I know that evolution doesn't have a goal. I may have antropomorphosized it too much, but my goal was to follow a sort of temporal/casual trail that we find interesting.

The way I imagine my idea is that, if you pick a species and its interesting descendants, and order on a timeline like such:

  A --> B --> C --> D

What really happens is more like:

  [bunch of other spinoffs]
    ^
    |
  A --> B --> ...

And that some of those spinoff could be viruses. Imagine a mutation to A causes a bunch of functionality in the organism to shut down (and in subsequent generations disappear), while leaving a viable organism. For a sufficiently uncomplicated A, the result of such mutation could be a bundle of genes wrapped in a shell with an injection mechanism. That's a virus.