Systems-chemistry approach to prebiotic evolution

The puzzle of the origin of life is grand. A major challenge is to understand the transition from a mixture of molecules into an entity with basic life faculties, such as a protocell, capable of self-replication and inheritance. Two major schools tackle this problem: the genetic, or replicator-first approach, and the metabolism-first approach. The replicator-first approach focuses on a single self-perpetuating informational biopolymer, e.g., RNA, as the first step, and it is thus often referred to as the “RNA world”. In contrast, the metabolism-first approach focuses on a network of chemical reactions among simpler chemical components that became endowed with some reproductive characteristics as the first step that led to a protocell. The lipid world scenario, largely initiated by our laboratory, delineates a specific example of metabolism first. It suggests that spontaneously forming assemblies of relatively simple molecules, such as mutually interacting lipids, that resemble primitive metabolism, are capable of storing and transmitting information similar to sequence-based polymeric RNA, except that in this case it is compositional information that is at work.

This thesis is about further exploration of the lipid world scenario, showing in more detail how a relatively simple chemical system can acquire features such as selection and evolution. This was accomplished by studying dynamical aspects of the graded autocatalysis replication domain (GARD) computer-simulation lipid world model, previously developed at our laboratory. GARD simulates the homeostatic growth of a compositional amphiphile assembly by reversible accretion from a buffered heterogeneous external pool. This process is governed by a network of mutually catalytic reactions, and exhibits quasi-stationary compositional states termed compotype, that may be regarded as GARD species.

I have demonstrated that that such GARD species exhibit positive as well as negative selection, an important prerequisite of a minimally living system. I further showed that when the catalytic network becomes dominated by mutual catalysis, as opposed to self-catalysis, selection is enhanced. When studying the dynamics of large populations of GARD assemblies under constant population conditions, I rewardingly found that they exhibit dynamics similar to natural ecosystem populations, e.g. similes of competition or predator-prey dynamics. I was able to establish relationships between a compotype’s internal molecular parameters (e.g. its molecular diversity) and population ecology behavior. In a separate vein, I have developed a new approach towards observing open-ended evolution, which enables asking whether there is an optimal level of open endedness in prebiotic evolution. Finally, I was able to show clear similarities between GARD compotypes and quasispecies in the Eigen-Schuster model for evolution, further underlining GARD’s capacity as an alternative to RNA World. Taken together, these results uncover quantitative aspects of the GARD model which in turn contribute towards our understanding of the origin of life via the lipid world scenario.