Prebiotic organic synthesis

Prebiotic organic synthesis

The plausible pathways by which the molecular precursors of life may have formed on the primitive Earth. Amino acids, the nitrogenous bases, and ribose phosphates can be prepared under conditions that might have prevailed on the primitive Earth. The linking together of amino acids to form polypeptides, and of nucleotides to form polynucleotides, has in principle been established.

Harold C. Urey's model of the primordial Earth postulates an atmosphere rich in methane, ammonia, water, and hydrogen. When this gas mixture is subjected to an electrical spark, analogous to the way that lightning may have initiated such syntheses 4 billion years ago, the identified products included several amino acids (glycine, alanine, and aspartic acid), the building blocks of proteins. This novel result lent credibility to a theory in which the origin of life was viewed as a cumulative, stepwise process, beginning with the gaseous synthesis of small molecules, which rained down into oceans, lagoons, and lakes. With water as a ubiquitous solvent, organic molecules could then react with one another to form larger molecules (biopolymers) and finally to assemble into primitive cells. This general scenario has guided the design of prebiotic simulations.

However, an accumulation of geophysical data and computational models has cast doubt on the relevance of the synthesis of amino acids to the primordial Earth. Hydrogen probably escaped rapidly as the Earth cooled, leaving an atmosphere in which methane and ammonia were virtually absent. As the input of hydrogen is diminished, the formation of biomolecules is inhibited. This problem has led some scientists to look for extraterrestrial sources of organic matter. For example, meteorites are known to contain a rich source of amino acids and other small biomolecules, and perhaps the infall of such cosmic bodies onto the young Earth gave life its start. Alternatively, there may have been localized environments on the Earth where methane and other hydrogen-rich precursors were abundant, such as deep-ocean hydrothermal vents, which would have been favorable for the formation of life. See Amino acids, Hydrothermal vent