One day, the people of a village in New Guinea decided to change their word for "no", to distinguish their speech from that of a neighbouring village. Another community swapped the masculine and feminine genders in its language to stand out from its neighbours. The principle of a local language for local people is cherished on the island, with the result that it is home to 800 of the world's 7,000 tongues. The New Guineans' ability to pass rules obstructing mutual comprehension might well be envied by European linguistic nationalists, whose powers are generally limited to words for things like computers and aeroplanes. Yet it is only a difference of degree. The New Guineans' attitude is instantly recognisable, because parochialism is universal.
Why so many languages? Mark Pagel, who heads the Evolution Laboratory at the University of Reading, once put that question to a nomadic pastoralist in northern Kenya who spoke six but had rarely gone more than 30 miles from his birthplace. It's a profoundly difficult one to answer in any language. Throwing into relief humankind's apparent compulsion to cluster in groups, it is one of the most significant questions we can ask.
We can start to answer by asking a more fundamental question: why do we uniquely have such large and capable brains? Pagel's view is that "we owe our big brains less to inventiveness than to conflicts of interest among social minds engaged in an arms race to be the best at manipulating others". It wasn't the computing power needed to make stone tools; it was the pressure of life as intensely social beings, depending on each other while competing with each other. Our distant ancestors needed unprecedented brainpower to understand and influence what was going on in the brains of those around them.
As Pagel's phrasing illustrates, this view of the evolved mind is not rose-tinted. Its fundamentals are competition and self-interest – but what fascinates it is how self-interest leads so abundantly to co-operation. Pagel condenses several decades of thinking about the social brain and the evolution of cooperation to tell a story in which the most sophisticated forms of learning – the ability to imitate another's actions, with a degree of insight into their purpose – are a relatively recent evolutionary arrival. This is a somewhat obtuse reading of the artefacts earlier hominins left, and what we can reasonably surmise about how they may have lived. But there's no doubt that anatomically modern humans brought about the first cultural spring the planet had ever seen. Art and adornment flourished; toolkits diversified.
Pagel does not see this as a universal explosion of creativity, though. We are most of us imitators, he argues; only a few are innovators. Our distinguishing intelligence is our ability to assess what others are doing and copy the best examples. Culture is principally an imitation system; and in such a system, nothing matters more than reputation.
That encourages cooperation, because what individuals gain in reputation by cooperating may well exceed the costs they incur by doing things for the benefit of others. The regard of others is of such paramount importance that language, Pagel argues, is "principally a social technology for managing and exploiting the benefits of reputation and the cooperation it enables".
Culture reduces conflicts of interest by aligning people's behaviour and values. The more you share somebody's culture, the better you will be able to predict their behaviour, and the easier it will be for you to decide whether you can trust them to act in your interests in any given situation. The stricter the culture's rules, the more predictable and trustworthy its members will be to each other – and the more suspicious they are likely to be of everybody else. Language can be used as a technology to strengthen bonds within a community by frustrating connections with outsiders. It promotes what Pagel calls tribalism.
He builds his argument the way it should be built, recognising that in the beginning there is self-interest, that culture should be considered as an evolutionary adaptation that promotes the replication of genes, and that to be human is to be social. It is a compelling account, though readers unfamiliar with the territory may have some difficulty seeing the wood for the trees, and will find little in the way of aesthetic nuance.
But having begun with a question, he leaves another one hanging. Encouraged by the cultural diversity of modern cities, he expresses his confidence that people are beginning to find new markers of trustworthiness to replace traditional ethnic and cultural ones. Let's hope he is right – but what exactly are these brave new markers that have the power to end our species's tribal history?
Marek Kohn is the author of 'Trust: Self-Interest and the Common Good' (Oxford).
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