This observation can be explained by the theory of endosymbiosis. From what scientists have discovered, it can be explained that the double membrane of both chloroplast and mitochondria are as a result of them been subjected to phagocytosis but they were not digested. Their DNA suggest that they were once free living prokaryotes. This agree with the endosymbiosis theory which states that eukaryotic cell organelles developed from prokaryotic cells.
