The diverse effects of side reactions on the persistence of autocatalytic cycles.

A key criticism of metabolism-first theories of abiogenesis is that, in the absence of specialized catalysts, side reactions would convert too many autocatalysts to inert by-products for autocatalytic systems to persist. Here, we use algebraic analysis and numerical simulations to explore the effects of side reactions on the persistence of simple autocatalytic systems. For an autocatalytic reaction coupled to a side reaction, whenever a component reaction is strictly irreversible, which is technically impossible but frequently considered an acceptable approximation, side reactions harm autocatalytic systems. But if all reactions are assumed reversible, perhaps with unequal forward and reverse rate constants, side reactions can have a wide range of effects-they can be harmful, neutral, beneficial, or harmful in the short term but beneficial in the longer term. These effects arise in the presence of reversible reactions, because side products can also serve as a reservoir from which autocatalysts can be re-derived. We then use algebraic analysis to confirm that such diverse effects also apply to a larger autocatalytic cycle coupled to more side reactions. Our results show that the effects of side reactions on autocatalytic systems do not provide sufficient grounds to reject metabolism-first models.