text: The prebiotic chemistry of life had to be reductive in order to obtain, e.g. Carbon monoxide (CO) and Hydrogen cyanide (HCN) from existing CO and N in the atmosphere. CO and HCN were precursor molecules of the essential biomolecules, proteins, lipids, nucleotides and sugars. However, atmospheric oxygen levels increased considerably, and it was then necessary for cells to have control over the reduction and oxidation of such small molecules in order to build and break down cells when necessary, without the inevitable oxidation (breaking down) of everything. Transition metal ions, due to their multiple oxidation states, were the only elements capable of controlling the oxidation states of such molecules, and thus were selected for.