Elucidation of the heme biosynthesis pathway involed some interesting detective work. David Shemin and David Rittenberg, who were among the first to use isotopic tracers in the elucidation of metabolic pathways, demonstrated, in 1945, that all of heme's C and N atoms can be derived from acetate and glycine. Only glycine, out of a variety of 15N-labeled metabolites they tested