Origins of Life: Chemistry and Evolution

The origins of biopolymers embody a central question about the origin of life and the nature of chemical sciences. Two conceptually distinct models, direct chemical synthesis and chemical evolution, have been advanced to explain the prebiotic emergence of RNA and other biopolymers, and they motivate different predictions and experimental approaches. In the direct chemical synthesis model, the inherent reactivities of prebiotic compounds on the ancient Earth led to the formation of RNA, which subsequently enabled Darwinian evolution. This model depends on the prebiotic availability of sugars (and their precursors), nucleic acid bases, amino acids, and other chemical species. In this framework, serial and parallel chemical reactions, governed primarily by the intrinsic properties of small-molecule feedstocks, combine stepwise to yield RNA. In a fundamentally different and more recent model, RNA is assumed to have emerged alongside other biopolymers during a prolonged interval of chemical evolution in which complex, diverse molecular ensembles were subject to dynamic and continually shifting selective pressures. In this evolutionary framework, RNA does not arise directly from simple building blocks in a prebiotic milieu but from iterative cycles of diversity generation, selective retention, and chemical transformation within heterogeneous organic mixtures. This model presages deep biological entanglement among different classes of biomolecules. It remains unknown whether direct chemical synthesis, chemical evolution, or some combination of these processes provides a viable prebiotic path to RNA.