RNA viruses are an integral part in evolution of all organisms.

RNA viruses are intracellular symbiotic obligate parasites, needing host factors and energy for their replication with forms of symbiosis ranging from antagonism (pathogenic, not contributing to host metabolism) to mutualism (contributing benefits to the host as well as making demands on host metabolism). As a group, they have several unusual features: a) metagenomic studies suggest that they are probably are the most common group of viruses infecting all organism species and are the most abundant biological entity on earth; b) they have existed ever since the Last Universal Common Ancestor from which all living organisms have evolved; c) a high proportion of their species have + strand RNA genomes, or are retroviruses, that replicate without proof-reading creating many variants (quasispecies); d) they replicate in organelles within the endoplasmic reticulum and other membranes which connect to other organelles and to membrane and metabolic network systems. This paper brings together these facts presenting the hypothesis that RNA viruses and retroviruses form host/mutualistic virus symbionts as an evolutionary unit with the viral responses to evolutionary stresses being rapid and linking closely with the slower host genomic responses. The hypothesis is presented with a background of evolution of organisms and viruses, drivers of evolution, and the evolutionary natural selection pathway from the sources of stresses to impact and molecular reactions to stresses entering the basic organism body, the cell.