There are viruses that enter a host, merge with its DNA, replicate along with it for generations just like "normal" DNA. Eventually, it can "wake up", cut itself out of the host genes, and start replicating as a virus again.
Some of our DNA is apparently made out of viruses that did this and got stuck.
The model that emerges from consideration of recent discoveries is something like, bacteria and viruses form a single global organism that is the primary resident of this planet (by mass, by throughput, etc.)
http://www.press.uchicago.edu/ucp/books/book/chicago/P/bo222...
There are viruses that enter a host, merge with its DNA, replicate along with it for generations just like "normal" DNA. Eventually, it can "wake up", cut itself out of the host genes, and start replicating as a virus again.
Some of our DNA is apparently made out of viruses that did this and got stuck.
The model that emerges from consideration of recent discoveries is something like, bacteria and viruses form a single global organism that is the primary resident of this planet (by mass, by throughput, etc.)