On their own, neither sulfite nor ferrocyanide are efficient sources of photochemically-generated electrons for the reductive homologation of hydrogen cyanide, but together they are.

A key challenge in origin-of-life studies is understanding the environmental conditions on early Earth under which abiogenesis occurred. While some constraints do exist (e.g., zircon evidence for surface liquid water), relatively few constraints exist on the abundances of trace chemical species, which are relevant to assessing the plausibility and guiding the development of postulated prebiotic chemical pathways which depend on these species. In this work, we combine literature photochemistry models with simple equilibrium chemistry calculations to place constraints on the plausible range of concentrations of sulfidic anions (HS−, HSO3−, SO32−) available in surficial aquatic reservoirs on early Earth due to outgassing of SO2 and H2S and their dissolution into small shallow surface water reservoirs like lakes. We find that this mechanism could have supplied prebiotically relevant levels of SO2-derived anions, but not H2S-derived anions. Radiative transfer modeling suggests UV light would have remained abundant on the planet surface for all but the largest volcanic explosions. We apply our results to the case study of the proposed prebiotic reaction network of Patel et al. (2015) and discuss the implications for improving its prebiotic plausibility. In general, epochs of moderately high volcanism could have been especially conducive to cyanosulfidic prebiotic chemistry. Our work can be similarly applied to assess and improve the prebiotic plausibility of other postulated surficial prebiotic chemistries that are sensitive to sulfidic anions, and our methods adapted to study other atmospherically derived trace species.

A key challenge in origins‐of‐life studies is estimating the abundances of species relevant to the chemical pathways proposed to have contributed to the emergence of life on early Earth. Dissolved nitrogen oxide anions ( NOX−), in particular nitrate ( NO3−) and nitrite ( NO2−), have been invoked in diverse origins‐of‐life chemistry, from the oligomerization of RNA to the emergence of protometabolism. Recent work has calculated the supply of NOX− from the prebiotic atmosphere to the ocean and reported steady state [ NOX−] to be high across all plausible parameter space. These findings rest on the assumption that NOX− is stable in natural waters unless processed at a hydrothermal vent. Here, we show that NOX− is unstable in the reducing environment of early Earth. Sinks due to ultraviolet photolysis and reactions with reduced iron (Fe2+) suppress [ NOX−] by several orders of magnitude relative to past predictions. For pH = 6.5–8 and T = 0–50 °C, we find that it is most probable that [ NOX−]  <1μM in the prebiotic ocean. On the other hand, prebiotic ponds with favorable drainage characteristics may have sustained [ NOX−]  ≥1μM. As on modern Earth, most NOX− on prebiotic Earth should have been present as NO3−, due to its much greater stability. These findings inform the kind of prebiotic chemistries that would have been possible on early Earth. We discuss the implications for proposed prebiotic chemistries and highlight the need for further studies of NOX− kinetics to reduce the considerable uncertainties in predicting [ NOX−] on early Earth.

Prebiotic nucleotide synthesis is crucial to understanding the origins of life on Earth. There are numerous candidates for life’s first nucleic acid, however, currently no prebiotic method to selectively and concurrently synthesise the canonical Watson–Crick base-pairing pyrimidine (C, U) and purine (A, G) nucleosides exists for any genetic polymer. Here, we demonstrate the divergent prebiotic synthesis of arabinonucleic acid (ANA) nucleosides. The complete set of canonical nucleosides is delivered from one reaction sequence, with regiospecific glycosidation and complete furanosyl selectivity. We observe photochemical 8-mercaptopurine reduction is efficient for the canonical purines (A, G), but not the non-canonical purine inosine (I). Our results demonstrate that synthesis of ANA may have been facile under conditions that comply with plausible geochemical environments on early Earth and, given that ANA is capable of encoding RNA/DNA compatible information and evolving to yield catalytic ANA-zymes, ANA may have played a critical role during the origins of life.

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Three 2-aminoazoles important for prebiotic chemistry undergo UV-driven photodegradation on timescales of 7–100 hours under solar irradiation.

a UV-driven photoredox processing of cyanocuprates can generate simple sugars necessary for prebiotic synthesis. We investigate the wavelength dependence of this process from 215 to 295 nm and generally observe faster rates at shorter wavelengths. The most efficient wavelengths are accessible to a range of potential prebiotic atmospheres, supporting the potential role of cyanocuprate photochemistry in prebiotic synthesis on the early Earth.

Ultraviolet (UV) light plays a key role in surficial theories of the origin of life, and numerous studies have focused on constraining the atmospheric transmission of UV radiation on early Earth. However, the UV transmission of the natural waters in which origins-of-life chemistry (prebiotic chemistry) is postulated to have occurred is poorly constrained. In this work, we combine laboratory and literature-derived absorption spectra of potential aqueous-phase prebiotic UV absorbers with literature estimates of their concentrations on early Earth to constrain the prebiotic UV environment in marine and terrestrial natural waters, and we consider the implications for prebiotic chemistry. We find that prebiotic freshwaters were largely transparent in the UV, contrary to assumptions in some models of prebiotic chemistry. Some waters, such as high-salinity waters like carbonate lakes, may be deficient in shortwave (≤220 nm) UV flux. More dramatically, ferrous waters can be strongly UV-shielded, particularly if the Fe 2+ forms highly UV-absorbent species such as . Such waters may be compelling venues for UV-averse origin-of-life scenarios but are unfavorable for some UV-dependent prebiotic chemistries. UV light can trigger photochemistry even if attenuated through photochemical transformations of the absorber ( e.g. , production from halide irradiation), which may have both constructive and destructive effects for prebiotic syntheses. Prebiotic chemistries that invoke waters that contain such absorbers must self-consistently account for the chemical effects of these transformations. The speciation and abundance of Fe 2+ in natural waters on early Earth is a major uncertainty and should be prioritized for further investigation, as it played a major role in UV transmission in prebiotic natural waters.