IN early times the earth had a reducing atmosphere, and an abundance of ferrous iron was present for incorporation into biologic molecules.¹ Later, with an increase in atmospheric oxygen, iron existed largely in its less available ferric form,² and special devices had to be developed by life forms for the acquisition of iron. Bacteria, for example, synthesize and excrete high-affinity chelating agents that extract otherwise unavailable iron from the surrounding environment.³ The roots of plants also secrete substances that augment iron absorption.⁴ In mammalian species a "shuttle" protein in the upper intestine — mucosal transferrin — appears to perform this . . .