A proposed bacterial evolutionary scheme based on physiological development



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An evolutionary scheme is proposed for bacteria which is based on physiological development of bioenergetically important electron transport systems. It is proposed, as has been done by others, that the 'most primitive' procaryotes are the obligately anaerobic heterotrophs, whose energy for growth is obtained directly by substrate level phosphorylations, via pre-glycolytic pathways. The origin of this bacterium is unknown, but one can predict its physiological characteristics if the environment can be chemically defined at the time of its origin, and if the physiological types of today are representative of those that existed on primitive earth. As the environmental conditions on earth changed, this physiological-type of organism underwent evolutionary changes, developing more complex physiological processes, which continued until electron-transport phosphorylation processes evolved. The electron transport process, particularly those that are membrane-associated are mandatory for both anoxygenic and oxygenic photosynthesis, as well as for oxidative phosphorylation. Such highly developed electron transport systems contain multiple oxidationreduction carriers, and components that are integrated into the photosynthetic or respiratory electron transport chain, which then allows for efficient energy conservation. As organic nutrients became depleted, and the atmosphere became less reduced, and then oxygenated, those life forms having efficient electron transport phosphorylation systems had more selective and adaptive advantages. How such electron transport systems evolved is discussed, the basic assumptions being that as bacteria evolved, the physiological processes became more complex, and more efficient bioenergetically. This development continued, until the ultimate was reached which today represents the obligate aerobic bacteria having highly evolved, and very active multiple terminal oxidases. The specific physiological types of bacteria for each of the presumed evolutionary stages, are named, and the developmental scheme assumes that organisms today represent survivor types that once were prevelant when different environmental conditions existed on earth. The atmosphere on earth played a major selective role, particularly the appearance of oxygen, in determining the physiological type that evolved. An attempt was also made to show how the proposed evolutionary scheme would fit for Gram-positive and negative bacteria.