When a Shiwiar gets hurt in this way, his or her kin group takes care of the person until they’re better. In one case, a young man who suffered nerve damage from a snakebite was shuttled from safehouse to safehouse by allies during wartime. Without this kind of help, many Shiwiar would have died from their injuries.

The example of the Shiwiar shows why we humans, reflexively, like to take care of our own. In our ancestral past, we couldn’t accumulate wealth or stockpile resources. We were hunter-gatherers, and once we had hunted and gathered all there was to hunt or gather in a given area, we had to move on. Social relationships were the only bank deposit you had. To keep your group from going extinct, you had to provide your group members with resources when they were struggling.

“Since networks of alliances are the only health insurance policy available in small-scale societies, it follows that, when the likelihood of illness increases, individuals should be motivated to ensure both that their premiums are paid and that their coverage is extensive,” wrote Carlos Navarrete and Daniel Fessler in a 2006 study.87 Your premiums are ties to other people. Your coverage is how they help you when times get tough.

Most researchers agree that the successful groups of our ancestral past were like the Shiwiar. They were basically like little hippie communes, with more, or perhaps less, nudity, depending on which particular communes and cavemen you’re picturing. They lived in close-knit communities and looked out for one another. There’s some debate, however, about how willing our ancestors were to help people beyond the Shiwiar down the block. Some anthropologists and psychologists argue that less porous groups were, in past millennia, more likely to survive and thrive than groups that would accept just about anyone. In other words, according to this theory, we humans have always favored our own little band of hunter-gatherers and remained wary88 of anyone else.89 Today, some people say illegal immigrants are taking “our jobs.” For thousands of years, according to this theory, we were worried they’d steal our freshly killed meat.

Others, though, paint a rosier picture of our Pleistocene forebears, suggesting that some of them were actually less fearful of outsiders than humans are today. Nomadic foragers, as these early people were called, lived in groups of about twenty or thirty, but they switched groups often and would frequently trade and interact with groups that lived even hundreds of miles away. A family might live with the parents of the wife for a while, then travel around with the parents of the husband for a bit.