Following hints from X-ray data (Ostermeier C et al., 1997, Proc Natl Acad Sci USA 94:10547–10553; Yoshikawa S et al., 1998, Science 280:1723–1729), chemical evidence is presented from four distantly related cytochrome-c oxidases for the existence of a copperB-coordinated His240–Tyr244) cross-link at the O2-activating Heme Fea3–CuB center in the catalytic subunit I of the enzyme. The early evolutionary invention of this unusual structure may have prevented demaging [bull ]OH-radical release at e−-transfer to dioxygen and thus have enabled O2 respiration.