Autocatalysis Before Enzymes: The Emergence of Prebiotic Chain Reactions

How could complex, enzymeor ribozyme-like molecules first have arisen on planet Earth? Several authors have suggested autocatalytic cycles as a partial answer to this question, since such reactions exhibit the life-like property of exponential growth while being composed of relatively simple molecules. However, a question remains as to the likelihood of an autocatalytic cycle forming spontaneously in the absence of highly specific catalysts. Here we show that such cycles form readily in a very simple model that includes no direct catalysis reactions. Catalytic effects nevertheless emerge as properties of the reaction network. This suggests that the conditions for the formation of such cycles are not difficult to achieve. The resulting cycles solve the problem of specificity not by being small and simple but by being large and complicated, suggesting that early prebiotic metabolisms could have been extremely complex. We predict that this phenomenon can be reproduced in wet chemistry. We discuss the challenges involved in this, as well as the implications for how we view the origins of life.