The endosymbiotic theory concerns the mitochondria, plastids (e.g. chloroplasts), and possibly other organelles of eukaryotic cells. 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,[1][2] or close relatives) and chloroplasts from cyanobacteria. Enjoy!