After the discovery of the vasodilatory functions of nitric oxide (NO), many signaling mechanisms involving NO were identified through experiments on mammals. NO activates soluble guanylyl cyclase to induce the formation of cGMP, stimulates ADP-ribosylation of GAPDH to alter cell energy production, and combines with superoxide to generate peroxynitrite. It then became clear that NO was a major messenger molecule in mammals, involved in the regulation of blood vessel dilatation, immune function and neurotransmission in the brain and peripheral nervous system. The wide spectrum of physiological effects of NO in mammals prompted researchers to look for the presence of NO in vertebrates and invertebrates. Parallel findings on the presence of NO signaling in vertebrates and invertebrates were observed, and then NO was found to be a signaling molecule widely spread throughout the metazoan kingdom and whose functions were highly conserved during evolution. These features were extended to the entire animal kingdom after the discovery of NOS activity in protozoa, yeasts and bacteria. Recently, the involvement of NO and NOS in plant disease resistance to infection was documented and many close similarities were detected between NO-dependent signaling mechanisms involved in plants and those identified in animals. All of these results indicated that NO is one of the earliest and most widespread signaling molecules in living organisms. This short review was aimed at marshalling recent information that led to this conclusion.