Fig. 1.

RNA nucleoside formation pathway. A geothermal environment provides the right set up for the depicted transformations by establishing wet–dry cycles. The prebiotic starting materials are produced from a prebiotic atmosphere and washed into an aqueous environment (e.g. by rain). Major atmospheric components are written in larger letters, whereas minor components are written in smaller letters. Transformations are taking place in different environments, illustrated by various rivers (in light blue). Each environment provides the right setup for different chemistries, leading to several different chemical transformations. This geochemical setup leads to a set of canonical and non-canonical RNA building blocks by continuous synthesis (6a, m¹G: R¹ = O, R² = Me, R³ = NH₂; 6b, ms²A: R¹ = NH, R² = H, R³ = SMe; 6c, A: R¹ = NH, R² = H, R³ = H; 6d, m²G: R¹ = O, R² = H, R³ = NHMe; 6e, m²₂G: R¹ = O, R² = H, R³ = N(Me)₂; 6f, G: R¹ = O, R² = H, R³ = NH₂; 6g, DA: R¹ = NH, R² = H, R³ = NH₂; 6h, m²A: R¹ = NH, R² = H, R³ = Me)