Evolution of the RNA world: From signals to codes.

The accumulated material in evolutionary biology, greatly enhanced by the achievements of modern synthetic biology, allows us to envision certain key hypothetical stages of prebiotic (chemical) evolution. This is often understood as the further evolution in the RNA World towards the RNA-protein World. It is a path towards the emergence of translation and the genetic code (I), signaling pathways with signaling molecules (II), and the appearance of RNA-based components of future gene regulatory networks (III). We believe that these evolutionary paths can be constructively viewed from the perspective of the concept of biological codes (Barbieri, 2003). Crucial evolutionary events in these directions would involve the emergence of RNA-based adaptors. Such adaptors connect two families of functionally and chemically distinct molecules into one functional entity. The emergence of primitive translation processes is undoubtedly the major milestone in the evolutionary path towards modern life. The key aspect here is the appearance of adaptors between amino acids and their cognate triplet codons. The initial steps are believed to involve the emergence of proto-transfer RNAs capable of self-aminoacylation. The second significant evolutionary breakthrough is the development of biochemical regulatory networks based on signaling molecules of the RNA World (ribonucleotides and their derivatives), as well as receptors and effectors (riboswitches) for these messengers. Some authors refer to this as the "lost language of the RNA World." The third evolutionary step is the emergence of signal sequences for ribozymes on the molecules of their RNA targets. This level of regulation in the RNA World is comparable to the gene regulatory networks of modern organisms. We believe that the signal sequences on target molecules have been rediscovered and developed by evolution into the gene regulatory networks of modern cells. In conclusion, the immense diversity of modern biological codes, in some of its key characteristics, can be traced back to the achievements of prebiotic evolution.

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Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.