In the world of cubicles and studio apartments, loneliness is everywhere. We find it in both crowds and empty rooms. We change cities and lose friends. Even in marriage, people can be strangers to one another. But things were very different for our ancestors. When humans were evolving in a prehistoric environment, they banded together for food and for protection. To be ostracized from your tribe was a death sentence, says Charles Raison, a psychiatrist at the University of Wisconsin, Madison who did not work on the study. "Literally they would die. There was no human way to live in isolation," he says.