Molecular Clocks and the Puzzle of RNA Virus Origins

Abstract

Although the ultimate origins of RNA viruses are uncertain, it seems reasonable to assume that these infectious agents have a long evolutionary history, appearing with, or perhaps before, the first cellular life-forms. While the RNA viruses we see today may not date back quite this far, the evidence that some DNA viruses have evolved with their vertebrate hosts over many millions of years make an equally ancient history for RNA viruses a natural expectation. Yet a very different picture of RNA virus origins is painted if their gene sequences are compared; by usint the best estimates for rates of evolutionary change (nucleotide substitution) and assuming an approximate molecular clock, it can be inferred that the families of RNA viruses circulaing today could only have appeared very recently, probably not more than about 50,000 years ago. Hence, if evolutionary rates are accurate and relatively constant, present-day RNA viruses may have originated more recently than our own species. [Goes on to discuss this problem and suggest future solutions.]