The reverse tricarboxylic acid (rTCA) cycle, also known as the reductive citric acid cycle, is an important metabolic pathway found in several anaerobic bacteria and archaea to fix carbon dioxide. This cycle has been proposed as a possible prebiotic pathway for uncatalyzed carbon fixation, owing to both its simplicity and its presence in phylogenetically ancient organisms. However, evolutionary history makes it challenging to infer prebiotic chemical pathways from modern biochemical processes, as these pathways may have been substantially changed.