The Natural Evolution of RNA Viruses Provides Important Clues about the Origin of SARS-CoV-2 Variants

Despite the recent pandemic, the origin of its causative agent, SARS-CoV-2, remains controversial. This study identifies several prototype SARS-CoV-2 variants (proto-variants) that are descendants of the Wuhan variant. A thorough evaluation of the evolutionary histories of the genomes of these proto-variants reveals that most mutations in proto-variants were biased toward mutations that change the amino acid sequence. While these nonsynonymous substitutions (N mutations) were common in SARS-CoV-2 proto-variants, nucleotide changes that do not result in an amino acid change, termed synonymous substitutions (S mutations), dominate the mutations found in other RNA viruses. The N mutation bias in the SARS-CoV2 proto-variants was found in the spike gene as well as several other genes. The analysis of the ratio of N to S mutations in general RNA viruses revealed that the probability that an RNA virus spontaneously evolves a proto-variant is between 1.5 × 10−9 and 2.7 × 10−26 under natural conditions. These results suggest that SARS-CoV-2 variants did not emerge via a canonical route.