There is compelling evidence that today's mitochondria and chloroplasts were once primitive bacterial cells. This evidence is described in the endosymbiotic theory. Symbiosis occurs when two different species benefit from living and working together. When one organism actually lives inside the other it's called endosymbiosis. In the 1960's, a number of scientists had proposed that eukaryotic cells evolved when one bacterium (a prokaryote) engulfed another and the two began living together. Over many generations, and through many smaller changes, the engulfed cell evolved into an organelle, like the mitochondrion. According to this idea, mitochondria look and act so much like bacteria because they once were bacteria. All but one statement below is further evidence in support of the endosymbiosis theory. That is