Adding to support for the RNA world hypothesis, Sidney Becker and colleagues have presented what's not been shown before - a single chemical pathway that could generate both the purine and pyrimidine nucleosides, the key building blocks of RNA. This unified synthesis was possible because of an approach called "wet-dry" cycling, the authors say. Previously, the RNA world hypothesis has struggled to describe the contemporaneous formation of all Watson-Crick bases in a single pot. "Disparate prebiotic syntheses have been demonstrated for the two classes of RNA nucleosides...but no single geochemical scenario has been able to generate both," write Nicholas Hud and David Fialho in a related Perspective. The RNA World Hypothesis suggests that RNA was the first living substance on Earth and that it proliferated before the evolution of DNA and protein. While the properties of RNA make the RNA world conceptually plausible, a compatible prebiotic geochemical scenario capable of creating RNA precursors has been difficult to describe. Building upon previous research, which demonstrated a plausible pathway for purine synthesis, Becker et al. now show that pyrimidine nucleosides can also be synthesized under the same geochemical conditions. The results reveal hydroxylamine, which can form under plausible early-atmospheric conditions, as a key starting material in the synthetic pathway. Cycling between wet and dry conditions provides the environmental states required to complete the synthesis of each of the complementary nucleotides, the authors show. "Unlike volcanic eruptions and meteorite impacts, events often proposed as drivers of prebiotic reactions, wet-dry cycles would have been regular events on all parts of exposed land of the prebiotic Earth," write Hud and Fialho. While the findings of the study offer a plausible solution to the formation of the Watson-Crick bases, the synthesis specifics of carbohydrates, which form the structural backbone of RNA, remain unclear, according to the authors.