Thermodynamic and kinetic selection in evolving chemical mixtures

Complex or even relatively simple mixtures undergoing chemical transformations tend to combinatorically explode, i.e., a large number of different chemical species arise due to the large number of ways to combine them. The rise of chemical selectivity was one of the most important steps towards life and its emergence presents one of the most challenging questions in the origins of life research. Nevertheless, recent empirical work has shown that under some conditions, combinatorial compression, i.e., a reduced number of species compared to that expected by combinatorics, is observed. The mechanisms underlying the observed compression in the chemical space are yet to be elucidated. In this paper we combined thermodynamic and kinetic theory together with computer simulations to track the evolution of species (i.e., changes in concentrations) under a wide range of parameter scenarios. We have studied and defined a set of rules that are required for compression: (i) chemical connectivity, (ii) thermodynamic or kinetic dominance, (iii) continuous feeding of the ‘compressor’, and (iv) appropriate temperature or reaction time. Our results shed new light on the way in which chemical evolution operates at the very fundamental level and can guide future experiments of chemical evolution towards generation of chemical spaces that can potentially self-maintain high reactivity and open-ended evolution.