text: 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.