The precellular scenario of genovirions.

A new hypothesis of the ancestors of contemporary viruses, the genovirions is proposed. This concept emphasizes the close connection between the evolution of viruses and the proto-cells. Recent and mounting evidences from comparative genomics indicate that both RNA and DNA viruses evolved from primordial genetic elements before proto-cells existed. The most persuasive discoveries that challenge the conventional virus concept include the giant virusphage Mimivirus, the insect polydnaviruses, some unusual archaeal viruses, and the large phycodnaviruses. The existence of several viral genes central to replication and structure shared with many viruses but not present in cellular genomes indicates that once an ancient virus era existed. As viruses are the only quasi-living beings that use RNA as genetic information and storage carrier, their ancestors were witnesses of the RNA world. The ancient virus scenario existed at the time of the RNA world before proto-cells developed hence these genetic elements were not cell parasites. Later on, retroviruses were responsible for the transition from the RNA to the DNA world.