Mitochondrion shows all sings of a regular prokaryotic cell: ringed naked dna, small ribosomes, metabolism.
The endosymbiosis theory is plausible. The double membrane shows that it was indeed a separate cell once. The twists of the inner membrane to maximize the surface area show that there is indeed some sort of exploitation taking place.
The bigger question today is the origin and nature of the nucleus. The nucleus very strangely also has double membrane. Viral origin of nucleus is somewhat plausible. The membrane of a eukaryotic cell is similar to bacterial membrane, not archaeal. This suggests that it was a bacterium cell in the beginning, not an archaeon. The phagocytosis is strictly associated with eukaryotic cell, which suggests that the nucleus appeared before the mitochondrion.
And a You for OP.
>>11198919 
