ON A SPECTRUM. “Eukaryogenesis is not necessarily different in kind from other symbiotic alliances that have occurred much more recently in evolutionary history, but it is a very ancient one, affected at a basic cellular level. Accordingly, it receives very high scores along each of the axes of integration. Some (but not all) of the other symbioses have lower scores along some axes.”
http://www.pnas.org/content/112/33/10278.full