The 1953 Miller Experiment and the Beginnings of Prebiotic Chemistry

On May 15, 1953, Stanley Miller published a short paper in SCIENCE describing the spark discharge induced synthesis of glycine, alanine and other amino acids from a mixture of reducing gases thought to represent the atmosphere of early Earth. This study legitimized the field of prebiotic chemistry and demonstrated that the basic molecules of life could, with relative ease, be synthesized from simple inorganic molecules. Darwin's “warm little pond” was no longer a hypothetical concept but a feasible scenario. One criticism of the original Miller experiment has been that the reducing gas mixture he used was not a realistic representation of the primitive atmosphere, now thought to have been dominated by CO2 and N2 which when used in the spark experiment yielded low levels of amino acids. However, recent studies of some of Miller's original extracts from another apparatus configuration, termed the “volcanic” apparatus, suggests that localized prebiotic synthesis could have been effective even if the overall atmosphere was not reducing in composition. Reduced gases and lightning associated with volcanic eruptions in hot spot, island arc type systems could have been prevalent on the early Earth before extensive continents formed. Amino acids formed in volcanic island arc systems could have been locally rained out into tidal areas, where they could be polymerized by carbonyl sulfide, a simple volcanic gas that has been shown to form peptides under mild conditions, as well as other polymerization processes. In addition, other recent experiments have shown that significant amounts of amino acids are produced from CO2 and N2 mixtures. The low yields previously reported for neutral gas mixtures appear to be the outcome of oxidation of the organic compounds during hydrolytic workup by nitrite and nitrate produced in the reactions. The yield of amino acids is greatly increased when oxidation inhibitors are added prior to hydrolysis. Organic synthesis from neutral atmospheres may have depended on the oceanic availability of oxidation inhibitors as well as on the nature of the primitive atmosphere itself. These results suggest that endogenous synthesis of compounds considered to be essential for the origin of life as we know was a robust process on the early Earth. How these simple compounds were polymerized into complex molecules with functions such as catalysis and replications, the hallmark of biochemistry, is an ongoing area of research.