculation, for we do not know that the Earth had a reducing atmosphere when it was formed. . . . No direct evidence has yet been found.”—Journal of the American Chemical Society, May 12, 1955.

Was evidence ever found? Some 25 years later, science writer Robert C. Cowen reported: “Scientists are having to rethink some of their assumptions. . . . Little evidence has emerged to support the notion of a hydrogen-rich, highly reducing atmosphere, but some evidence speaks against it.”—Technology Review, April 1981.

And since then? In 1991, John Horgan wrote in Scientific American: “Over the past decade or so, doubts have grown about Urey and Miller’s assumptions regarding the atmosphere. Laboratory experiments and computerized reconstructions of the atmosphere . . . suggest that ultraviolet radiation from the sun, which today is blocked by atmospheric ozone, would have destroyed hydrogen-based molecules in the atmosphere. . . . Such an atmosphere [carbon dioxide and nitrogen] would not have been conducive to the synthesis of amino acids and other precursors of life.”

Why, then, do many still hold that earth’s early atmosphere was reducing, containing little oxygen? In Molecular Evolution and the Origin of Life, Sidney W. Fox and Klaus Dose answer: The atmosphere must have lacked oxygen because, for one thing, “laboratory experiments show that chemical evolution . . . would be largely inhibited by oxygen” and because compounds such as amino acids “are not stable over geological times in the presence of oxygen.”

Is this not circular reasoning? The early atmosphere was a reducing one, it is said, because spontaneous generation of life could otherwise not have taken place. But there actually is no assurance that it was reducing.

There is another telling detail: If the gas mixture represents the atmosphere, the electric spark mimics lightning, and boiling water stands in for the sea, what or who does the scientist arranging and carrying out of the experiment represent?

[Footnote]

Scientist Stanley L. Miller, working in the laboratory of Harold Urey, took hydrogen, ammonia, methane, and water vapor (assuming that this had been the primitive atmosphere), sealed these in a flask with boiling water at the bottom (to represent an ocean), and zapped electric sparks (like lightning) through the vapors. Within a week, there were traces of reddish goo, which Miller analyzed and found to be rich in amino acids—the essence of proteins. You may well have heard of this experiment because for years it has been cited in science textbooks and school courses as if it explains how life on earth began. But does it?

Actually, the value of Miller’s experiment is seriously questioned today. (See “Classic but Questionable,” pages 36-7.)

In the years since, however, that optimism has evaporated. Decades have passed, and life’s secrets remain elusive. Some 40 years after his experiment, Professor Miller told Scientific American: “The problem of the origin of life has turned out to be much more difficult than I, and most other people, envisioned.”

After Miller and others had synthesized amino acids, scientists set out to make proteins and DNA, both of which are necessary for life on earth. After thousands of experiments with so-called prebiotic conditions, what was the outcome? The Mystery of Life’s Origin: Reassessing Current Theories notes: “There is an impressive contrast between the considerable success in synthesizing amino acids and the consistent failure to synthesize protein and DNA.” The latter efforts are characterized by “uniform failure.”

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“[The smallest bacterium] is so much more like people than Stanley Miller’s mixtures of chemicals, because it already has these system properties. So to go from a bacterium to people is less of a step than to go from a mixture of amino acids to that bacterium.”—Professor of Biology Lynn Margulis