Evolution of the virus world: origin of the main lineages from the primordial gene pool. Characteristic images of RNA and protein structures are shown for each postulated stage of evolution, and characteristic virion images are shown for the emerging classes of viruses. Thin arrows show the postulated movement of genetic pools between inorganic compartments. Block arrows show the origin of different classes of viruses at different stages of pre-cellular evolution.
Koonin et al. Biology Direct 2006 1:29 doi:10.1186/1745-6150-1-29