Avenues for emergent ecologies

Abstract

In this work, we present some fascinating behaviour emerging from a simple synthetic chemistry model. The results of Ono and Ikegami (2001) demonstrated the spontaneous formation of primitive, self-reproducing cells from a random homogeneous mixture of chemical components. Their model made use of a simple, artificial reaction network. Discrete particles were placed on a triangular lattice and the dynamics consisted of the following particle transitions: translation over one lattice spacing and chemical transformation. The primary particle types were membrane-forming particles, catalysts and water. The membrane particles formed structures akin to lipid bilayers. Their synthesis was stimulated by the catalyst particles, which were also capable of template self-replication using precursors. The system readily exhibits protocell formation from a random initial condition. These protocells form, grow, divide and eventually decay in a continuous cycle. Such emergent dynamics were an illuminating result given that the simulation itself only defines local interactions between particles and a set of physical transition rules. The protocell structures are not explicitly represented or built into the model. Hence it demonstrated a basic physical logic wherein the concepts of self-maintenance and self-reproduction could arise spontaneously from a set of simpler, lower level rules. In essence, it was an in silico realisation of the principle of autopoiesis.

We decided to extend this work by augmenting the particle species repertoire. An additional catalyst was added, which did not stimulate the synthesis of membrane particles, but rather stimulated their decay. It was expected that this would reduce the rate of protocell formation. However a surprising dynamic was uncovered with this new system. As one might expect the protocells did not arise in abundance as in the original model. Instead they formed in small, isolated colonies since this was the only means by which they could avoid the destructive effects of the new catalyst. However because this toxic particle was also autocatalytic (like the other, constructive catalyst), its concentration rose sharply in regions confined by membrane particles since the membranes slowed their outward diffusion. Thus membranes actually created a niche for the toxic catalyst. This in turn produced a predator-prey dynamic with clouds of the toxic particle growing near protocells and protocells being forced to grow in the opposite direction to avoid the destructive effects of the new particle. These results reveal that high level, ecological phenomena can manifest themselves even in simple physico-chemical systems. They demonstrate that ideas of natural selection and fitness are intimately bound with the basic principle of free energy minimisation. We have also now enhanced the model further by adding a second reaction network. It is similar, but independent to the first and allows for two "species" of protocell. It is also possible for hybrids to form, comprised of mixtures of the membrane particles from the two reaction networks. Results from this new version are currently being gathered and analysed

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