Mitochondria and chloroplasts both possess a double membrane and are roughly the same size as anaerobic bacteria, which supports the endosymbiotic theory. They also replicate through binary fission, a method used by bacteria.
This video explains the origin of eukaryotic cells through the theory of serial endosymbiosis. It details how a smaller prokaryote was ingested by a larger prokaryote, surviving digestion and eventually forming a mutually beneficial relationship that led to the development of organelles like mitochondria and chloroplasts. The video also presents evidence supporting this theory.