Structural changes involving new neurons can occur through stem cell-driven neurogenesis and late-maturing immature neurons, namely undifferentiated neuronal precursors frozen in a state of arrested maturation. The latter exist in the cerebral cortex, being particularly abundant in large-brained mammals. Similar cells have been described in the amygdala of some species, although their interspecies variation remain poorly understood. Here, their occurrence, number, molecular expression, and morphology were systematically analyzed in eight diverse mammalian species widely differing in neuroanatomy, brain size, lifespan, and socioecology. We show remarkable phylogenetic variation of the immature neurons in the amygdala, with a significantly greater prevalence in primates. The cells are associated with the amygdala basolateral complex that in evolution has expanded in primates in conjunction with cortical projections, thus mimicking the general trend of the neocortex. These results support the emerging view that large brains performing complex socio-cognitive functions rely on wide reservoirs of immature neurons.