The Ecopoesis model is a hypothesis about the origin of life on Earth that proposes that the geochemical cycles of biogenic elements (or biogeochemical cycles) preceded the appearance of organisms. The existence of such cycles in a prebiological world implies the presence of strong electron donors (reductants) and acceptors (oxidants), replenished by abiotic reactions in the earth's primitive aqueous environment. Cycling of carbon, nitrogen and sulfur compounds would take place in the oceans, an aqueous environment in contact with a lithosphere rich in reducing ions such as ferrous (divalent) iron and sulphide, and an atmosphere containing substantial amounts of molecular (free) oxygen, generated by the atmospheric photolysis of water vapour. The redox transitions pertaining to living processes would thus be primordial attributes of the nascent ecosphere, determining the nature of materials and energy-yielding processes that would gradually shape the evolution of organismality. High atmospheric levels of carbon dioxide would account for some essential reactive properties of this protobiological world, such as the possibility of carbon fixation, and chiral propagation.