Primaeval pollution

Many billions of years ago, the atmosphere of the Earth was not composed of nitrogen, oxygen and carbon dioxyde as today, but of ammonia, hydrogen and methane. Then, the first algae appeared, and they begun to consume all the carbon dioxyde, and to produce huge amounts of heavily polluting oxygen. Oxygen, in fact, is highly toxic for cells, and probably aging, in all roganisms, is a consequence of the aggression of oxygen to the organic structures of the cell. The ecosystem did not collapse because of exhaustion of carbon dioxyde because heterotrophs begun to consume oxygen and to give back carbon dioxyde. The key heterotrophs are not animals, that in ecosystems are somewhat useless, but fungi. Nowadays, still plants produce oxygen and consume carbon dioxyde, and decomposers, mainly fungi, consume oxygen and give back carbon dioxyde, mantaining a frail equilibrium.