It must have evolve early on and practitioners of methanogenesis cannot mess with the genes too much, lest they die. Above is shown the use of hydrogen as the source of electrons. They are the only group of microbes that produce a hydrocarbon as major end product of their metabolism. Another group of carbonate reducing microbes are the homoacetogens. They utilize hydrogen as the electron source and use it to reduce CO 2 to acetic acid.
Anaerobic respiration is a common occurrence .
Microbes that carry out this process are common in the environment and their activities greatly influence the global cycling of elements. With the exception of methanogenesis these catabolic capabilities are not associated with any phylogenetic group , but are found scattered among the various groups of bacteria. All of these processes are less efficient that oxidative phosphorylation . This may explain why aerobic organisms dominate the earth. That and the toxic effects of oxygen.