Evolution of metal ions in biological systems refers to the incorporation of metallic ions into living organisms and how it has changed over time. Metal ions have been associated with biological systems for billions of years, but only in the last century have scientists began to truly appreciate the scale of their influence. Major (iron, manganese, magnesium and zinc) and minor (copper, cobalt, nickel, molybdenum, tungsten) metal ions have become aligned with living organisms through the interplay of biogeochemical weathering and metabolic pathways involving the products of that weathering. The associated complexes have evolved over time.
Natural development of chemicals and elements challenged organisms to adapt or die. Current organisms require redox reactions to induce metabolism and other life processes. Metals have a tendency to lose electrons and are important for redox reactions. Metals have become so central to cellular function that the collection of metal-binding proteins (referred to as the metallomes) accounts for over 30% of all proteins in the cell. Metals are known to be involved in over 40% of enzymatic reactions, and metal-binding proteins carry out at least one step in almost all biological pathways.Metals are also toxic so a balance must be acquired to regulate where the metals are in an organism as well as in what quantities. Many organisms have flexible systems in which they can exchange one metal for another if one is scarce. Metals in this discussion are naturally occurring elements that have a tendency to undergo oxidation. Vanadium, molybdenum, cobalt, copper, chromium, iron, manganese, nickel, and zinc are deemed essential because without them biological function is impaired.
Origins
The Earth began as an iron aquatic world with low oxygen. The Great Oxygenation Event occurred approximately 2.4 Ga (billion years ago) as cyanobacteria and photosynthetic life induced the presence of dioxygen in the planet's atmosphere. Iron became insoluble (as did other metals) and scarce while other metals became soluble. Sulfur was a very important element during this time. Once oxygen was released into the environment, sulfates made metals more soluble and released those metals into the environment; especially into the water. Incorporation of metals perhaps combatted oxidative stress.
The central chemistry of all these cells has to be reductive in order that the synthesis of the required chemicals, especially biopolymers, is possible. The different anaerobic, autocatalysed, reductive, metabolic pathways seen in the earliest known cells developed in separate energised vesicles, protocells, where they were produced cooperatively with certain bases of the nucleic acids.
Hypotheses proposed for how elements became essential is their relative quantity in the environment as life formed. This has produced research on the origin of life; for instance, Orgel and Crick hypothesized that life was extraterrestrial due to the alleged low abundance of molybdenum on early Earth (it is now suspected that there were larger quantities than previously thought). Another example is life forming around thermal vents based on the availability of zinc and sulfur. In conjunction with this theory is that life evolved as chemoautotrophs. Therefore, life occurred around metals and not in response to their presence. Some evidence for this theory is that inorganic matter has self-contained attributes that life adopted as shown by life's compartmentalization. Other evidence includes the ready binding of metals by artificial proteins without evolutionary history.
Importance of metal ions in evolution
Catalysis
Redox catalysts
The prebiotic chemistry of life had to be reductive in order to obtain, e.g. Carbon monoxide (CO) and Hydrogen cyanide (HCN) from existing CO2 and N2 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.
Condensation and hydrolysis
O-donors such as HPO were abundant in the prebiotic atmosphere. Metal ion binding to such O-donors was required to build the biological polymers, since the bond is generally weak, it can catalyze the required reaction and dissociate after (i.e. Mg2+ in DNA synthesis).