Human nature contains both good and evil. We observe displays of extreme prosociality and substantial cruelty in human societies around the world. To answer the question of how these two extremes can co-exist, and why we observe any prosociality at all, given its costs, I study the origins of human sociality. If we understand where our social preferences and behaviours come from, we can then understand why people do or do not care about one another, and why there exist high levels of heterogeneity in sociality across societies, across individuals within a society, and across time and space within an individual. My dissertation, therefore, centres around exploring the origins of and the variation in human sociality. In my projects, I study how humans evolved to be ultra-social while the extent of sociality in other animals, and especially in non-human primates, remained relatively restricted. In doing so, I focus on the mechanisms that can explain the co-existence of good and evil in us, and look for unique elements in human social interactions, or a combination thereof, to pinpoint the cause of divergence in human sociality.