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.