The mitochondrion is a tiny rod -shaped structure inside the cell. Its main function is to oxidize organic fuels (food) to capture energy in chemical form. This energy is used to drive the life processes of the cell. There are many mitochondria in each cell. Every mitochondrion contains a short ring of DNA that controls the development of some parts of the mitochondrion. The mitochondrion is a very complex membrane-bound organelle in eukaryotic cells and there are no good explanations as to how such a structure might have evolved. The endosymbiosis theory is the most popular evolutionary explanation for the appearance of mitochondria. However, endosymbiosis theory exhibits a lot of assumptions that are not at all reasonable or realistic.