human capacities for norm following and sanctioning depend not on a single but on a plurality of cognitive and motivational mechanisms. As I see it, a natural history of hierarchies explains how hierarchical or egalitarian social outcomes relate to the biological evolution of these mechanisms. In this chapter and the following, I argue that social norms and sanctions as we know them in modern Homo sapiens result from a few significant cognitive changes in the human lineage and that these changes led to the elimination of dominance hierarchies as they exist in nonhuman primates.

To make my point, I first need to explain what dominance hierarchies are (2.1) and how their existence must be understood within the context of nonhuman primates' social cognition (2.2) and affiliative behavior (2.3). I then describe the mechanism of joint attention, which is specific to humans, and explain how it gives rise to the normative expectations discussed in the previous chapter (2.4). This comparative discussion is not meant to suggest that nonhuman primates can be taken as a portrait of our last common ancestors with them. My point is simply that a trait shared by two related species – biologists qualify such traits as “homologous” – has a high probability of having been present in their last common ancestors. In the last two sections, I delve into the archaeological data to identify major behavioral changes since our last common ancestor with chimpanzees and other apes (2.5).