This study adopts the basic premise that the Earth was without a primordial atmosphere and that its secondary atmosphere has arisen primarily from local heating and volcanic action associated with continent building. Since no oxygen can be derived in this way, the initial formation of oxygen from photochemical dissociation of water vapor is found to provide the primitive oxygen in the atmosphere. Because of the Urey self-regulation of this process by shielding H2O vapor with O2, O3, and CO2, primitive oxygen levels cannot exceed O2∼0.001 present atmospheric level (P.A.L.). The analysis of photochemistry of the atmospheric constituents is made possible by measurements of solar radiation with space vehicles and the now excellent data on uv absorption. The rates of oxidation of lithospheric materials are examined in this primitive atmosphere and, because of active species of oxygen present, found adequate to make unnecessary the usual assumption of high oxygenic levels in the pre-Cambrian eras to account for such lithospheric oxides. The appearance of an oxygenic atmosphere awaits a rate of production that exceeds O2 photodissociation and loss. The rise of oxygen from the primitive levels can only be associated with photosynthetic activity, which in turn depends upon the range of ecologic conditions at any period. Throughout the pre-Cambrian, lethal quantities of uv will penetrate to 5 or 10 meters depth in water. This limits the origin and early evolution of life to benthic organisms in shallow pools, small lakes or protected shallow seas where excessive convection does not bring life too close to the surface, and yet where it can receive a maximum of non-lethal but attenuated sunlight. Life cannot exist in the oceans generally and pelagic organisms are forbidden. Atmospheric oxygen cannot rise significantly until continental extensions and climatic circumstances combine to achieve the necessary extent of this protected photosynthesis, over an area estimated at 1 to 10 per cent of present continental areas. When oxygen passes ∼0.01 P.A.L., the ocean surfaces are sufficiently shadowed to permit widespread extension of life to the entire hydrosphere. Likewise, a variety of other biological opportunities arising from the metabolic potentials of respiration are opened to major evolutionary modification when oxygenic concentration rises to this level. Therefore, this oxygenic level is specified as the “first critical level” which is identified by immediate inference with the explosive evolutionary advances of the Cambrian period (−600 m.y.). The consequent rate of oxygen production is expedited. When oxygen passes 0.1 P.A.L., the land surfaces are sufficiently shadowed from lethal uv to permit spread of life to dry land. This oxygenic level is specified as the “second critical level” and by immediate inference is identified with the appearance and explosive spread of evolutionary organisms on the land at the end of the Silurian (−420 m.y.). Subsequently, oxygen must have risen rapidly to the Carboniferous. Because of the phase lag in the process of decay, the change of atmospheric oxygen may have fluctuated as a damped saw-toothed oscillation through late Paleozoic, Mesozoic, and even Cenozoic times in arriving at the present quasi-permanent level. The physical and evolutionary evidence concerning the development of the Earth appears fully to support such a model, which removes the so-called “puzzle” of the Cambrian evolutionary explosion and of certain subsequent radical evolutionary advances. In particular, consideration of the rise of oxygen permits a view of the history of the Earth in a rather new and more advanced perspective. It seems reasonable that the atmosphere of Mars appears similar to the above model of the Earth's in its primitive state. Life on Mars would therefore be subject to the same restrictive ecology as existed on Earth during Archaeozoic or early Proterozoic eras.