Earth's early history may have been characterized by coevolution of microbial metabolism and atmospheric composition. Metabolic developments affected the composition of the atmosphere and the resultant changes in the atmosphere stimulated the evolution of new metabolic capabilities. The first organisms were presumably fermenting heterotrophs, exploiting organic molecules abiotically synthesized. These organisms multiplied, developing new biosynthetic capabilities to overcome deficiencies in the abiotic supply of particular compounds, until their growth was limited by the energy source provided by abiotic synthesis of fermentable organic compounds. Further growth required a new energy source, which may have been the chemical energy represented by the mixture of carbon dioxide and hydrogen in the primitive atmosphere. Chemotrophic organisms resembling methane bacteria may have evolved to exploit this source. They would have flourished, along with the heterotrophs that fed on them, until they had decreased the level of atmospheric hydrogen to the point where further extractions of chemical energy from the atmosphere was not possible. Once again, the expansion of life was limited by the availability of energy. The origin of bacterial photosynthesis overcame the second energy crisis. Photosynthetic bacteria could exploit the abundant energy of sunlight while using atmospheric hydrogen and reduced compounds derived from it only as electron donors. Life flourished again, drawing atmospheric hydrogen (replenished only by volcanoes) down to levels so low as to limit even bacterial photosynthesis. Before the full potential of photosynthesis could be exploited the evolution of the metabolic apparatus to process an electron donor of unlimited abundance was necessary. This donor, of course, was water, and the new metabolic process was algal photosynthesis. The oxygen released changed the world from anaerobic to aerobic and made possible the last great advance in energy-yielding metabolism, aerobic respiration.