A small aerobic prokaryote was engulfed in and lived in a larger anaerobic prokaryote. These prokaryotes provide a way to synthesize energy. This is called endosymbiosis, where "endo," or "endon," means within, "syn" means together, and "biosis" means ways of life. The prokaryote later evolved into modern mitochondria. (For more information, see the event Lynn Margulis' Experiment).