Evolution of the prokaryotes.
Abstract A scheme for the evolution of the prokaryotes is proposed. This is based on the assumptions that (1) the main course of early prokaryotic evolution after the development of an effective genetic apparatus was determined by the problem of generating metabolically usable energy under anerobic conditions and (2) that each of the major metabolic types of anerobic bacteria existing today is mainly composed of surviving representatives of the corresponding stages in prokaryote evolution. In this paper, known species of bacteria are arranged in a postulated phylogenetic sequence which begins with the simplest anerobes and progresses through a series of relatively small, discrete steps to the photosynthetic bacteria and the blue-green algae. The latter, by introducing substantial amounts of oxygen into the biosphere, radically altered the possibilities available for energy production, and led to a second major era of prokaryote evolution with the appearance of the aerobes. Some of the implications of this hypothesis for the phylogeny and taxonomy of the prokaryotes are discussed.