Evolution of Genetic Alphabet and of Amino Acid Code

On considering chemical evolution of the Earth since the time of its appearance when its composition was similar to the elementary composition of star substance, a tentative hypothesis has been put forward that molecular evolution of the four-letter genetic alphabet includes two periods: I (pre-oxygen) and II (oxygenated) periods of chemical evolution. At the period I, in the primary Earth atmosphere the first nitrogen base, adenine (A), containing no oxygen appeared. The period II, during which three other nitrogen bases appeared in the atmosphere, consisted of three stages; at the first stage, guanine (G) appeared, at the second, cytosine (C), and at the third stage, uracyl (U). In accordance with the above periods, formation of codons and amino acids in nature was taking place presumably by the following way: at the period I, the first and the only codon AAA appeared, to which the amino acid lysine (Lys) corresponded; at the first stage of the period II, 7 codons and 3 amino acids (Arg, Glu, Gly) appeared; at the second stage, 19 codons and 8 new amino acids (Asn, Gin, Ser, Asp, Thr, Ala, His, Pro) appeared; at the third stage, 37 codons and more 8 new amino acids (Trp, Tyr, Cys, Ile, Met, Val, Leu, Phe) appeared. Thereby, in the course of biochemical evolution, 20 amino acids and 64 codons appeared in nature.