Atmospheric Ozone and the History of Life

Abstract

Biological evolution, in its early stages, may have been constrained by the solar ultraviolet radiation that would have penetrated to the ground if the atmosphere at this time contained little or no oxygen and therefore little or no ozone. In order to provide a more quantitative basis for the examination of this possibility we have used a simple photochemical model to evaluate ozone densities in atmospheres resembling the present atmosphere in most respects, but containing less oxygen. As the amount of oxygen in the atmosphere is reduced, we find that the ozone column density initially increases. Only when the amount of oxygen in the atmosphere falls below 10⁻¹ of its present level does the ozone column density begin to decrease. Our results suggest that very little atmospheric oxygen (possibly 10⁻³ of the present amount) is required to produce a biologically effective ozone screen.