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Endosymbiosis refers to a symbiotic association in which one organism lives within the other. In the process,one cell engulfs another cell to generate with time a coevolved association in which neither cell could thrive alone.

Theenergy-producing organelles within theeukaryotic cells like mitochondria and chloroplasts originated via the endosymbiotic events. The nuclear genome of eukaryotes is most commonly associated with the Archaea, therefore, it may have been an initial archaean, which may have engulfed the cell of a bacteria, which evolved into a mitochondrion.

Chloroplasts would have originated as acyanobacterium, while mitochondria seem to have originated from an alpha-proteobacterium.

There is also some proof of secondary endosymbiotic events. Other components of the cell would have also originated from the endosymbiotic events.