When did Life Likely Emerge on Earth in an RNA-First Process?

The widespread presence of ribonucleic acid (RNA) catalysts and cofactors in Earth’s biosphere today suggests that RNA was the first biopolymer to support Darwinian evolution. However, most “path-hypotheses” to generate building blocks for RNA require reduced nitrogen-containing compounds not made in useful amounts in the CO2-N2-H2O atmospheres of the Hadean. We review models for Earth’s impact history that invoke a single ~10^23 kg impactor (Moneta) to account for measured amounts of platinum, gold, and other siderophilic (“iron-loving”) elements on the Earth and Moon. If it were the last sterilizing impactor, Moneta would have reduced the atmosphere but not its mantle, opening a “window of opportunity” for RNA synthesis, a period when RNA precursors rained from the atmosphere to land holding oxidized minerals that stabilize advanced RNA precursors and RNA. Surprisingly, this combination of physics, geology, and chemistry suggests a time when RNA formation was most probable, ~120 +/- 100 million years after Moneta’s impact, or ~4.36 +/- 0.1 billion years ago. Uncertainties in this time are driven by uncertainties in rates of productive atmosphere loss and amounts of sub-aerial land.

💡 Research Summary

The paper tackles the long‑standing “RNA‑first” hypothesis by embedding it within a realistic model of early Earth’s impact history and atmospheric chemistry. It begins by noting that modern biosphere abundance of ribozymes implies RNA was the earliest biopolymer capable of Darwinian evolution, yet most prebiotic pathways to RNA building blocks require reduced nitrogen species that are scarce in a CO₂‑N₂‑H₂O Hadean atmosphere. To resolve this, the authors invoke a single, massive impactor—named Moneta—with a mass of roughly 10^23 kg. Geochemical evidence from the Earth‑Moon system (elevated platinum, gold, and other siderophile elements) is used to argue that Moneta was the last sterilizing impact, capable of reducing the atmosphere without substantially altering the mantle.

Following the impact, the atmosphere becomes strongly reducing, generating large amounts of NH₃, HCN, CH₄, and related precursors. The authors model atmospheric loss processes (photochemical escape, sputtering, impact erosion) and estimate the exposed sub‑aerial land area composed of oxidized silicates and iron oxides. These minerals act as sinks and catalysts for the newly formed RNA precursors, which rain down as a weak acidified precipitation. The “window of opportunity” for productive RNA synthesis is defined as the interval during which the reduced atmosphere persists while sufficient land is available to capture and stabilize the precursors.

Numerical simulations indicate that this window peaks roughly 120 ± 100 million years after the Moneta impact, corresponding to a time of 4.36 ± 0.1 billion years ago. The uncertainty is dominated by the rates of atmospheric loss and the amount of exposed oxidized crust. Within this window, the combination of abundant reduced nitrogen compounds, metal‑ion catalysis on mineral surfaces, and protection from rapid degradation makes RNA polymerization and accumulation most probable.

The authors discuss the sensitivity of the result to key parameters: faster atmospheric escape shortens the window, while a larger exposed land area extends it. They also highlight the need for geological evidence of oxidized surfaces at that epoch and for experimental validation of precursor rain and mineral‑mediated stabilization. In conclusion, the study provides a physically grounded temporal framework that aligns a single giant impact with the most favorable conditions for RNA‑based life to emerge, thereby strengthening the plausibility of the RNA‑first scenario.