Evolutionary hypotheses about human development raise philosophical questions about human nature, the origin of morality, and the causes that made our complex social world possible. The Australian philosopher and evolutionary theorist Kim Sterelny thinks that answers to these questions require an account of how changes in culture and biology interacted in the course of hominin history. In The Pleistocene Social Contract, he tells a story about hominin development using evolutionary theory, evidence from archaeology, and anthropologists’ accounts of life in hunter–gatherer societies.

How did creatures with mental capacities similar to chimpanzees at the beginning of the Pleistocene manage to develop big brains and a complex social life? The answer, Sterelny thinks, was their increasing ability to cooperate. Cooperation can produce great advantages for participants, but it is vulnerable to bullies who try to monopolise benefits and to cheats who try to get them without doing their share. Cooperators must be able to accept and enforce norms of reciprocity. Philosophers justify these norms by arguing that rational individuals ought to agree to a social contract that gives each a fair share of benefits and burdens. Sterelny’s task is to explain from an evolutionary point of view how the norms required by such a contract became possible.

The first step, according to his story, was taken by hominid groups that hunted large game with stone weapons. Everyone participated and everyone got a share. Slacking off was not possible, and bullies were discouraged by the fact that their fellows carried axes. The next stage, he thinks, resulted from the invention of projectile weapons that enabled people to hunt in smaller groups. Successful hunters brought back their kill to the group, shared it with those who were not successful, and expected to get a share when others succeeded. Reciprocity of this nature requires trust and acceptance of the norms that make that trust possible. Gossip, Sterelny thinks, played a crucial role. Group members shared information about what others did. A good reputation became important to individual fitness, and community members internalised and enforced the norms governing their exchanges. In the course of this development, according to Sterelny, hominins were becoming language users with mental capabilities not all that dissimilar to modern humans. Hominin social life in the later Pleistocene, he supposes, resembled relationships in forager communities described by anthropologists. By means of contacts between male kin communities were able to cooperate with each other. Community members shared resources more or less equally and violent conflict between groups was infrequent.

Sterelny’s next task is to explain how many humans, starting late in the Pleistocene, came to live in unequal, authoritarian societies that made war on each other. He speculates that the change occurred when humans were able to store food, farm, and settle down. Farming went along with private property and inheritance – a recipe for increasing inequality. Violent takeover of the land of others was now rewarding, and negotiations between the ‘Big Men’ of different groups became vital for group survival. These individuals rose to a position of power aided by priests and shamans who found ways of justifying the authority of leaders.

Sterelny’s account of the Pleistocene social contract is not meant to condone authoritarianism or inequality. Theorists now reject the idea that evolution justifies a belief in the superiority of modern civilisation. We are free to conclude that the Pleistocene social contract turned out to be a bad deal for most people. Nevertheless, Sterelny’s story reproduces some of the questionable assumptions made by earlier theorists.

One problem is his focus on the development of the interpersonal norms that we call ‘morality’. What he leaves out are the norms that arose from an interaction of hominins with their natural environment – what Aboriginal people call ‘law’. These norms seem to be universal in societies described by anthropologists and motivate cooperation within and between groups. An evolutionary story that ignores their development is inadequate. Also largely missing from Sterelny’s account are women. The important changes, according to his story, came from hunting large game, the connections between male kin and the increasing domination of Big Men. Sterelny’s social contract, like that of classical philosophers, is largely men’s business. Perhaps he is right. But it’s also possible that he, like earlier theorists, has not paid sufficient attention to the knowledge and skills more likely to be acquired by women.

People in many forager societies know how to make poisonous plants edible through complicated processes. They have learned to make medicines out of plants. These discoveries, probably made by women who used them to raise healthier children, require an ability to distinguish between what appears to the senses and an underlying reality that can only be exploited by following rules. Could these discoveries have been the basis of a belief in the existence of a law governing relations with nature? Could they have led to the developments in brain power that made scientific thought possible? This is merely speculation. We will never know what happened so long ago. But one of the virtues of Kim Sterelny’s approach is that it couples science with storytelling. We cannot experience what it was like to be a Pleistocene hominin, and we will probably never know how our hominin ancestors became modern humans, but we can use what we know to tell plausible stories that bring them closer to us.