Iron has played a critical role in the evolution of life. The ancient Greeks, believing iron to be a special gift sent to earth by one of the gods, named it sideros, or star (Liebel, Greenfield, and Pollitt 1979). As the second most common metal, iron accounts for 5 percent of the earth’s crust; it is also found in both sea- and freshwater (Bernat 1983). Scientists believe that the earth’s atmosphere was originally a reducing one with very low oxygen pressure. As a result, large amounts of reduced iron would have been available for living organisms (Bothwell et al. 1979). Iron is an essential element for all organisms, with the possible exception of some Lactobacillus (Griffiths 1987; Payne 1988). In animals, the processes of DNA replication, RNA synthesis, and oxygen and electron transport require iron. Today most iron in the environment exists in an oxidized state and is less available to organisms. However, the problems of extracting insoluble iron have been overcome during evolution. A variety of sophisticated mechanisms have evolved that are specific to different kingdoms and/or different species (e.g., mechanisms plants use to be able to live in acidic or iron-poor environments) (Bothwell et al. 1979). Such mechanisms in animals include iron complexing agents, which transport iron and deliver it to cells, and low-molecular-weight compounds, such as fructose and amino acids, that reduce iron into a soluble form (Griffiths 1987; Simmons 1989: 14).

Metabolic processes within humans involve the presence of free radicals, that is, substances that are reactive because of instability in the arrangement of electrons (Wadsworth 1991). Iron may be present as a free radical. Such instability makes iron highly likely to donate or accept electrons.As a result, iron is versatile and able to serve a number of functions within cells. These functions include acting as a catalyst in electron transport processes and serving as a transporter of oxygen. Iron is a key component of hemoglobin, the oxygen carrier found in red blood cells. It is involved in many other extracellular processes as well (Woods, DeMarco, and Friedland 1990). Iron also is required for collagen synthesis, the production of antibodies, removal of fats from the blood, conversion of carotene to vitamin A, detoxification of drugs in the liver, and the conversion of fuel nutrients to energy (Long and Shannon 1983). In addition to its importance in the maintenance of normal metabolic processes, iron involvement in pathological change and initiation of disease is a critical facet of host defense.