In 1966, american biologist lynn margulis proposed the theory of endosymbiosis, or the idea that mitochondria are the descendants of symbiotic, aerobic eubacteria. what does the theory of endosymbiosis explain?

Its explain why mitochondria in animals and plastids in plants have its own DNA and double membrane. The theory postulates that this is due to a symbiotic relationship that began with the history of life when protists probably engulfed bacteria (most probably by phagocytosis). The bacteria offered energy while it was sheltered. The relationship evolved to become facultative.