When did we begin to adopt the perspective of others? When did empathy cross the threshold into mutual care? When did we first begin to emphasize our place in society and develop unique identities? These questions concern the architecture of the human mind as it encounters other minds, the cognitive and emotional machinery that evolution built over hundreds of thousands of years, long before the first city wall, the first law code, the first throne. Every institution we have ever implemented rests on that older foundation. From a multi-disciplinary perspective, the volume addresses questions that span from the origins of human group size, demography, and sexual behaviour to questions at the very frontier of what science can currently answer. Drawing on material culture, developmental psychology, comparative primatology, and evolutionary modelling, the adressed questions push toward a richer account of what it means, and what it costs, to become social. In doing so, they will bring us closer to understanding the deepest paradox of human nature, i.e., that the same evolved capacity for sociality underlies both our most profound acts of care and our most devastating acts of violence.