One Species' Entire Genome Discovered Inside Another's:

http://www.sciencedaily.com/releases/2007/08/070830150118.htm

Very interesting stuff. We ourselves have mitochondria, another organism, within ourselves and it has been shaping our evolution for a very long time. Anyway, this article suggest basically that other similar situations have been occurring in other bacteria and such, and is really just an interesting read about evolution theory. It's good to wonder how we got here today.