Complex life also requires a certain minimum biological support system through the activity of autotrophs, organisms that synthesize organic molecules from simple inorganic matter. For example, photosynthetic algae and some bacteria synthesize food from such inorganic materials as carbon dioxide, nitrogen, methane, hydrogen, and various minerals. These algae and bacteria, and their organic products, then become food for other organisms that require organic food—heterotrophs like us. Some environments might be able to support low-level microbial life, but if it lacks the energy to sustain an abundant autotroph population, it won’t allow for larger, more complex organisms.
Life relies on chemical energy for its immediate metabolic needs, and chemical energy is all about the exchange of electrons. The most energy is released when elements located on opposite ends of the periodic table exchange electrons. Oxygen is second only to fluorine in the amount of chemical energy released when it combines with other elements. Hydrogen, and carbon combined with hydrogen, or hydrocarbons, are the best substances to combine with oxygen. All complex life forms use such oxidation reactions (other common reactions yield far less chemical energy). And not incidentally, the products of oxidation are water and carbon dioxide, the nontoxic and essential components of the climate regulation system. So hydrogen, carbon, and oxygen, together, offer the best source of chemical energy.
Gonzalez, Privileged planet, page 37