17 Origin of life by metabolic avalanche breakthrough in an iron–sulfur world

Abstract

Origin-of-life research is polarized on two levels. On the methodological level, we distinguish two heuristics: (a) the conventional heuristic of parallel back extrapolations into the chaos of a prebiotic broth and (b) the heuristic of convergent biochemical retrodiction of a ‘pioneer organism’. On the theoretical level, we distinguish two testable, explanatory theories: (1) The genetics-first RNA world theory postulates an origin by polymer replication, that is, by random aqueous polycondensations in a slowly accumulating, cold prebiotic ocean. The biochemical system is truncated to C–H–N–O–P–Mg. Explanatory power is meek and experiments can do no more than rationalize partial aspects of the low-probability overall scheme. (2) The metabolism-first Fe–S world theory postulates an origin by carbon fixation, that is, by directional aqueous, synthetic redox reactions in a hot volcanic-hydrothermal fluid flow. The reactions are driven by the chemical potential of quenched volcanic gases and catalyzed by transition metal centers of crustal minerals. The full biochemical system C–H–N–O–S–Se–P–Mg–Fe–Co–Ni–W is assumed. Experiments have demonstrated reductive carbon fixation pathways to a wide panel of highly functionalized organic compounds (e.g. thioesters, hydroxyl/amino acids, peptides). The pioneer organism reproduces and evolves by ligand effects of its organic products. Specifically, organic products of a reaction turn into activity-enhancing ligands of catalytic transition metals. By ligand feedback effect, an organic product enhances a catalyst of the pathway, whence it derives (metabolic reproduction). By ligand feed-forward effect, an organic product enhances a catalyst for transforming itself (or another organic reaction product) into a new organic product (metablic innovation = evolution). Any feed-forward effect weakens itself by weakening the pathway, whence it derives, unless it is accompanied by sufficiently strong feedback effects. From the point of view of a pre-established metabolic network, a feedback into a peripheral branch pathway is detrimental by weakening its mother network (virulyst effect). This metabolic virus effect has to be compensated by sufficient positive feedback into the mother network (vitalizer effect). These simple considerations lead us to two consequences: (1) at least some organic products should become enhancing ligands for two or more transition metal catalysts, and (2) at least some transition metal centers should catalyze two or more synthetic steps. These two conditions amount to self-accelerating metabolic expansions (avalanche breakthrough). By these ligand effects, the origin of life in a Fe–S world is a unique, complexity-increasing, intrinsically synthetic, and directional process, preordained in the universal laws of chemistry and definitely knowable.