Describe how some cells came to have chloroplasts, according to the theory of endosymbiosis.

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!

According to the Theory of Endosymbyosis chloroplasts were "primitive" cells that formed a symbiotic relationship with other prokaryotic cells. Some cells have chloroplasts because it was believed by "compelling evidence" that the organelles of chloroplasts could undertake photosynthesis.