Fig. 1

Three major theories of virus origin. Arrows show the direction of evolutionary changes. a. According to the virus-first hypothesis at the dawn of life there were no cellular forms but only first RNA molecules possessing enzymatic activities and capable of self-replication, also called selfish genetic elements. b. According to the escape hypothesis viruses derived from cellular RNA or/and DNA fragments such as plasmids and transpozons. During asymmetrical cell fission a vesicle (smaller cell-like entity) could have formed engulfing a self replicating RNA and a coat encoding RNA segment. c. According to the reduction hypothesis viruses come from small primordial cells (not necessarily primitive), which lost their cellular elements in the course of evolution. They maintained, however, their genetic material and certain elements needed for replication. Proto-cells presented in this picture already contained ribosomes (black small plain circles) and were able to produce proteins/capsids, whereas cells containing a nucleus correspond to modern cells, which descended from LUCA. Eukaryotic cells were used to depict all three hypotheses of virus origin and underline a possible involvement of viruses in eukaryogenesis