Where did viruses (biological) originate from?
It is very quickly apparent from sequence studies that there can have been no single origin of viruses as organisms. For instance, there is no obvious way one can relate viruses of the size and complexity of the Poxviridae [double-stranded linear DNA,130-375 kb, 150-300 genes] with viruses like the tobamoviruses [ss linear RNA, 6-7 kb, 4 genes], or either of these with the Geminiviridae [ss circular DNA, 2.7 – 5.4 kb, 3-7 genes]. Thus, there can be no simple “family tree” for viruses; rather, their evolutionary descent must resemble a number of scattered “bushes”. Viruses as a class of organism must be therefore be considered to be polyphyletic in origins: that is, having a number of independent origins, almost certainly at different times, usually from cellular organisms. What they have in common is a role as the ultimate “stripped-down” parasites: organisms which can only undergo a life cycle inside the cells of a host organism, using at the very least the metabolic enzymes and pathw
