Prebiotic Oligomer Assembly: What Was the Energy Source?

Polymerization of nucleotides and amino acids to form large, complex, and potentially functional products was an early and essential event on the paths leading to life's origin. The standard Gibbs energies of the condensation reactions are uphill, however, and at equilibrium will yield only declining sequences of small, nonfunctional oligomers. Geochemically produced condensing agents such as carbonyl sulfide, cyanamide, and polyphosphates have been proposed to invert the unfavorable condensation Gibbs energies and thereby activate exergonic condensation. We argue, however, that although activators may provide modest yields of oligomers, the inherently episodic nature of their sources throttles their effectiveness, and the fundamental hydrolytic instabilities of oligonucleotides and peptides ultimately prevail to yield decreasing product sequences. Notably, the Gibbs energy governing oligomer formation is antientropic. Accordingly, we propose that declining progression can be surmounted in evaporating pools in which a favorable entropy change is produced when high surface/volume ratios concentrate reactants at the air/water interface in continuous cycles of wetting and drying. The severely reduced configurational freedom of the solutes then inverts the antientropic nature of the condensation reactions, pivoting them to exergonic states and thus to the production of ascending sequences of complex polymeric products.