This is not exactly physics, but I'm interested in a physicists opinion: Oxygen is considered a very active substance, very dangerous to many physiological processes of the cell. Even though or maybe as a result of this fact, we know that all life on earth thrives on reach in oxygen atmospheres. Aerobic surroundings are inhabited by forms of the simplest bacteria, Why is this so? Can complex organisms evolve in oxygen lacking atmospheres, or is there some rule that prevents this? If so, is there another kind of element that can substitute oxygen in it's role in the evolution of life?