Mitochondria and chloroplasts are thought to have once been free-living bacteria that invaded another cell.

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Quizplus · Accepted Answer

This is known as the endosymbiotic theory and is widely accepted in the scientific community. Both mitochondria and chloroplasts have their own DNA and ribosomes, similar to bacterial cells. They also have a double membrane, which is consistent with the idea that they were engulfed by a larger cell.

Mitochondria and Chloroplasts Are Thought to Have Once Been Free-Living