Abstract: Humans have evolved from a great ape-like ancestor 5-7 mio years ago. During this time, we not only evolved our uniquely human cognitive abilities, but also started to systematically share offspring care in the social group (i.e. engage in cooperative breeding). According to the cooperative breeding model of human evolution, this breeding system played a key role for the emergence of human hyper-sociality and cognition. I will present comparative data from nonhuman primates that support the cooperative breeding model, and show that across primates, shared offspring care is indeed associated with hyper-sociality, cooperation, and increased performance in socio-cognitive tasks. In the second part, I will zoom in on proximate, underlying mechanisms of hypersociality and cooperation in callitrichid monkeys (marmosets), who are the only other cooperatively breeding primates besides humans. The comparison of these proximate mechanisms in humans and cooperatively breeding primates are mandatory to fully understand the impact of cooperative breeding for human evolution, as well as for the use of marmosets as emerging neuroscientific model system.