text: The reaction is a possible candidate for prebiotic early-Earth conditions and, therefore, is of interest in the research of the origin of life. It has been found that some non-consecutive steps of the cycle can be catalyzed by minerals through photochemistry, while entire two and three-step sequences can be promoted by metal ions such as iron (as reducing agents) under acidic conditions. In addition, these organisms that undergo photochemistry can and do utilize the citric acid cycle. However, the conditions are extremely harsh and require 1 M hydrochloric or 1 M sulfuric acid and strong heating at 80–140 °C.
  Along with these possibilities of the rTCA cycle contributing to early life and biomolecules, it is thought that the rTCA cycle could not have been completed without the use of enzymes. The kinetic and thermodynamic parameters of the reduction of highly oxidized species to push the rTCA cycle are seemingly unlikely without the necessary action of biological catalysts known as enzymes. The rate of some of the reactions in the rTCA cycle likely would have been too slow to contribute significantly to the formation of life on Earth without enzymes. Considering the thermodynamics of the rTCA cycle, the increase in Gibbs free energy going from product to reactant would make pyrophosphate an unlikely energy source for the conversion of pyruvate to oxaloacetate as the reaction is too endoergic. However, it is suggested that a nonenzymatic precursor to the Krebs cycle, glyoxylate cycle, and reverse Krebs cycle might have originated, where oxidation and reduction reactions cooperated. The later use of carboxylation utilizing ATP could have given rise to parts of reverse Krebs cycle.  
  It is suggested that the reverse Krebs cycle was incomplete, even in the last universal common ancestor. Many reactions of the reverse Krebs cycle, including thioesterification and hydrolysis, could have been catalyzed by iron-sulfide minerals at deep sea alkaline hydrothermal vent cavities. More recently, aqueous microdroplets have been shown to promote reductive carboxylation reactions in the reverse Krebs cycle.