A Hadean timeline for the emergence of the RNA World

Estimates for when life first appeared range from ca. 3.8 Ga (Eoarchean) to sometime in the Hadean eon (≥4.0-4.5 Ga). Such approximations arise from interpretations of the oldest (albeit fragmentary) paleontological records, cryptic isotopic traces arguably interpreted as biosignatures, and phylogenetic analyses of extant microbial genomes. Here, we infer that a proposed early stage of life – the RNA World – could emerge at ca. 4.33 Ga by modelling the near-surface thermal stability of key biomolecules comprising postulated RNA organisms. We apply a global three-dimensional numerical model to analyse the thermal evolution profile of Hadean crust in response to late accretion bombardment in the time interval 4.5-3.5 Ga. Results show that frequent global sterilization events persisted up until 4.4 Ga owing to hot ejecta deposition, rock vapour condensation, and enhanced geothermal gradients. Later, conditions at or near the Hadean surface became favourable to persistent biochemistry by harbouring interconnected zones of biomolecule stability and extensive energy-rich hydrothermal oases. We identify an optimal time (a temporal ‘sweet spot’) for the RNA World that dovetails with the emergence of the Last Universal Common Ancestor (LUCA) population 150 Myr later.