The function of the citric acid cycle is to convert efficiently the energy released during the combustion of acetate into the energy stored in the pyrophosphate bonds of ATP. The cycle is almost twice as efficient as feasible alternatives of acetate combustion, such as a direct pathway via glycollate, glyoxylate, formaldehyde and formate. The reason is that the first stage of acetate degradation cannot be a dehydrogenation; it must be an oxygenation by molecular oxygen. Thus the cycle must have evolved because in a competitive environment the chances of survival are greatest if resources are optimal. Analogous considerations apply to the evolution of other metabolic cycles.