It was carried out the analysis of literature data on free (molecular) oxygen contribution to the processes of arising and evolution of the life. The dual role of oxygen in these processes was demonstrated. The chemical evolution became possible owing to the unoxygen medium and the absence of azone layer. The biological evolution was due to the photosynthetic oxygen, which had allowed the organisms to pass from fermentation process to respiration. The biochemical reactions with the assistance of active forms of oxygen and their contribution in gene mutations was also considered. It was shown the evolution of antioxygen protective systems of the organisms.
