Verbal and mathematical models that consider the costs and benefits of behavioral strategies have been useful in explaining animal behavior and are often used as the basis of evolutionary explanations of human behavior. In most cases, however, these models do not account for the effects that group structure and cultural traditions within a human population have on the costs and benefits of its members' decisions. Nor do they consider the likelihood that cultural as well as genetic traits will be subject to natural selection. In this paper, we present an agent-based model that incorporates some key aspects of human social structure and life history. We investigate the evolution of a population under conditions of different environmental harshness and in which selection can occur at the level of the group as well as the level of the individual. We focus on the evolution of a socially learned characteristic related to individuals' willingness to contribute to raising the offspring of others within their family group. We find that environmental harshness increases the frequency of individuals who make such contributions. However, under the conditions we stipulate, we also find that environmental variability can allow groups to survive with lower frequencies of helpers. The model presented here is inevitably a simplified representation of a human population, but it provides a basis for future modeling work toward evolutionary explanations of human behavior that consider the influence of both genetic and cultural transmission of behavior.

Introduction

Many animals are adapted to survive in variable and challenging environments, but only humans can make a living in such a wide range of settings, from the savannah to the tropics, from the scorching desert to the frozen tundra. Our ability to do so stems not from our physical or mental prowess as individuals, but from our ability to organize collectively, to cooperate, and to learn from one another in a manner that produces cumulative improvements and considerable diversity between human populations [1], [2]. Much theory and research into human origins is now focused on the characteristics that make it possible for human groups to generate and share such a complex culture, characteristics such as our highly developed ability to socially learn and our inclination to share resources [3], [4]. Social institutions and psychological norms of behavior have guided our ability to organize into cohesive cultural groups, shaping effective interactions both within and between groups [5], [6]. Increasingly, human parenting behavior, which has been described as “cooperative breeding” because allomaternal parental care is essential for the raising of human young, is recognized as having played an important role in the evolution of these characteristics [7]–[11]. Humans have a number of adaptations that facilitate the creation and maintenance of family groups whose members contribute to the raising of young. For example, humans have a unique life history; it is common for females to live many years after they have ceased to be fertile [12]. Thus human families usually include at least one experienced mother with time to help look after other women's children. The longevity that makes grandparenting possible is almost certainly underpinned by genetic adaptations. However, other adaptations that serve to ensure mothers have help raising young appear to be mostly cultural, such as the marriage customs through which a sexual relationship and responsibility for offspring are formally acknowledged. Caspari and Lee [13] have also argued that the increased longevity of modern humans was driven primarily by cultural and demographic factors rather than genetic factors, as indicated by the human fossil record.

Understanding the features and dynamics of family structure and group organization, with attention paid to lifespan stages, is essential for gathering a complete picture of how modern humans evolved. It is increasingly recognized that theoretical models complement empirical work on evolution. Models provide a formalization of theory and boundary conditions for hypothesis formation. Analytical models typically focus on one small piece of the ecological puzzle, while ignoring many environmental, structural, and psychological details. This is partly for the purposes of tractability but also in the interest of simplicity, both of analysis and of parsimony. Nevertheless, the use of more complex computational models can shed light on processes of organization and evolution that are missed by simpler models. Spatial structure, temporal changes in social networks, lifespan-dependent aspects of individual behavior, or extensive heterogeneity in the population play important roles in the historical and evolutionary trajectories of human groups.

A striking example of the importance of including heterogeneity comes from a computational modeling study of the Anasazi, a now extinct society native to the North American Southwest [14]. A detailed agent-based model was built that was able to re-create details of the population expansion and subsequent collapse. However, the re-creation was only possible when age onsets for fertility and death were made heterogeneous across agents. Thus, a complex computational model may be justified when details such as individual heterogeneity and sociospatial structure are important to the dynamics under investigation.

