What brought the adaptive immune system to vertebrates?--The jaw hypothesis and the seahorse

A hypothesis is discussed that the adaptive immune system of vertebrates evolved in the gastrointestinal regions of primitive jawed fish (placoderms) due to increased localized injuries and infections which were inadvertently brought about by the novel jaw structures and the predatory life style. The question whether the modern jawless fish, cyclostomes, have adaptive immunity or not is briefly but critically reviewed. The discovery that the gut-associated immune tissues in mammals constitute the primary immune tissues for the local T cells and that some epithelial gamma delta T cells have a unique propensity is summarized and discussed in relation to the jaw hypothesis. Initial study of the seahorse (Hippocampus) indicates that the gut-associated immune tissues may be absent in this teleost species, suggesting an evolutionary link between the adaptive immune system and the jaw structure or eating habit.