The theory suggests that chloroplasts and mitochondria were once free living organisms and were engulfed by larger cells through Phagocytosis. This is the process by which a larger cell devours a smaller cell. They eventually became functioning parts of the larger cell. The theory is supported by the fact that both mitochondria and chloroplasts have their own membrane systems that is separate from the cellular system and they also both have DNA that is different from that of the nuclear DNA.