Evolution of the first genetic cells and the universal genetic code: A hypothesis based on macromolecular coevolution of RNA and proteins

Abstract

A qualitative hypothesis based on coevolution of protein and nucleic acid macromolecules was developed to explain the evolution of the first genetic cells, from the likely organic chemical-rich environment of early earth, through to the Last Universal Common Ancestor (LUCA). The evolution of the first genetic cell was divided into three phases, proto-genetic cells I, II and III, and the transition to each milestone is described, based on development of chemical cross-catalysis, bio-cross-catalysis, and the universal genetic code, respectively. Selection of macromolecular properties of both peptides and nucleic acids, in response to environmental factors, was likely to be a key aspect of early evolution. The development of hereditable nucleic acids with various key functions; translation, transcription and replication, is described. These functions are envisaged to have coevolved with protein enzymes, from simple organic precursors. Genetically heritable nucleotides may have developed after the local earth environment had cooled below 63°C. Around this temperature G–C bases would have been preferentially utilized for nucleotide synthesis. Under these conditions RNA type nucleotides were then likely selected from a range of different types of nucleotide backbones through template-based synthesis. Initial development of the genetic coding system was simplified by the availability of proto-messenger RNA sequences that contained only G and C bases, and the need to encode only four amino acids. The step-wise addition of further amino acids to the code was predicted to parallel the growing metabolic complexity of the proto-genetic cell. On completion of this evolutionary process the proto-genetic cell is envisaged to have become the LUCA, the last common ancestor of bacteria, eukaryote and archaea domains. Key issues addressed by the model include: (a) the transition from non-hereditable random sequences of peptides and nucleic acids to specific proteins coded by hereditable nucleotide sequences, (b) the origin of homochiral amino acids and sugars, and (c) the mutation limits on the sizes of early nucleic acid genomes. The first genome was limited to a size of about 200 base pairs.