It is argued that nitrogenase originated monophyletically in obligate anaerobes similar to Clostridia. The enzyme system was later inherited, without much change, by photosynthetic bacteria, by prokaryotic plants (blue-greens) and by aerobic bacteria. The hydrogenase function of the enzyme complex preceded the nitrogenase function, and was useful in hydrogen fermentations. The consumption of ATP served to assure disposal of electrons in the form of hydrogen gas. The present need of the enzyme system, whether acting as a hydrogenase or as a nitrogenase, for ATP may be a relic from the period when the biosphere was still reducing.