Lynn Magulis in the 1960s introduced the theory of endosymbiosis. Vaious evidence supports this theory that the cell organelles, such as mitochondria and chloroplasts once used be independent living organisms. Both of these organelles have their own DNA. The mitochondria cannot divide to form chloroplasts, and vice versa.
Although, the fact that the mitochondria are present in all the eukaryotic cells, but the chloroplast is present only in some cells, suggests that the mitochondria evolved much earlier than the chloroplasts.