The anthropoids, that is monkeys, apes and humans, are highly adaptable. This enhanced adaptability depends on the development of new areas in the prefrontal cortex. In the human brain these regions support reasoning and decision-making. We need to explain how the prefrontal regions of our ancestors were co-opted to allow this. We take up this challenge by adopting a neuroecological perspective. We take advantage of three lines of evidence: evolutionary biology elucidating the foraging niches of early anthropoid primates; comparative neuroscience comparing the human brain with that of other living primates; and computational neuroscience detailing the computations underlying free-ranging behaviour. We demonstrate that the enhanced adaptability of anthropoids depends on their ability to understand abstract concepts such as number, order, and identity. The selective advantage is that when foraging they can solve problems rapidly if they have an abstract similarity to ones they have met previously. We show which prefrontal areas are homologous in humans and monkeys. Though the human prefrontal cortex is especially expanded, it has the same fundamental organisation as in macaques. We conclude that, through our common ancestry, humans have co-opted the abilities that macaques use for foraging to support reasoning and decision-making.