Selection of Early Life Codons by Ultraviolet Light

How life developed in its earliest stages is a central
but notoriously
difficult question in science. The earliest lifeforms likely used
a reduced set of codon sequences that were progressively completed
over time, driven by chemical, physical, and combinatorial constraints.
However, despite its importance for prebiotic chemistry, UV radiation
has not been considered a selection pressure for the evolution of
early codon sequences. In this proof-of-principle study, we quantified
the UV susceptibility of large pools of DNA protogenomes and tested
the timing of evolutionary incorporation of codon sequences using
a Monte Carlo method utilizing sequence-context-dependent damage rates
previously determined by high throughput sequencing experiments. We
traced the UV-radiation selection pressure on early protogenomes comprising
a limited number of codon sequences to late protogenomes with access
to all codons. The modeling showed that in just minutes under early
sunlight, the choice of the first codons determined whether most of
the protogenomes remained intact or became damaged entirely. The results
correlated with earlier chemical models of the evolution of the genetic
code. Our results show how UV could have played a crucial role in
the evolution of the early genetic code for a DNA-based genome and
provide the concept for future RNA-based studies.