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Summary. Factors involved in the selection of the 20 protein L-a-amino acids during chemical evolution and the early stages of Darwinian evolution are discussed. The selection is considered on the basis of the availabili-ty in the primitive ocean, function in proteins, the stabi-lity of the amino acid and its peptides, tability to race-mization, and stability on the transfer RNA. We con-clude that aspartic acid, glutamic acid, rginine, lysine, serine and possibly threonine are the best choices for acidic, basic and hydroxy amino acids. The hydrophobic amino acids are reasonable choices, except for the puzz-ling absences of a-amino-n-butyric a id, norvaline and norleucine. The choices of the sulfur and aromatic amino acids seem reasonable, but are not compelling. Asparagine and glutamine are apparently not primitive. If life were to arise on another planet, we would expect that the catalysts would be poly-a-amino acids and that about 75 % of the amino acids would be the same as on the earth. Key words: Amino acids-Molecular evolution-Genetic Code-Protein synthesis-Prebiotic synthesis There are only twenty amino acids that are coded for in protein synthesis, along with about 120 that occur by post-translational modifications (Uy and Wold 1977). Yet there are over 300 naturally-occurring amino acids known (Mooz 1976), and thousands of amino acids are possible. The question then is- why were these parti-cular 20 amino acids selected uring the process that led to the origin of the most primitive organism and during the early stages of Darwinian evolution. This problem Offprint requests to: Stanley L. Miller Dedicated to the memory of Harold C. Urey has not been explicitly discussed previously although there are extensive discussions of the related problem of the origin of the genetic ode (Crick 1968; Crick et |
