The theory of endosymbiosis claims that millions of years ago, aerobic bacteria were taken inside of anaerobic cells through endophagocytosis. The aerobic bacteria then fed on the half-digested food molecules in the cell's cytoplasm and gained a great amount of energy, which, in turn, benefited the host cell. Since both organisms benefited, they formed an endosymbiotic relationship, the bacteria evolved into organelles, such as mitochondria and chloroplasts, and the aerobic cells evolved into eukaryotic cells. Which of the following can be cited as evidence for the theory of endosymbiosis?