On the Origin and Planetary Distribution of Life

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Abstract
The present paper is a discussion of current opinion and speculation concerning the origin and early history of life on Earth, with particular emphasis on the role that radiation may have played, and with application to the problem of extraterrestrial life. The following general picture is drawn: Organic molecules were produced in high yield by ultraviolet light in the primitive reducing atmospheres of the Earth and other planets. Subsequent heating and irradiation, and solution of the products in water, produced amino acids, sugars, purines, and pyri-midines. If there existed a process for the preferential production or concentration of deoxyribonucleoside triphosphates from these precursors, it is likely that polynucleotides were synthesized in geologically short periods of time, even in the absence of primers or contemporary polymerases. In such an aqueous environment, the synthesized polynucleotides would be self-replicating, and natural selection would have begun. The origin of life can be identified with this event, and can be dated at 4.0 [plus or minus] 0.5 billion years age. It is suggested that an atmospheric window permitted near UV radiation to penetrate to the ocean surface, and that much of the energy for biological synthetic processes in primitive times was obtained from UV-absorbing purines and pyrimidines. In order to avoid lethal or highly mutagenic UV doses, and still accumulate UV-activated molecules before de-excitation, the first organisms must have resided deep in the oceans, at a depth estimated as between 40 and 100 meters. To avoid the mutagenic effects of UV-produced oxidants, such peroxide reducers as catalase must have been selected in early times; and it is suggested that the cytoplasm originally evolved to deal with oxidative mutagens, and with the chemical consequences of the new oxidizing atmosphere. There is no entirely convincing direct evidence for extraterrestrial life, although a variety of observations of Mars strongly points to an indigenous water-dependent hydrocarbon-based microflora. Venus seems much less hospitable, and Jupiter much more hospitable, for life than has been suggested in the past.

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