Evolutionary and comparative aspects of nitric oxide, carbon monoxide and hydrogen sulfide

► Nitric oxide (NO), carbon monoxide (CO) and hydrogen sulfide (H2S) have been intimately linked with evolution. ► NO, CO and H2S are retained as signaling molecules in modern day animals. ► Many of these signaling pathways are important in regulating oxygen delivery. ► Comparative Physiology permits a evolutionary view of how these signaling pathways developed and offers a more thorough understanding of their utility and adaptability.