Tetrapyrroles are essential molecules in living organisms and perform a multitude of functions in all kingdoms. Their synthesis is achieved in cells via a complex biosynthetic machinery which is unlikely to be maintained, if unnecessary. Here we propose that ancient hemes, such as the d(1)-heme of cd(1) nitrite reductase or the siroheme of bacterial and plant nitrite and sulphite reductases, are molecular fossils which have survived the evolutionary pressure because their role is strategic for the organism where they are found today. The peculiar NO-releasing propensity of the d(1)-heme of P. aeruginosa NIR, recently shown by our group is, in our opinion, an example of this strategy. The hypothesis is that the d(1)-heme structure might be a pre-requisite for the fast rate of NO dissociation from the ferrous form, a property which is crucial to enzymatic activity and cannot be achieved with a more common b-type heme.