Prebiotic organic matter: possible pathways for synthesis in a geological context

Abstract

The variety of pathways for synthesis and chemical evolution of organic matter involved in the origins of life on the primitive Earth must have been constrained by the early geophysical-geochemical evolution of the planet that resulted from accretion and core formation. These processes would have strongly influenced the temperature and composition of the atmosphere, surface and interior over time. Yet large gaps persist in our understanding of how Earth accreted and differentiated into the core-mantle-crust-atmosphere system. Moreover, major uncertainties exist about the time-span over which Earth acquired its volatiles, the elemental abundances in its volatiles inventory, and the conditions under which outgassing of these volatiles occurred to form the primitive atmosphere. Within the context of existing models for Earth accretion and early evolution these uncertainties allow a wide range of possible prebiotic atmospheric compositions at the time and temperature when liquid water appeared and thermally-labile organic compounds could survive. These compositions range from strongly reducing, dominated by high abundances of H 2. CO and CH 4, to mildly reducing, containing mostly N 2 with minor to trace amounts of CO 2 and H 2. Laboratory experiments indicate that prebiotic synthesis of organic matter would have occurred readily in strongly reducing atmospheres. The possibilities in mildly or non-reducing atmospheres may have been more limited, but they merit much more study than they have received. The conversion of N 2 to nitrogen-containing organic compounds in any prebiotic atmosphere could have been difficult to achieve by atmospheric photochemical processes; electric discharges or other processes may have been more effective. Prebiotic organic syntheses need not have occurred only in the atmosphere; they could have occurred on land, in the seas, and at the interfaces between atmosphere, seas, and land surfaces. The involvement of inorganic matter in the origin of life is expected to have been a natural consequence of the geological context within which atmospheric and organic chemical evolution occurred. Metal ions and minerals, particularly clays, could have served as reactants, catalysts, and even templates for prebiotic organic synthesis. Thus far, considerable success has been achieved in producing the monomeric and oligomeric building blocks of present day proteins and nucleic acids in putative prebiotic syntheses. But the connections between the model environmental conditions implicit in many of the syntheses and the geologic and meteorologic realities of the prebiotic Earth may be tenuous and remain to be established clearly. Until tighter constraints can be imposed on the range of possible prebiotic atmospheric compositions and surface environments, and in the absence of direct evidence of terrestrial organic chemical evolution, it is important to explore and assess pathways for organic synthesis in all model environments that continue to be consistent with evidence unveiled in the cosmochemical, geological and biological records.