Co-operation, paternal care and the evolution of hominid social groups
Humans are social animals. Human societies emerge from vast networks of cooperative interactions between many different individuals. In this respect, humans are similar to most other primates. However, human societies are unusual among primates in the number of different types of cooperative relationships that are involved. In humans, males and females form strong pair bonds within large multimale, multi-female societies in which many other cooperative relationships are also important. How and when did human social systems arise? Do males and females use different types of cooperative strategies? Under what conditions does paternal care evolve? Do males and females have different constraints, and how do these affect the types of social strategies they employ? How do factors such as environment quality and seasonality modify these strategies? This thesis seeks answers to these questions using computer simulations based on the iterated Prisoner's Dilemma. The hypotheses generated by these models are tested using data from living primates. They are then used to investigate the kinds of societies that our hominid ancestors may have lived in. The theoretical and empirical evidence presented in this thesis suggests that sex differences in the energetic cost of reproduction determine the cooperative strategies, and ultimately the types of social groups, that evolve. It is proposed that during hominid evolution female energetic costs increased greatly, in comparison to male energetic costs, due to changes in body size dimorphism, diet and brain size. A two-stage model of hominid social structure is developed. The first stage, at the transition from the australopithecines to Homo erectus, would have involved an increase in female cooperation, especially food sharing. The second stage, occurring between 500,000 and 100,000 years ago, would have involved male care giving, the formation of pair bonds and the sexual division of labour within the context of a wider cooperative network.