Glowacki provides a unique perspective on the evolutionary foundations of human peace – carefully evaluating how harmonious relationships between groups can arise, and the conditions required to maintain a peaceful status quo. In a synthesis of human and nonhuman ethnography and game theory, his work generates novel hypotheses on the emergence of human peacemaking. The crux of Glowacki's argument is that humans did not evolve an innate capacity for peace but have rather developed unique cultural mechanisms to maintain positive-sum relationships between groups. Providing a detailed and careful consideration of the interplay between the mechanisms leading to conflict and peacemaking, Glowacki's framework lays the groundwork for testable novel hypotheses. However, we argue that a more accurate representation of the social systems and between-group dynamics of broader taxa is needed before we can reconstruct potential pathways to peace and understand the processes leading to human's remarkable societies.

Glowacki defines peace through two parallel processes: The expectation of harmonious relationships between groups and the overall rarity of expression of aggression and violence. When violence does occur, it is expected to be quickly resolved. Following this definition, it is clear why the presence of cooperative relationships between groups is not a sufficient condition for peace. However, whether the absence of war and violence between regularly interacting groups is a sufficient determinant of peaceful relationships remains unclear. Between-group tolerance in the absence of violence is commonly observed in nonhumans. Taxa as varied as primates, elephants, and cetaceans (Grueter, Chapais, & Zinner, Reference Grueter, Chapais and Zinner2012; Nandini, Keerthipriya, & Vidya, Reference Nandini, Keerthipriya and Vidya2018; Schreier & Swedell, Reference Schreier and Swedell2009; Stead & Teichroeb, Reference Stead and Teichroeb2019; Whitehead et al., Reference Whitehead, Antunes, Gero, Wong, Engelhaupt and Rendell2012) live in multilevel societies where distinct groups separate and merge across time and space to maximize the benefits of social living. In these societies, violence is rare and between-group tolerance provides clear benefits to participating social units that would not otherwise be conferred (Grueter et al., Reference Grueter, Qi, Zinner, Bergman, Li, Xiang and Swedell2020). What can these social systems teach us about pathways to peace? Must conflict occur and be resolved for us to identify peace in a society? At the minimum, the between-group connections and rarity of conflict in these systems permit the investigations of diverging evolutionary trajectories that lead to similar outcomes – tolerant systems that support information flow and exchange.

One may argue, however, that it is unclear how group identities play out in multilevel systems. If individuals perceive everyone as in-group, such systems will be disqualified as examples for the presence of peace because peace can only be defined as a between-group currency. Here, bonobos (Pan paniscus), one of our closest living relatives, offer valuable insights into a system of between-group tolerance among socially and culturally distinct groups (Samuni, Langergraber, & Surbeck, Reference Samuni, Langergraber and Surbeck2022; Samuni, Wegdell, & Surbeck, Reference Samuni, Wegdell and Surbeck2020). Bonobos live in large, male-philopatric groups that regularly interact. Members of different groups forage and hunt together (Lucchesi et al., Reference Lucchesi, Cheng, Deschner, Mundry, Wessling and Surbeck2020; Sakamaki, Ryu, Toda, Tokuyama, & Furuichi, Reference Sakamaki, Ryu, Toda, Tokuyama and Furuichi2018; Samuni et al., Reference Samuni, Wegdell and Surbeck2020), jointly mob predators (Samuni & Surbeck, Reference Samuni and Surbeckunpublished data), support one another in conflict (Tokuyama, Sakamaki, & Furuichi, Reference Tokuyama, Sakamaki and Furuichi2019), and even share meat and other high-quality foods (Fruth & Hohmann, Reference Fruth and Hohmann2018). The remarkable between-group cooperation that bonobos exhibit among non-kin is only second to humans (Samuni & Surbeck, Reference Samuni and Surbeckunpublished data).

