Hydrogenosomes are not the same. They evolved several times — independently — from mitochondria or the common ancestor of hydrogenosomes and mitochondria. This process, in general, involved the loss of the organellar genome together with the mitochondrial electron transport chain, and metabolic adaptations to anoxic environments such as the use of protons as terminal electron acceptors. Substantial differences in the physiological capacities of the various hydrogenosomes reflect their independent evolution through evolutionary tinkering. Notably, even the common denominator of these organelles, i.e., the production of hydrogen, can become marginal in certain hydrogenosomes.