These hunter-gatherer groups lived free from leaders or borders. Population density was low, so if you encountered someone, your first impulse was not to hate them. There’s little evidence that we waged war until about ten thousand years ago, says Douglas Fry, an anthropologist at the University of Alabama. “We’d rather party than fight,” is how he put it to me.

We saw this party preference among foraging groups even in recent centuries.90 Nineteenth-century accounts describe a group of Aborigines in Australia who lit a huge signal fire whenever they found a beached whale.91 The signal was meant to draw people from all over. It meant, Fry says, “Hey! Here’s a dead whale, plenty of food for all of us. We’ll get to renew acquaintances, we’ll get to have sex with other people, we can exchange art objects.” (Like I said, commune.)

So if this was our past, when did we become so hateful toward outsiders? How did we get from whale party to fascism? Apparently, it was when we started farming, about ten thousand years ago. Farming involved settling on patches of land and interacting less with outside groups. When we did encounter outsiders, the differences between us and them seemed more significant. Plus, unlike a forest full of berries, a carefully tended field was something we felt was rightfully ours. The earth’s climate changed. Social hierarchies developed in which some had more, and some had less, and you could fight your way from one rank to another.

Then came organized religion, money, and rulers—all of which diced people up into subgroups and, in some cases, pitted them against one another. Warfare, according to some scholars, has actually increased since “civilization” took hold. Even as society grew supposedly more sophisticated, humans were becoming more ethnocentric and parochial.

Our blissed-out ancestors would have been so disappointed.

More recent history, of course, has supported Tajfel’s theory: We tend to sort people into groups based on their differences, even when those differences are laughably minor. Then, we rank the groups in relation to our own.

In New York in the late nineteenth century, doctors and charity workers focused their anxieties on the influx of Eastern European immigrants, and particularly, on one of these newcomers’ troubling habits: eating pickles.92 According to the food writer Jane Ziegelman, pickles were seen as too pungent and, for the families who ate them, oddly addictive. This supposed “addiction” was apparently evidenced by the, uh, relish with which poor immigrant kids snapped them up. More likely, penny pickles were about the only luxuries they could afford.