It is supposed that the last universal common ancestor had many iron-sulfur clusters.
[4] Iron–sulfur clusters occur in many biological systems, often as components of electron transfer proteins.
[6] The [4Fe–4S] clusters occur in two forms: normal ferredoxins and high potential iron proteins (HiPIP).
[7] The relevant redox couple in all Fe–S proteins is Fe(II)/Fe(III).
[7] Many clusters have been synthesized in the laboratory with the formula [Fe4S4(SR)4]2−, which are known for many R substituents, and with many cations.