6. In the face of constant competition for the limited amounts of organic matter, certain bacteria (if we may now call them that) found ways to enhance their survival by using metalloporphyrins and similar delocalized ring molecules to absorb solar energy. Perhaps at first the absorbed energy was used only as heat to accelerate all reactions uniformly. Later, this electronic excitation of chlorophyll molecules was coupled to the production of ATP and NADH. Two of the best reducing agents that were available, H2 and H2S, were used to supply reducing equivalents for making NADH. Carbohydrates were synthesized from this ATP, NADH, and atmospheric CO2 by taking some of the reactions of gluconeogenesis and turning them into the Calvin cycle. The stage of the present-day green and purple sulfur bacteria had been reached.

7. Sulfate, although not a substance that would have been present in quantity on the primitive Earth, was given off as a waste product from bacterial photosynthesis. The ancestors of Desulfovibrio developed the ability to squeeze a little more energy out of their foods by oxidizing them with this sulfate. Colonies of green sulfur bacteria and sulfate-respiring bacteria could have existed in close symbiotic association, as they sometimes do today, passing oxidized and reduced sulfur compounds back and forth and drawing their common support from the sun. Respiration had been invented, although not the kind that was to dominate the planet in later years.

8. The slow development of a citric acid cycle as an alternate source of NADH gradually liberated the purple sulfur bacteria from their dependence on H2S and noncyclic photosynthesis. The ancestors of the purple nonsulfur bacteria arose, which were dependent mainly on cyclic photophosphorylation for ATP energy.