Reduced substances are those that can be oxidized by the free dioxygen from the air, so they are reactive and unstable in the presence of air. In the presence

Reduced substances are those that can be oxidized by the free dioxygen from the air, so they are reactive and unstable in the presence of air. In the presence of air, oxidized substances, like silicates and the other abundant components of stones and soil, are the substances that are non-reactive and stable. For many billions of years, even long before the evolution of the kind of phototrophy (a.k.a. photosynthesis) that produces free dioxygen by oxidizing water, the living beings had to continuously produce reduced (and reactive) forms of carbon, nitrogen and sulfur from the oxidized (and non-reactive) forms of carbon, nitrogen and sulfur from the environment. These biological reduction processes have also used solar energy a very long time before the evolution of the variant that produces free oxygen, and before that they have used free dihydrogen, which is produced naturally by the reaction between the partially reduced iron, Fe(II), from volcanic rocks, with water, which oxidizes it to Fe(III), releasing reduced free dihydrogen as a consequence of the reaction. Here the origin of the energy that powers this process is the internal heat of the Earth, because at the higher internal temperatures the substances that are in chemical equilibrium are not the same that are in chemical equilibrium at low temperatures. So when surface rocks are created by volcanism, they are not in chemical equilibrium and the reaction between their reduced components with water can produce the energy that has fed the first forms of life until they have evolved the means for capturing solar energy. Probably the most important development in human technology has been the discovery of how to transform the non-reactive oxidized forms of metals from the environment into reduced forms of metals, which are reactive, therefore they are easily corroded, but they are very useful materials. For many millennia, the reduced metals have been produced with the help of another reduced substance, i.e. charcoal, whose ultimate origin is in the reducing processes by which living beings produce reduced carbon from the oxidized carbon dioxide.