Recently, Smaldino, Schank, and McElreath [15] presented a spatial agent-based model of the evolution of cooperation in harsh environments, in which increased environmental harshness was modeled as an increased energy deduction (a “cost of living”) incurred by each agent at each time step. The model demonstrated that, although harsher environments caused cooperators to fare worse than freeloaders in the short run, individuals with the highest fitness were observed in emergent groups with high concentrations of cooperators. Harsh conditions led to local extinctions in regions where cooperators were scarce or not well assorted. Most importantly, harsher environments led to higher long-term frequencies of cooperators, lending theoretical support to Kropotkin's [16] proposal that harsh environments should select for cooperation. Although harsh environments have previously been shown to select for reduced parasitism [17], [18], this was the first model to show an increase in cooperation with increased environmental harshness in which cooperative acts yielded positive payoffs and defection consistently outperformed cooperation in single interactions. These results depended on a model that incorporated aspects of life history (mobility, varied social partners, decoupled birth and death) and spatial structure, and as such contained somewhat more biological realism than many game theoretic models. Nevertheless, the model was still abstract in many ways; sociospatial structure arose through random movement and resources were obtained only through pairwise interactions.

The study of recent human evolution, including our cultural evolution, is likely to benefit from the development of a family of models which are even more complex, incorporating aspects of individual life history, social structure, social learning, and behavioral institutions. Ideally, these models would be unified by a common framework, which could be modified or adapted to investigate a wide range of questions about how the evolution of humans and their groups (families and wider groupings) resulted in an animal with the characteristics we observe in modern humans. The model we present in this paper is our first step toward developing such a framework. It incorporates representations of many elements of human life history and social structure found in small-scale societies [19]. We use this model to study the evolution of cooperation in a context which has a direct effect on fitness: in the caring of young. We then define environmental harshness as increasing costs of raising young relative to the availability of resources.

Below, we will first discuss the role of cooperative breeding in human evolution in more detail, followed by an overview of our model structure. We will then present the model in more detail. We will show that this more realistic model replicates the findings of the previous model, but also illustrates how factors such as genetic adaptability and seasonal variability in available resources complicate the picture. We will conclude with a broad discussion of future directions.

Human Cooperative Breeding We can define cooperative breeding behavior as individuals exerting costly parenting effort to contribute to the developmental success of an infant or juvenile that is not their own offspring [20], [21]. Among terrestrial vertebrates, this behavior is more common in birds than mammals but has evolved independently in a number of mammalian families [22], [23]. Several authors argue that humans can be considered cooperative breeders because, although parenting behavior is highly culturally variable, in no culture do mothers raise their children without help from others [7]–[10], [24]. Hill and Hurtado [24], in their studies of contemporary South American hunter-gatherer societies, found not only that cooperative breeding behavior was ubiquitous, but also, crucially, that husband-wife pairs were physically incapable of procuring sufficient food for their offspring and themselves without help from others. Moreover, they found that meat acquisition of Ache hunters over a given 90-day period was often highly variable for any given individual, as a result of illness, injury, or luck. Sharing food resources between nuclear families was therefore necessary to ensure the survival of young children. These observations suggest that humans are more accurately described as “cooperative breeders” than “biparental carers” as has been suggested by several influential evolutionary psychologists (e.g., [25]). Research in a number of small-scale subsistence communities has shown that the death or absence of a child's father often had no effect on the survival or welfare of the child [10], [26]. Presumably, in these subsistence societies contributions from the child's other kin and individuals who are unrelated but allied with the family are able to compensate for the lack of paternal contributions. For many parents living in economically developed large-scale societies, extended kin groups are no longer essential for raising children, but technology and the extensive networks of cooperation and division of labor in these societies have reduced the physical effort required for acquiring food. Also, state institutions such as education and health care systems provide considerable assistance to mothers raising their children. It is likely that hominins have been raising their children cooperatively for some time [8]. The large brains of humans inevitably make our offspring costly to raise because the growth and development of brain tissue requires high levels of energy and nutrients [27]. Van Schaik and colleagues have argued that the increased encephalization of the hominin line would not have been possible unless females were receiving help provisioning their young [28]. In particular, cooperative breeding has been identified as a potentially crucial factor in the evolution of human prosociality and our tremendous cognitive advantage over our nearest relatives, the great apes [8], [29]. Early Homo fossils are also the earliest hominin fossils to be found associated with the drier, more heterogeneous environments which began to expand in Africa at the beginning of the Pleistocene [30], [31]. In these harsher environments natural selection might have favored cooperation in the raising of young. Scarcity of water sources would have placed considerable stress on lactating females because human milk, like the milk of all primates, is very dilute [32], [33].