On the prebiotic role of iron and sulfur.

Thermodynamic data (Sillen, L. G.: 1966, Arkiv Kemi, 25, 159) indicate that there is little support for the idea that metabolic sulfur cycles were involved in prebiotic and early life processes, since under reducing conditions the 'equilibrium' concentration of sulfur in the primordial seas should have been very low, < 10(-8) M. However, it is suggested that metabolic sulfur cycles may have become important when oxygen evolved, when iron(II) ions disappeared from the seas, and when large amounts of sulfur were released from their iron sulfide sources.