Prebiotic synthesis on minerals: bridging the prebiotic and RNA worlds.

An array of experimental findings suggest that the simple molecules required for the origin of life may have been available on the primitive Earth. The well-known MillerUrey electric discharge experiment yields HCN, amino acids, and carboxylic acids (1). Carbonaceous meteorites contain over 70 amino acids, some heterocyclic compounds, including purines, and carboxylic acids. Other organics may have been synthesized under the reducing conditions present in hydrothermal systems (2). Although some of these organics may have been present on the primitive Earth, there is no agreement on which was the most important for their formation. The Miller-Urey experiment requires the presence of either hydrogen gas or reduced carbon and hydrogen compounds, such as methane, carbon monoxide, and ammonia, in the atmosphere of the primitive Earth. Most geochemists believe that the Earth’s crust was not reducing when life originated, so the volcanic emissions that resulted in the primitive atmosphere were also not reducing. The notion that comets, meteorites, and asteroids delivered reduced organics to the surface of the primitive Earth is not favored by those who believe that, when these bodies hit the Earth’s atmosphere, the organic compounds present on them would have been pyrolyzed. The hydrothermal synthesis of reduced organics is a relatively new hypothesis that is just now undergoing experimental evaluation. Assuming that the requisite starting materials were available on the primitive Earth (2), and providing conditions that plausibly occurred at that time, I have undertaken to investigate how monomers, particularly mononucleotides,