Able to swap and repair genes, some microbes evolve ways to tolerate oxygen by forming protective enzymes. These enzymes react with dangerous radicals produced by oxygen, converting them to innocuous compounds.
Other microbes develop a radical approach to oxygen, which both protects them and provides a powerful new means of energy transformation. They consume the oxygen produced by photosynthesis. Aerobic respiration commences: controlled combustion breaks down organic molecules for energy and gives off energy-poor carbon dioxide and water.
This innovation energizes life. Fermenting a single sugar molecule produces two molecules of ATP, the primary energy carrier for cell metabolism and motility. Processing the same sugar molecule through respiration yields as many as 36 energizing ATP molecules.