The place of viruses in the "tree of life".

Ribozymal entry into vesicle containing autocatalytically replicating oligopeptides engendered RNA proliferation and enzyme synthesis within units whose RNA genomes derived from ancestors of viroids. There is good reason to consider the coexistence of proto- or spheroplastic forms of ancient prokaryotes and archaeons. Predecessors of extant mycoplasmavirus L3 or archaeal fuselloviruses could induce cell fusions among these entities. The possibility that the first eukaryotic cells arose consequentially to virally mediated fusions of prokaryotic and archaeal proto- or spheroplasts is presented. Retrotransposons and endogenous retroviruses might have emerged in theropod dinosaurs when Aves evolved; and directed the development of syncytiotrophoblasts in the placentae of the first mammals. As viruses coevolved with their hosts descendants of ancient viruses diverged from one another. Certain phenotypical features could connect extant phages and eukaryotic viruses to common ancestors.