While anthropologists have debated the origins and evolution of war extensively, peace remains a neglected topic, and is often simply taken for granted as the situation that would prevail in the absence of war. Glowacki provides a welcome focus on this topic, highlighting the unusual character of peaceful interactions among human groups compared to other animals. Glowacki's field experience with cattle raiders provides important insights into the challenges of maintaining peaceful intergroup relations, given the incentives for individuals to conduct raids or otherwise break the peace. I agree with Glowacki's main points, and focus on two questions: (1) Is human intergroup peace unique? (2) When and why did our ancestors evolve the capacity for intergroup peace?

Glowacki states that “humans are alone in having durable, positive-sum, interdependent relationships across unrelated social groups” (target article, Abstract). Are humans truly alone in this respect? And if so, why?

To answer this question, Glowacki focuses mainly on our two closest living relatives, chimpanzees and bonobos. While studies of these species provide valuable insights, no living species is likely to be a perfect model of our extinct ancestor. Our ape cousins exhibit various traits that appear derived compared to our last common ancestor (Hunt, Reference Hunt2020). Looking more broadly across animals, many species aggregate peacefully: Shoals of fish; flocks of birds; and herds of bison and gelada monkeys. Competition occurs within these aggregations, but members do not attempt to attack other groups or aggressively exclude outsiders. Such peaceful aggregations can be explained through economics: When resources are dispersed or transient, it is not feasible for any one group to monopolize them (Brown, Reference Brown1964). Primates tend to defend their home ranges as territories only when it is economical to do so (Mitani & Rodman, Reference Mitani and Rodman1979). Bonobos may have more peaceful intergroup interactions than chimpanzees because their home ranges are not economically defendable (Pusey, Reference Pusey2022).

As Glowacki notes, peaceful interactions in humans involve not just tolerance, but also positive-sum interactions. Intergroup relations among chimpanzees involve mainly zero-sum games. Apart from immigration by dispersing females, male chimpanzees benefit from intergroup encounters only insofar as they can impose costs on the other group, by deterring them from entering their own territory, forcing rivals to cede territory, and/or killing rivals to reduce their coalition strength (Wilson, Reference Wilson and Fry2013). While bonobos sometimes have peaceful intergroup interactions, only rarely do beneficial exchanges such as food sharing occur (Fruth & Hohmann, Reference Fruth and Hohmann2018). Aggregating fish, birds, and grazers may gain mutual benefits, including access to mates, information about food, and increased safety from predators. Humans demonstrate an even greater capacity to engage in mutually beneficial intergroup interaction, shown to an extreme degree by the intensely interconnected global economy, but also discernible among foragers, who exchange trade goods, information, permission to use water holes and hunting grounds, and marriage partners (Kelly, Reference Kelly2013). As Glowacki notes, the first evidence of intergroup trade dates back to around 300,000 years ago. But were these the first such interactions? And why did humans, but not other primates, evolve this capacity?

Glowacki claims that “the preconditions for peace only emerged in the past 100 thousand years” (target article, Abstract). While key cultural tools for building peace likely did arise recently, I propose that the roots of peace extend much deeper than that. At least two key preconditions likely have a long history in hominin evolution: Multilevel social organization and collective foraging, which promotes a psychology of sharing and exchange.

Glowacki notes that humans are “members of multiple social groups simultaneously with overlapping nonexclusive boundaries” (target article, Introduction, para. 3), which makes it difficult to define the boundaries of our groups. As such, our societies resemble the multilevel societies of some monkeys (Grueter, Chapais, & Zinner, Reference Grueter, Chapais and Zinner2012; Layton, O'Hara, & Bilsborough, Reference Layton, O'Hara and Bilsborough2012; Swedell & Plummer, Reference Swedell and Plummer2019). How, why, and when our ancestors evolved multilevel societies remains unknown. The multilevel societies of geladas and hamadryas baboons may depend on two key factors. First, sparse and seasonally variable food supplies prevent individuals from foraging in large groups year-round; instead, in some seasons, individuals must forage in smaller groups. Second, in open habitats, sleeping sites safe from predators are scarce, limited to cliffs and groves of tall trees. Such sleeping sites serve as gathering points for subgroups that disperse throughout the day to forage. Agent-based modeling supports the hypothesis that when females forage in smaller groups, males can sire more offspring by monitoring and monopolizing access to a few females, rather than constantly searching for fertile females in the population at large (Crouse, Miller, & Wilson, Reference Crouse, Miller and Wilson2019). If multilevel societies originated early in human evolution, then the capacity for interacting peacefully across a range of social groupings would be an ancient trait of hominins (Wilson & Glowacki, Reference Wilson, Glowacki, Muller, Wrangham and Pilbeam2017).

The psychological capacity for trading goods between groups likely depends upon a long evolutionary history of trade within groups. Hunter–gatherers forage collectively, bringing food back to camp to cook and share with their families and other group members. Psychological traits exhibited even by small children differ profoundly from the more self-centered psychology of nonhuman apes. We are more other-minded, with a greater capacity to control impulses to achieve future goals (Hrdy, Reference Hrdy2009; Tomasello, Reference Tomasello2014). These psychological traits underpin the collective foraging of hunter–gatherers, and suggest a long evolutionary history of sharing and exchange, going back perhaps to the origins of Homo, or even earlier hominins such as Australopithecus. Key foods proposed for australopiths include the underground storage organs of plants: Corms, roots, and tubers (Laden & Wrangham, Reference Laden and Wrangham2005). Simple sticks and stones arguably enabled early hominins to extract hidden and protected foods, including roots, tubers, nuts, marrow, and brains. Such extracted foods potentially enabled foragers to produce a surplus to be shared with others, such as mothers sharing with offspring. Our effort to model the origins of hominin food sharing found that long-term mating bonds between males and females would promote sharing of extracted foods by females to males, so long as males provided females with other benefits, such as protection from predators, infanticide, and theft (Alger, Dridi, Stieglitz, & Wilson, Reference Alger, Dridi, Stieglitz and Wilson2023). The roots of intergroup peace in humans may thus originate with a root-based economy.