One of the most striking aspects of the energy-extracting and -storing machinery of living creatures is its universality. Some processes are shared by all forms of life, and we can assume that this is because they are very old components of a common metabolic heritage of related organisms. Other reactions and processes are possessed only by one branch or another of the living family, and by peeling these layers of metabolism back and looking for similarities to other reactions, we may be able to decide how the chemical machinery that we see today first evolved.

The first living organisms probably were ATP-using, judging from the universality that ATP holds as a short-term energy-storage molecule. We can imagine primitive one-celled creatures evolving glycolysis to make more ATP when competition had depleted the natural supply. This may or may not be true, but it is plausible. In any event, glycolysis as a means of extracting energy from glucose proved so beneficial that it, too, became fixed in the chemistry of life. Some bacteria, such as the strictly anaerobic Clostridia, never progressed beyond this stage, and are found today fermenting in anaerobic pockets of our world, away from the oxygen gas that is deadly to them, although it is the breath of life to most organisms.

Photosynthesis broke the dependence on the environment for high-free-energy molecules. Bacteria that could absorb light energy and use it to make their own glucose henceforth were freed from the constraints of a scavenging existence. They trapped light with chlorophyll and took hydrogen from H₂S to make NADH and ATP, and used these products of the light reactions to drive a dark-reaction synthesis of glucose, with the aid of a set of reactions that look very much like glycolysis in reverse. But by turning gluconeogenesis into a cyclic process involving a five-carbon sugar as a "carrier" molecule, these bacteria found a way to begin the synthesis at the one-carbon C0₂ stage, instead of the three-carbon pyruvate stage that was the starting point of the older mechanism.

The blue-green algae changed the light reactions of photosynthesis from a one-photocenter process that uses a good but scarce hydrogen donor, H₂S, into a two-photocenter process that uses a poor but exceedingly common donor, water. This increased by many times the amount of life this planet could support. The oxygen that this kind of photosynthesis released is believed to have permanently changed the character of the atmosphere of the planet. We will come back to this important point in Chapter 26 when we discuss the origin of life on Earth.