Tidal chain reaction and the origin of replicating biopolymers

Template-directed polymer assembly is a likely feature of prebiotic chemistry, but the product blocks further synthesis, preventing amplification and Darwinian selection. Nucleic acids are unusual because charge repulsion between opposing phosphates permits salt-dependent association and dissociation. It was postulated (Lathe, R. (2004). Fast tidal cycling and the origin of life. Icarus 168, 18–22) that tides at ocean shores provide the driving force for amplification: evaporative concentration promoted association/assembly on drying, while charge repulsion on tidal dilution drove dissociation. This permits exponential amplification by a process termed here the tidal chain reaction (TCR). The process is not strictly contingent upon tidal ebb and flow: circadian dews and rainfalls can produce identical cycling. Ionic strength-dependent association and dissociation of nucleic acids and possible prebiotic precursors are reviewed. Polymer scavenging, chain assembly by the recruitment of pre-formed fragments, is proposed as the primary mechanism of reiterative chain assembly. Parameters determining prebiotic polymer structure and amplification by TCR are discussed, with the suggestion that Darwinian selection may have operated on families of related polymers rather than on individual molecules.