In the epoch of geochemical energy 3.7 billion years ago, the first living organisms “fed” on molecules like hydrogen and methane that formed in reaction between water and rocks. They wrung energy out of chemical bonds. It was not very efficient—the biosphere’s productivity then was an estimated a thousand to a million times less than it is today.
Sunlight, of course, was shining on Earth all along. When microbes that can harness sunlight finally evolve, the productivity and diversity of the biosphere leveled up. One particular type of bacteria, called cyanobacteria, hits upon a way of harnessing the sun’s energy that makes oxygen (O2) as a byproduct, and with profound consequences: The planet gets an ozone (O3) layer that blocks UV radiation, new minerals through oxygen reactions, and an atmosphere full of highly reactive O2.
Which brings us to the epoch of oxygen. Given an opportunity, oxygen will steal electrons from anything it finds. New oxygen-resistant organisms evolve with enzymes to protect them from oxygen. They have advantages too: Because oxygen is so reactive, it makes the metabolism of these organisms much more efficient. In some conditions, organisms can get 16 times as much energy out of a glucose molecule with the presence of oxygen than without.
With more energy, you can have motion and so in the epoch of flesh, highly mobile animals become abundant. They can fly, swim, ran to catch prey. “Flesh” is source of concentrated energy, rich in fats and protein and carbon.
Then one particular type of animal—those of the genus Homo—figure out fire.