  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.
