The non-genetic dawn of life: a review of the contemporary standing of the metabolism-first approach to the origin of life enigma
Meijenfeldt, F.A.B. von
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The origin of life on early Earth is unresolved still. Two approaches to the problem dominate the debate. Replicator-first proponents search for the very first replicator molecule. A much envisioned molecule in this regard is RNA. Metabolism-first adherents deem it unlikely that a replicator molecule emerged de novo on early Earth, and instead search for a network of small molecules that could have seeded modern life. Three metabolism-first scenarios are discussed. The elaborate lipid world scenario proclaims the Darwinian selection of early metabolic networks. It is shown that even though persisting entities may have come into existence, heredity in ensemble replicators is probably too inaccurate to allow for their selection, and even if accurate enough inherently attractor based, posing severe problems for their subsequent evolution. The problems are exemplary for scenarios that pose the evolution of metabolic networks. The only demonstration to date of the accumulation of novel mutations in ensemble replicators is provided in a far-off from real chemistry polymer model. The evolutionary dynamics encountered in the model depend heavily on the combinatorial chemistry of polymers, and cannot be generalised with the same success to monomer models. Moreover, many open questions remain. It is concluded that the standing of Darwinian selection in metabolism-first scenarios is problematic. The proposition that life started with the reverse tricarboxylic acid cycle that arose due to geochemical ordering is discussed as an example of a preparatory metabolism theory. As yet, it has not been shown that the cycle arises spontaneously in geothermal environments. The repertoire of catalysis by small organic molecules and mineral surfaces seems to be a crucial factor. A thermodynamic framework for non-equilibrium systems is lacking. However, experiments in hydrothermal reactors might provide the answer: the possibility that part of metabolic complexity is merely the result of thermodynamic ordering makes such experiments worthwhile the effort. It is concluded that the claim that metabolism-first theories provide a robust alternative for replicator-first scenarios cannot be supported yet based on the three hypotheses. Questions posed in the present paper may provide guidance for research to come.