Advances in molecular biology and comparative genomics over the last twenty years have raised evolutionary problems that can only be tackled by partly speculative approaches. The aim is to produce evolutionary scenarios of varying degrees of credibility, with the aim of reconstructing a history of living organisms that we hope will be close to historical reality. Some scenarios are more or less unanimously accepted. For example, it seems that the three types of informative macromolecules present in the living world appeared according to the RNA-protein-DNA sequence. Here we develop the argument that the transition from the RNA world to the present-day world required the existence of sophisticated RNA cells capable of synthesizing complex proteins (ribonucleotide reductases, reverse transcriptase, thymidylate synthases).
Why did these cells disappear, and why were they replaced by DNA cells? According to Darwinian theory, this replacement could only have occurred if strong selection pressure favored the first DNA-containing organisms (first U-DNA, then T-DNA). In my view, the extreme diversity of DNA replication mechanisms in viruses (plasmids) and the atypical distribution of the enzymes involved in these mechanisms along the universal tree of life are clues to answering these questions, while proposing a coherent and explanatory scenario. DNA and its replication mechanisms first appeared within an ancient virosphere of viruses infecting RNA cells.
