Hunter-gatherers maintain assortativity in cooperation despite high-levels of residential change and mixing

Abstract

SUMMARYWidespread cooperation is a defining feature of human societies from hunter-gatherer bands to nation states. But explaining its evolution remains a challenge. While positive assortment – of cooperators with cooperators – is recognized as a basic requirement for the evolution of cooperation, the mechanisms governing assortment are debated. Moreover, the social structure of modern hunter-gatherers, characterized by high mobility, residential mixing and low genetic relatedness, undermine assortment and add to the puzzle of how cooperation evolved. Here, we analyze four years of data (2010, 2013, 2014, 2016) tracking residence and levels of cooperation elicited from a public goods game (PG), in Hadza hunter-gatherers of Tanzania. Data were collected from 56 camps, comprising 383 unique individuals, 137 of whom we have data for two or more years. Despite significant residential mixing, we observe a robust pattern of assortment necessary for cooperation to evolve: In every year, Hadza camps exhibit high between-camp and low within-camp variation in cooperation. We further consider the role of homophily in generating this assortment. We find little evidence that cooperative behavior within individuals is stable over time or that similarity in cooperation between dyads predicts their future cohabitation. Both sets of findings are inconsistent with homophilic models that assume stable cooperative and selfish types. Consistent with social norms, culture and reciprocity theories, the data suggest that the strongest predictor of an individual’s level of cooperation in any given year is the mean cooperation of their campmates in that year. These findings underscore the adaptive nature of human cooperation – particularly its responsiveness to social contexts – as a feature important in generating the assortment necessary for cooperation to evolve.

Human Cooperation: The Hunter-Gatherer Puzzle

“…If camps with more cooperative norms persist longer, as has been shown in some hunter-gatherer populations [20], then these learning dynamics could sustain substantial levels of cooperation within the population as a whole. Consistent with this, Apicella and colleagues [4] show that, while their prior behavior is not instructive, the single best predictor of how much a…”

“…Many plants experience natural fluctuations in the saline and osmotic environments around roots and it is not surprising that most have evolved a range of tolerance mechanisms of varying effectiveness. The mechanisms underlying responses to increased salinity and tolerance of moderate salinity have been well studied in model plant systems, such as Arabidopsis and certain crop plants [3,4]. Such studies have shown key roles of a number of transporters for Na + , Cl À and K + in facilitating Na + exclusion from the cytoplasm.…”

“…The analyses by Apicella and colleagues [4] reveal highly fluid groups and no temporal consistency in people's cooperativeness. That is, not only are camps constantly remixing, but knowing how many honey sticks someone gave previously tells you essentially nothing about what they'll do later.…”

“…This rapid transformation suggests that at the core of this mystery lies the social life of hunter-gatherers, a lifestyle that includes many elements that stretch back two million years. Probing this puzzle in this issue of Current Biology, Coren Apicella and her team [4] analyzed rich longitudinal data that combine population dynamics with ecologicallyrelevant experimental measures of cooperation from one of the last remaining populations of hunter-gatherers.…”

“…Apicella and colleagues [4] conducted a six-year study among hundreds of Hadza hunter-gatherers in Tanzania (Figure 1). Despite the encircling global markets, this population continues to live in nomadic camps and subsist by hunting wild game and foraging for tubers, roots and berries.…”