Several invaluable observations are available to constrain early Earth conditions. Given the multiple processes that contribute to the balance of H 2, CH 4, and CO 2 in the early Earth atmosphere, including volcanic outgassing, asteroid impacts, hydrothermal activity in undersea vents, hydrogen escape from the atmosphere, and rain-out, what are the yields of biomolecules in specific environments? However, such experiments do not address the whole planetary and geochemical context of an evolving planet and its atmosphere, nor do they address what conditions actually lead to RNA synthesis sufficient for an RNA world. It is the HCN produced by electrical discharges, once dissolved in the water reservoir in the Miller–Urey apparatus, that produces the plethora of amino acids (Miller 1957b Bada 2013) and nucleobases (Ferus et al. Reduced carbon (e.g., CH 4, CH 3), on the other hand, directly reacts to produce HCN (Pearce et al. This is because oxygen must be removed from oxidized carbon (i.e., CO 2 and CO) before it can react to form HCN, which is energetically expensive. The famous Miller–Urey experiments showed that reducing atmospheres rich in H 2 and CH 4 are favorable for HCN production, whereas oxidizing atmospheres rich in CO 2 do not produce as much HCN (Miller 1957a Schlesinger & Miller 1983 Benner et al. One advantage of HCN over other more complex biomolecule precursors is that there are multiple favorable reaction pathways for its production directly from the dissociation products of common atmospheric gases, i.e., N 2, CH 4, and H 2 (Pearce et al. HCN is a key biomolecule precursor because in aqueous solution it reacts with itself and other small molecules such as formaldehyde to produce several relevant biomolecules for the origin of RNA-widely thought to have been critical for the first life on Earth (Rich 1962 Gilbert 1986). If not, then life's origin presumably depended on the delivery of biomolecules via external agents, such as carbon-rich meteorites (Chyba & Sagan 1992 Pearce et al. A fundamental question about the origin of life such as our own is whether biomolecular building blocks critical to creating information polymers such as RNA and proteins can be synthesized in situ on a habitable planet (Miller 1953). Our work points to an early origin of RNA on Earth within ∼200 Myr of the Moon-forming impact.Īstrophysical, geophysical, and fossil evidence suggests that life on Earth emerged in the interval of 4.5–3.7 bya (billion years ago) (Pearce et al. The source of HCN is predominantly from UV radiation rather than lightning. The early evolution of the atmosphere is dominated by the decrease in hydrogen due to falling impact rates and atmospheric escape, and the rise of oxygenated species such as OH from H 2O photolysis. Meteorite delivery of adenine to WLPs can provide boosts in concentration by 2–3 orders of magnitude, but these boosts deplete within months by UV photodissociation, seepage, and hydrolysis. We find that 4.4 billion years ago the limit of adenine concentrations in ponds for habitable surfaces is 0.05 μM in the absence of seepage. We then use a comprehensive numerical model of sources and sinks to compute the resulting abundances of nucleobases, ribose, and nucleotide precursors such as 2-aminooxazole resulting from aqueous and UV-driven chemistry within them. This allows us to calculate the rain-out of HCN into warm little ponds (WLPs). The atmosphere is supplied with hydrogen from impact degassing of meteorites, water evaporated from the oceans, carbon dioxide from volcanoes, and methane from undersea hydrothermal vents, and in it lightning and external UV-driven chemistry produce HCN. Here, we construct a robust physical and nonequilibrium chemical model of the early Earth's atmosphere. The basic building blocks of RNA could have been delivered by carbon-rich meteorites or produced in situ by processes beginning with the synthesis of hydrogen cyanide (HCN) in the early Earth's atmosphere. The origin of life on Earth involves the early appearance of an information-containing molecule such as RNA.
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