The atmosphere that supported the Earth's first living organisms was very different to the one we live in now. In contrast to the oxygen-rich atmosphere of today, billions of years ago the Earth's atmosphere was reducing in character, containing ammonia, carbon dioxide, methane and hydrogen sulfide in addition to nitrogen. Metal ions were mainly present in their lower oxidation states, typically existing in the form of sulfides.
Under these conditions, iron(II) was soluble and readily available and so became incorporated into organisms as a biometal, which could catalyse redox processes in the active sites of many proteins and enzymes.
The evolution of siderophores
As oxygen began to accumulate in the atmosphere, owing to photosynthesis, the availability of iron changed drastically. In the presence of oxygen, reasonably water soluble iron(II) sulfide was oxidised to iron(III) oxide and hydroxide, becoming insoluble at neutral and basic pH.