We have seen up to this point how the atmosphere of the Earth originated and we have vaguely mentioned that water was liberated from the impacts of planetesimals and may be comets creating a steam atmosphere that with the cooling of the planet resulted in a liquid mass, the oceans. We need now to know some proof of this theory together with the knowledge of the chemical evolution of the oceans, how they acquired and maintained the salinity. Oceans probably contributed also to the change in the composition of the atmosphere (i.e., the rise of oxygen) and finally to the origin of life.