Evolution of the first metabolic cycles.

TLDR

The origin of life may be heterotrophic or autotrophic, and the key challenge for an autotrophic origin is the initial carbon‑fixation step. I propose that the first carbon‑fixation process is an autocatalytic cycle derived from the reductive citric acid cycle by replacing thioesters with thioacids and using pyrite formation for reducing power. The proposed cycle is strictly chemoautotrophic, catalyzes pyrite formation, and is autocatalytic for its own multiplication, being constructed from the reductive citric acid cycle by replacing thioesters with thioacids. The cycle cannot exist in isolation but must be part of a network of concatenated homologous cycles, from which all anabolic pathways appear to have sprung.

Abstract

There are two alternatives concerning the origin of life: the origin may be heterotrophic or autotrophic. The central problem within the theory of an autotrophic origin is the first process of carbon fixation. I here propose the hypothesis that this process is an autocatalytic cycle that can be retrodictively constructed from the extant reductive citric acid cycle by replacing thioesters by thioacids and by assuming that the required reducing power is obtained from the oxidative formation of pyrite (FeS2). This archaic cycle is strictly chemoautotrophic: photoautotrophy is not required. The cycle is catalytic for pyrite formation and autocatalytic for its own multiplication. It is a consequence of this hypothesis that the postulated cycle cannot exist as a single isolated cycle but must be a member of a network of concatenated homologous cycles, from which all anabolic pathways appear to have sprung.