Endosymbiont

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An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically, the two organisms are in a mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), which live in the root nodules of legumes, single-cell algae inside reef-building corals, and bacterial endosymbionts that provide essential nutrients to insects. According to the theory of symbiogenesis, endosymbiosis played key roles in the development of eukaryotes. Roughly 2.3 billion years ago an archaeon (likely within the "Asgard" superphylum) absorbed an alphaproteobacterium through phagocytosis, that eventually became the mitochondria that provide energy to almost all living eukaryotic cells. Approximately 1 billion years ago, some of those cells absorbed cyanobacteria that eventually became chloroplasts, organelles that produce energy from sunlight. Approximately 100 million years ago, a lineage of amoeba in the genus Paulinella independently engulfed a cyanobacterium...

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