One of the challenges in understanding chemical evolution is the large number of starting organics and environments that were plausible on early Earth. Starting with realistic organic mixtures and using chemical analyses that are not biologically biased, understanding the interplay between organic composition and environment can be approached using statistical analysis. In this work, a mixture of 73 organics was cycled through dehydrating conditions five times, considering environmental parameters of pH, salinity, and rehydration solution. Products were analyzed by HPLC, amide and ester assays, and phosphatase and esterase assays. While all environmental factors were found to influence chemical evolution, salinity was found to play a large role in the evolution of these mixtures, with samples diverging at very high sea salt concentrations. This framework should be expanded and formalized to improve our understanding of abiogenesis.

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.

Some of the most interesting open questions about the origins of life and molecular sciences center on chemical evolution and the spontaneous generation of complex and functional chemical species. The processes that generated the spectacular biopolymers that underlay biology demonstrate an untapped, by modern science, creative potential. We have established a robust, facile, and generally applicable platform for observing and analyzing chemical evolution using complex mixtures. While previous studies have characterized the formation of proto-polymers via chemical reactions, we systematically studied the process itself. We report empirical outcomes that were not foreseen or predicted. We have constructed an experimental platform to study the evolution of chemical systems that: (i) undergoes continuous recursive change with transitions to new chemical spaces while not converging throughout the course of the experiment, (ii) demonstrates chemical selection, during which combinatorial explosion is avoided, (iii) maintains synchronicity of molecular sub-populations, and (iv) harvests environmental energy that is stored in chemical energy. We have established some general guidelines for conducting chemical evolution. Our results suggest that chemical evolution can be adapted to produce a broad array of molecules with novel structures and functions.

The emergence of chemical selectivity presents one of the greatest challenges in the origins-of-life research. Complex or even relatively simple chemical mixtures undergoing chemical transformations tend to combinatorically explode. Large numbers of different chemical products arise because of the large number of ways by which mixtures of reactants can combine. Recent empirical work has shown that, under kinetic control conditions, combinatorial compression, i.e., a reduced numbers of species compared to those expected by combinatorics, can be observed. The mechanisms underlying combinatorial compression are yet to be elucidated. In this paper, we combine transition state theory with computer simulations to track the evolution of chemical species (i.e., changes in concentrations) under a wide range of parameter scenarios. Our study reveals that the experimentally observed combinatorial compression requires (i) chemical connectivity, (ii) kinetic dominance by an especially reactive ‘compressor’, and (iii) appropriate temperatures and reaction times. Our results shed new light on mechanisms of chemical evolution and can guide future experiments.