G'day Libra 278, Thank you for your question. According to Wikipedia, the origin of viruses is not clear and one mechanism may not explain them all. "Small viruses with only a few genes may be runaway stretches of nucleic acid originating from the genome of a living organism. Their genetic material could have been derived from transferable genetic elements such as plasmids or transposons, which are prone to moving around, exiting, and entering genomes." "Viruses with larger genomes, such as poxviruses, may have once been small cells which parasitised larger host cells. Over time, genes not required by their parasitic lifestyle would have been lost in a streamlining process known as retrograde-evolution or reverse-evolution. The bacteria Rickettsia and Chlamydia are living cells that, like viruses, can only reproduce inside host cells. They lend credence to the streamlining hypothesis, as their parasitic lifestyle is likely to have caused the loss of genes that enabled them to survive outside a host cell." I have attached sources for your reference. Regards Reference Wikipedia Virus http://en.wikipedia.org/wiki/Virus Origins of Virus http://www.mcb.uct.ac.za/tutorial/virorig.html Answers.com Viral evolution http://www.answers.com/topic/virus-evolution Bloomberg http://www.bloomberg.com/apps/news?pid=20601086&sid=a6S3ZQwqZkS4&refer=latin_america

Three different theories have been proposed to explain the origin of virues. The first is the regressive theory of virus origins. This proposes that viruses originated from free-living organisms like bacteria that have progressively lost genetic information to the point where they become intracellular parasites dependent upon their hosts to supply the functions they have lost. The second theory proposes that viruses originated from the host cell DNA or RNA molecules, which gain a self-replicative but parasitic existence. One or few genes, or the mRNA from one or few genes acquired the ability to replicate and evolve (change its neuclotide sequences or organization) independantly of its host cell. The third theory proposes that viruses originated and evolved along with the most primitive molecules that first contained self-replicating abilities . While some of the molecules were eventually collected into units of organization and duplication termed cells, other molecules were packaged onto virus particles that co-evolved with cells and parasitized them.