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Abstract
Workers from the same research group as Xu et at. had previously identified4 a network of reactions promoted by ultraviolet light that resulted in the synthesis of two of the standard nucleosides found in RNA: uridine (U) and cytidine (C), which are collectively known as pyrimidines (Fig. 1). In the present work, Xu et at. revisited compounds produced as intermediates in the previously established reaction network4 that synthesizes U and C. They identified a pathway in which a key intermediate of pyrimidine-nucleoside synthesis, ribo-aminooxazoline (RAO; Fig. 1), can also be converted into two purine DNA nucleosides, deoxyadenosine (dA) and deoxyinosine (dl, which is not one of the standard nucleosides found in modern DNA). In the current work, the authors show that the four nucleosides can indeed be produced through processes that could reasonably be expected to have occurred on early Earth (such as hydrolysis, drying and UV irradiation), and provide plausible synthetic pathways that could supply the reactions with their required starting materials.