The Theme of Evolution in Microorganisms: Viruses
Viruses are interesting to consider in the model of evolution. When most scientists talk about evolution, they usually refer to living organisms, though it is hotly debated whether viruses are in fact living organisms (it's one of those nerdy arguments, like whether Dick York or Dick Sargeant was better in Bewitched).
The currently popular theory on how life originated was there was a pool of "proto-cells" with self-replicating RNA molecules surrounded by a lipid membrane. It is possible that the first viruses originated at this time, as a parasitic protocell that took the activity of the self-replicating RNA to replicate its own RNA. It is conceivable that while life diversified and became more complex, the same happened with these parasitic protocells. Though, because they became parasitic, they did not require developing the same level of complexity as actual living organisms.
This is why viruses appear to be so basic, including maintaining an RNA genome. Because they have always relied on a host to do their dirty work, they have never actually had a chance to develop. This is why if you want something done right, you gotta do it yourself.