  Prior to the origin of simple cellular life, the building blocks of RNA
(nucleotides) had to form and polymerize in favourable environments on the
early Earth. At this time, meteorites and interplanetary dust particles
delivered organics such as nucleobases (the characteristic molecules of
nucleotides) to warm little ponds whose wet-dry cycles promoted rapid
polymerization. We build a comprehensive numerical model for the evolution of
nucleobases in warm little ponds leading to the emergence of the first
nucleotides and RNA. We couple Earth's early evolution with complex prebiotic
chemistry in these environments. We find that RNA polymers must have emerged
very quickly after the deposition of meteorites (< a few years). Their
constituent nucleobases were primarily meteoritic in origin and not from
interplanetary dust particles. Ponds appeared as continents rose out of the
early global ocean but this increasing availability of "targets" for meteorites
was offset by declining meteorite bombardment rates. Moreover, the rapid losses
of nucleobases to pond seepage during wet periods, and to UV photodissociation
during dry periods means that the synthesis of nucleotides and their
polymerization into RNA occurred in just one to a few wet-dry cycles. Under
these conditions, RNA polymers likely appeared prior to 4.17 billion years ago.
  Significance: There are two competing hypotheses for the site at which an RNA
world emerged: hydrothermal vents in the deep ocean and warm little ponds.
Because the former lacks wet and dry cycles, which are well known to promote
polymerization (in this case, of nucleotides into RNA), we construct a
comprehensive model for the origin of RNA in the latter sites. Our model
advances the story and timeline of the RNA world by constraining the source of
biomolecules, the environmental conditions, the timescales of reaction, and the
emergence of first RNA polymers.
