Surface solar ultraviolet radiation for paleoatmospheric levels of oxygen and ozone

A hypothesis on the role of pressure in the origin of life

The necessity for long time spans in models on the origin of life leads to a major difficulty in that under the environmental conditions existing today biological macromolecules are inherently unstable. The present hypothesis suggests that life arose under a set of environmental conditions whereby polymerization was thermodynamically favored. In particular, increased pressure when coupled with low water activity and high temperature should stabilize polymer bond formation. Three implications of this pressure stabilization theory are presented: (1) The necessary conditions for stabilization are similar to some of the ecological niches occupied by representatives of the archaebacteria. It is suggested that the harsh and unusual habitats of the archaebacteria reflect in part prebiotic environmental conditions. (2) Biological optical activity would be generated if, for instance, L-L peptide bonds were stabilized to a greater degree than L-D peptide bonds. This type of selective stabilization would provide for the maintenance of molecular asymmetry as well as the creation of molecular asymmetry. (3) Conditions necessary for generating the requisite pressure may concurrently have provided protection from prebiotic ultraviolet radiation.

The photochemistry of the paleoatmosphere

The ideas of Harold Urey on the origin and evolution of the atmosphere have dominated thinking in this area for 3 decades. Recent progress in this area is reviewed, with particular emphasis on photochemical modeling studies of atmospheric evolution. Research into the paleoatmosphere can be divided into 3 distinct areas: (1) The photochemistry/chemistry of the prebiological paleoatmosphere, (2) the evolution of oxygen and the transition to an oxidizing atmosphere, and (3) the origin and evolution of ozone. Photochemical calculations indicate that the stability of a heavily reducing paleoatmosphere of CH4--NH3 was extremely shortlived, if such a prebiological atmosphere ever existed at all. A more mildly reducing early atmosphere of CO2--N2 is favored by photochemical considerations. Recent calculations of O2 in the prebiological paleoatmosphere vary from less than 10(-14) of present atmospheric level (PAL) to 10(-1) PAL. Clearly, additional work is indicated. The evolution of O3 as a function of O2 level has been investigated with increasingly detailed photochemical models that have included the photochemistry/chemistry of the oxygen, hydrogen, nitrogen, carbon, and chlorine species, as well as the effects of eddy transport, the rainout of water-soluble species, dry deposition and lightning as a source of trace atmospheric gases.