It's not really a hypothesis anymore. Mitochondria have their own DNA so we can put them on a phylogeny. The closest living relatives to mitochondria are http

It's not really a hypothesis anymore. Mitochondria have their own DNA so we can put them on a phylogeny. The closest living relatives to mitochondria are http://en.wikipedia.org/wiki/Alphaproteobacteria http://en.wikipedia.org/wiki/Alphaproteobacteria Similarly, plastids are most closely related to cyanobacteria.