Evolution of vertebrate survival circuits

Evolution selects those adaptive features that increase reproductive probabilities and facilitate survival. Analysing the brain circuits mediating risk-avoidance (e.g. defense) and those allowing reward-seeking (motivated) behaviours in different vertebrates leads to several main conclusions. First, circuits mediating risk-avoidance are similar in all studied vertebrates, where they include amygdala homologues located in the posterior half of the cerebral hemispheres, in close relationship with the chemosensory systems. Second, in all vertebrates, reward-seeking behaviours involve the activity of tegmento-striatal dopaminergic pathways, plus other inputs to the ventral striatum, including amygdalo-striatal glutamatergic projections. Third, output structures in these forebrain circuits for both risk-avoidance and reward-seeking behaviours occupy the caudal and rostral poles of the ventral striato-pallidum, namely the central amygdala and nucleus accumbens-olfactory tubercle respectively. This brain configuration was already present in at least the ancestral amniote, likely also in anamniotes. Finally, social behaviours (sexual, agonistic-territorial, parental) are fundamental for reproduction and survival. Consequently, the so-called socio-sexual brain network that governs these conducts is closely related with brain centres mediating motivation (maybe also risk-avoidance). Central nonapeptidergic circuits are apparently required for endowing social stimuli with rewarding (attractive) properties. More studies in non-mammals are required to further test and expand these ideas.