Figure 4.

Outline of most significant events leading to the Last Universal Common Ancestor (LUCA). The events that occurred during the same time period are shown vertically. Transition from non-coded to coded peptides gave rise to Darwinian evolution as described in Figure 3 and continue ever since. Proteins develop from small sized peptides with only a fraction of coded amino acids to formation of motifs and domains with increased coded content. At a later stage the peptides became able to fold and form catalytic centers leading to formation of proteins with fixed 1st and last amino acids. At the same time standard genetic code expands to cover more codons for at least 20 amino acids. By the time when the first proteins are fully formed the genetic code is established as we know it. During the period of short peptides and lack of folded peptides/proteins (RNA-peptide world) certain RNAs with catalytic functions were selected (ribozymes) to perform critical catalytic functions. This is time when the ribosome obtained its basic shape. By the time the peptides begun to fold and acquire new enzymatic functions, the ribozymes were already established and could not be replaced, but started to co-exist together with proteins. The genome size increased proportionally covering the information for all proteins and ribozymes. The Last Universal Common Ancestor (LUCA) emerged when all of its components—the ribozymes, genetic code, and proteins were functioning in full synchrony.