Bonobo cross-group interactions are not always tolerant or cooperative, and aggression and nonlethal violence are part of the repertoire of bonobo between-group interactions (Cheng, Samuni, Lucchesi, Deschner, & Surbeck, Reference Cheng, Samuni, Lucchesi, Deschner and Surbeck2022; Tokuyama et al., Reference Tokuyama, Sakamaki and Furuichi2019). In fact, the perception that physical aggression commonly occurs when bonobo groups meet has led Glowacki to argue that humans are alone in establishing and maintaining peace. But how frequent or violent should aggression be to be considered common enough to disqualify peace in bonobos? For such a critical determinant of the evolutionary timescale of peace emergence, Glowacki fails to offer objective measures by which the presence of peace in our closest living relatives can be quantified, thereby subjectively (and prematurely) discounting the bonobo as a taxon with likely homologous capacity for peace.

Like humans, bonobo populations show large variation in the amount of time that different groups spend together and the quality of these interactions – from rare and largely aggressive between-group interactions in LuiKotale (Hohmann & Fruth, Reference Hohmann, Fruth, Boesch, Hohmann and Marchant2002) to prolonged and predominantly tolerant interactions in Wamba (Tokuyama et al., Reference Tokuyama, Sakamaki and Furuichi2019) and Kokolopori (Lucchesi et al., Reference Lucchesi, Cheng, Deschner, Mundry, Wessling and Surbeck2020). Variation in between-group interactions is also evident within bonobo populations, with some groups spending days and weeks together while others only rarely meet (Samuni, Langergraber, & Surbeck, Reference Samuni, Langergraber and Surbeck2022). Here, we focus on behavior between bonobo groups that exhibit prolonged and frequent interactions, the type of connections relevant for discussions on peace.

When bonobo groups meet, disputes that can escalate into physical aggression and injuries may occur. Nonetheless, bonobo between-group aggressions are typically short, include mild threats or displays without any physical contact, are quickly resolved, and are not known to escalate into severe injuries. While the rates and intensity of aggressions tend to increase when bonobo groups meet, this heightened aggressive behavior is directed similarly toward out-group and in-group individuals (Cheng et al., Reference Cheng, Lucchesi, Mundry, Samuni, Deschner and Surbeck2021, Reference Cheng, Samuni, Lucchesi, Deschner and Surbeck2022; Tokuyama et al., Reference Tokuyama, Sakamaki and Furuichi2019). This is the key point – bonobo aggression between groups is similar to within-group expressions of aggression and does not resemble the levels of between-group violence observed in humans or in their sister species chimpanzees (Pan troglodytes) (Wilson et al., Reference Wilson, Boesch, Fruth, Furuichi, Gilby, Hashimoto and Wrangham2014; Wrangham, Reference Wrangham2018; Wrangham & Glowacki, Reference Wrangham and Glowacki2012). Therefore, bonobo aggressive tendencies toward out-groups occur at a sustainable level that is not expected to precipitate a breakdown of relationships between those groups. Despite having the same social structure thought to promote proactive violence and warfare in humans and chimpanzees, bonobo between-group violence is greatly reduced. This suggests the presence of mechanisms that foster bonobo between-group tolerance and cooperation, while restraining destructive conflict.

Reconsidering the bonobo as a species that can maintain peaceful between-group relationships requires a rethinking of the preconditions of peace and the evolutionary timescale of peace emergence. Variation in the expressions of violence and tolerance across bonobo populations offers a practical avenue for investigating the mechanisms underlying peaceful capacities in human-adjacent societies. How can bonobos maintain peace in the absence of social norms or institutional control? In humans and chimpanzees, expressions of proactive aggression are closely linked with their extreme tendencies for between-group violence (Wrangham, Reference Wrangham2018). Proactive aggression, however, is greatly reduced in bonobos (Wrangham, Reference Wrangham2018). Following Glowacki's rationale – that the factors promoting destructive violence in humans are also the ones contributing to the emergence of peace – we suggest that it is the lack of extreme violence in bonobos that enables “simpler” solutions to peace without relying on norm psychology or culture.