Illustration depicting endosymbiosis. Endosymbiosis is a relationship where one organism lives inside another for mutual benefit. The endocytosis of a bacterium (red, Step 1 and 2) is shown before its transformation into mitochondria (red, Step 2). The endosymbiotic theory states that some organelles in today's eukaryotes (cells containing membrane-bound organelles) were once prokaryotes (cells that do not contain membrane-bound organelles) that found their way into the first eukaryotic cells by endocytosis. Mitochondria are thought to have formed when bacteria capable of aerobic respiration were ingested, then survived within the organism and developed a symbiotic relationship.