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Endosymbiont Endosymbiont The serial endosymbiont theory proposes that mitochondrion and plastids such as chloroplasts of the modern day eukaryotic cell evolved from different types of bacteria that were engulfed by prokaryotes through endophagocytosis. An endosymbiont is any organism that lives within another cell or organism. It is believed that the mitochondrion of today’s organisms was.