Towards RNA life on Early Earth: From atmospheric HCN to biomolecule production in warm little ponds

TL;DR

This study models early Earth's atmosphere and aqueous environments to assess the synthesis and accumulation of RNA precursors, suggesting an origin of RNA within 200 million years after the Moon-forming impact.

Contribution

It provides a comprehensive physical and chemical model of early Earth's atmosphere and ponds, quantifying the production and stability of RNA building blocks.

Findings

Adenine concentrations could reach 0.05μM in ponds, sustained for over 100 million years.

Meteorite delivery can temporarily boost nucleotide concentrations by 2-3 orders of magnitude.

RNA precursors likely originated within ~200 million years of the Moon-forming impact.

Abstract

The origin of life on Earth involves the early appearance of an information-containing molecule such as RNA. 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. Here, we construct a robust physical and non-equilibrium chemical model of the early Earth atmosphere. The atmosphere is supplied with hydrogen from impact degassing of meteorites, sourced with water evaporated from the oceans, carbon dioxide from volcanoes, and methane from undersea hydrothermal vents, and in which lightning and external UV-driven chemistry produce HCN. This allows us to calculate the rain-out of HCN into warm little ponds (WLPs). We then use a comprehensive sources and sinks numerical model to compute the resulting abundances of nucleobases, ribose, and nucleotide precursors such as 2-aminooxazole resulting from aqueous and UV-driven chemistry within them. We find that at 4.4 bya (billion years ago) the limits of adenine concentrations in ponds for habitable surfaces is 0.05$\mu$M in the absence of seepage. These concentrations can be maintained for over 100 Myr. 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. The early evolution of the atmosphere is dominated by the decrease of hydrogen due to falling impact rates and atmospheric escape, and the rise of oxygenated species such as OH from H2O photolysis. Our work points to an early origin of RNA on Earth within ~200 Myr of the Moon-forming impact.