Figure 1.

A model for evolution of life on Earth. Evolution of tRNA and translation systems leads to the first cells. A small number of ribozymes appears sufficient to generate tRNA from pre-tRNA sequences that are known because they are conserved in tRNAs. Red type indicates ribozymes generated in vitro. Evolution of tRNA leads to evolution of translation systems and the genetic code. Triangles indicate the increases in biological potential associated with advances in coding [1]. Abbreviations: PTC, peptidyl transferase center; LUCA, last universal common cellular ancestor. A version of this figure was published in [1] and is reprinted here with permission.