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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“We have our little theory on all human and divine things. Poetry, the workings of genius itself, which, in all times, with one or another meaning, has been called Inspiration, and held to be mysterious and inscrutable, is no longer without its scientific exposition. The building of the lofty rhyme is like any other masonry or bricklaying: we have theories of its rise, height, decline and fall—which latter, it would seem, is now near, among all people.”
—Thomas Carlyle (1795–1881)