““One of the most studied organisms in this context is the tiny polyp Hydra, which possesses only a hundred thousand cells. Its neural network is concentrated in its head and foot: a first evolutionary step toward developing a brain and spinal cord. Hydra’s nervous system contains a chemical messenger—a minuscule protein—that resembles two of our own: vasopressin and oxytocin. A protein of this kind is called a neuropeptide. In vertebrates, the gene for this particular neuropeptide first doubled and then mutated in two places, creating the two closely related but specialized neuropeptides vasopressin and oxytocin, which have recently become the focus of interest, partly because of their important role as messengers in our social brains (see chapter 9).””
