The endosymbiotic theory argues that mitochondria, plastids (e.g. chloroplasts), and possibly other organelles of eukaryotic cells, originate through symbiosis between multiple microorganisms. According to this theory, certain organelles originated as free-living bacteria that were taken inside another cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales, the SAR11 clade, or close relatives) and chloroplasts from cyanobacteria.
Read more about Endosymbiotic Theory: History, From Endosymbionts To Organelles, Evidence, Secondary Endosymbiosis, Extensions
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“It makes no sense to say what the objects of a theory are,
beyond saying how to interpret or reinterpret that theory in another.”
—Willard Van Orman Quine (b. 1908)