Early molecular evolution, from first codons to first proteins.

 

Edward N. Trifonov 

 

Genome Diversity Center, Institute of Evolution, University of Haifa 

 

Origin and evolution of the triplet code, codon by codon, are reconstructed on the basis of consensus temporal 
order of appearance of amino acids deduced from over 80 different expert opinions. From the resulting chart
 several important predictions follow, which are confirmed by computational sequence analyses. The first amino 
acids of the tripet code history, alanine and glycine, have been encoded by complementary GCC and GGC codons,
 as today. They were succeeded, respectively, by A- and G-series of amino acids, encoded by pyrimidine-central
 and purine-central codons. The earliest proteins should have had structure An and Gn. This is confirmed by 
protein sequence analysis. Their length n is found to be 6-7 residues. This size of the earliest proteins (peptides)is 
confirmed also by detected traces of ancient hairpins in mRNA. Vestiges of Last Universal Common Ancestor (LUCA) 
can be found in extant proteins in form of entirely conserved short sequences present in all or almost all
 sequenced prokaryotic proteomes (omnipresent motifs). The most conserved motifs found have the size 7-8 residues.
 The functions of the topmost conserved octamers are not involved in the basic elementary syntheses. This suggests
 an initial abiotic supply of amino acids, bases and sugars.