In the early 1980s, the anthropologist Hilly Kaplan visited Paraguay to study a hunter-gatherer tribe called the Ache. He found a moral code that, by western standards, seemed too good to be true: Ache hunters shared with open hands, giving away 90 per cent of their meat and 80 per cent of the grubs and fruit they gathered. The Ache believed that a hunter who ate his own kill would be cursed.

As the years passed, Kaplan saw this sharing culture disappear. The rainforest was being hacked back, so the Ache found that foraging would no longer sustain them. They put down roots - literally - and began to farm. And as they started farming, they stopped sharing.

This sad story makes perfect sense. Hunters have enormously volatile incomes: one day they may catch more than they can possibly eat; the next day, nothing. Kaplan discovered that Ache hunters came home empty-handed slightly more often than not. At such a strike rate a hunter would expect to do without food for an entire week about once every year.