Current thought is that mitochondria were smaller lifeforms first "absorbed" by larger more complex cell (alternatively: the mitochondria "hijacked" the larger cell) and live in symbiosis.
Sounds weird, but also, we have more bacteria living in and on our bodies than we have cells with "our own" DNA.
Makes you wonder where we each really start and stop.
To be honest I'm surprised that looking at this picture the first thing that came to my mind was mitochondria and not amoeba. I don't even remember the last time I had any lessons about biology.