In animals, plants and unicellular eukaryotes, mitochondria perform many tasks such as the Krebs cycle, β-oxidation, and oxidative phosphorylation. The “endosymbiotic hypothesis” proposed by Lynn Margulis suggests that this organelle originated from the relationship of an endosymbiotic bacterium and a host-cell. As their relationship evolved, these organisms faced adverse environmental conditions together, each profiting from the other. During coevolution, the endosymbiont transferred genes to the host thus progressing to obligate intracellular and eventually lost all independence, to become a mitochondrion. This organelle shares diverse traits with modern bacteria proving its origin. In addition, modern endosymbiotic bacteria such as Wolbachia sp. and their hosts are coevolving, becoming more interdependent and expressing different metabolic routes in a process that illustrates the pathway that ancient mitochondrial ancestors may have followed.