text: Evidence suggests that manganese (Mn) was first incorporated into biological systems roughly 3.2–2.8 billion years ago, during the Archean Period. Together with calcium, it formed the manganese-calcium oxide complex (determined by X-ray diffraction) which consisted of a manganese cluster, essentially an inorganic cubane (cubical) structure. The incorporation of a manganese center in photosystem II was highly significant, as it allowed for photosynthetic oxygen evolution of plants. The oxygen-evolving complex (OEC) is a critical component of photosystem II contained in the thylakoid membranes of chloroplasts; it is responsible for terminal photooxidation of water during light reactions.
  The incorporation of Mn in proteins allowed the complexes the ability to reduce reactive oxygen species in Mn-superoxide dismutase (MnSOD) and catalase, in electron transfer-dependent catalysis (for instance in certain class I ribonucleotide reductases) and in the oxidation of water by photosystem II (PSII), where the production of thiobarbituric acid-reactive substances is decreased. This is due to manganese's ability to reduce superoxide anion and hydroxyl radicals as well as its chain-breaking capacity.