1 Before RNA and After: Geophysical and Geochemical Constraints on Molecular Evolution

Abstract

1. INTRODUCTION This chapter offers a description of some of the physical and chemical settings for the origin of life on Earth. Considering the topic of this book, particular attention is given to the conditions for a precursor RNA World; an ab initio system based on phosphate-sugar backbone structures in linear polymers and currently with nitrogen bases as recognition molecules. Both the appeal and the uncertainties in the assumption of an RNA World are obvious. The biochemical advantage of this model has geochemical and cosmochemical complements such as the abundance in the universe of simple aldehydes, “the sugar of space.” With their relatively high oxidation state, the aldehydes are compatible with plausible models of early terrestrial atmospheres dominated by CO 2 , H 2 O, N 2 , and small mixing fractions of CO, CH 4 and other reductants. Further suggestions of an RNA World come from observed and inferred sources of active oligophosphates and from concentration mechanisms based on the molecular charge conferred by phosphate esters. Finally, the facile formation of ribose phosphate has been successfully modeled under mild aqueous conditions in the laboratory. Obstacles to progress in prebiotic synthesis leading to the inception of an RNA World yet remain. The concentration of neutral molecules such as formaldehyde and glycolaldehyde required to permit the phosphorylation and sugar phosphate formation found in the laboratory remains an impediment to modeling molecular evolution. Furthermore, the oligomerization of nucleosides or nucleotides without the aid of artificial activating groups, and the synthesis and attachment of nitrogen bases in a formaldehyde environment...