9. Close relatives of these photosynthetic bacteria found a way, via a second chlorophyll photocenter, to absorb two photons of light where one had been absorbed before, and to use the extra energy to make an acceptable reducing agent out of H2O. Instead of abandoning a scarce reducing agent, H2S, they managed to trade it for a much more plentiful one, H2O. In these ancestors of the blue-green algae, green-plant photosynthesis was born. This step may have been reached as early as 3 billion years ago.

10. Oxygen began to accumulate locally around these photosynthetic organisms. They and the purple nonsulfur bacteria learned to use O2 with NADH from their citric acid cycle to obtain much more energy than ever before. The sequence of glycolysis-citric acid cycle-respiration, familiar in eucaryotes today, was complete. As today, blue-green algae and purple nonsulfur bacteria made relatively little use of respiration, depending mainly on photosynthesis for ATP energy, but the facility was there. Oxygen respiratior need not have required more than local concentrations of free O2, just as the earlier sulfate respiration would have required only local concentrations of sulfate around green and purple sulfur bacteria.

11. The great efficiency of water-splitting photosynthesis led to an explosion of life on the planet, and this may be why we see fossil remains for the first time in the Fig Tree cherts. With oxygen respiration still of minor importance, excess O2 gradually accumulated in the atmosphere, changing it slowly from reducing to oxidizing. This development had three important consequences for the future evolution of life. An ozone shield in the upper atmosphere blocked off the shorter ultraviolet wavelengths, thereby ending one source of nonbiological synthesis of organic molecules as possible foods for living organisms. Free oxygen in the atmosphere hastened the destruction of those organic molecules that already had been synthesized, with the result that for all time to come, organic compounds would be associated almost entirely with living organisms. Lastly, with the lethal ultraviolet radiation screened out, life could come up from the lower depths to inhabit the upper ten meters of the seas and, eventually, the land itself.