Excellent article by Judith Shulevitz at The New Republic The Lethality of Loneliness: We now know how it can ravage our body and brain. She employs an origin story of the importance of being interconnected with a social group, and the panic and danger of being ejected from one's tribe. In her version, she indicates that we relatively weak human set up watch systems to look for danger--I don't know that this would be necessary.  We rapidly communicate danger when any one of us comes under attack--we scream and make other noises that alert our group, to the extent that we are part of a social group.   I think this description is of high relevance to the compulsion many people feel to be part of a religious group and also of the agony of being ostracized from one's group, for instance by excommunication.  Here's an excerpt:

[N]atural selection favored people who needed people. Humans are vastly more social than most other mammals, even most primates, and to develop what neuroscientists call our social brain, we had to be good at cooperating. To raise our children, with their slow-maturing cerebral cortexes, we needed help from the tribe. To stoke the fires that cooked the meat that gave us the protein that sustained our calorically greedy gray matter, we had to organize night watches. But compared with our predators, we were small and weak. They came after us with swift strides. We ran in a comparative waddle. “The very fact that [loneliness] can affect the genes like that—it’s huge,” Suomi says. “It changes the way one thinks about development.” So what would happen if one of us wandered off from her little band, or got kicked out of it because she’d slacked off or been caught stealing? She’d find herself alone on the savanna, a fine treat for a bunch of lions. She’d be exposed to attacks from marauders. If her nervous system went into overdrive at perceiving her isolation, well, that would have just sent her scurrying home. Cacioppo thinks we’re hardwired to find life unpleasant outside the safety of trusted friends and family, just as we’re pre-programmed to find certain foods disgusting. “Why do you think you are ten thousand times more sensitive to foods that are bitter than to foods that are sweet?” Cacioppo asked me. “Because bitter’s dangerous!” One of those alone-on-the-savanna moments in our modern lives occurs when we go off to college, because we have to make a whole new set of friends. Back in the mid-’90s, when Cacioppo was at Ohio State University (he is now at the University of Chicago), he and his colleagues sorted undergraduates into three groups—the non-lonely, the sort-of-sometimes lonely, and the lonely. The researchers then strapped blood- pressure cuffs, biosensors, and beepers onto the students. Nine times a day for seven days, they were beeped and had to fill out questionnaires. Cacioppo also kept them overnight in the university hospital with “nightcaps” on their heads, monitoring the length and quality of their rest. He took saliva samples to measure levels of cortisol, a hormone produced under stress. As expected, he found the students with bodily symptoms of distress (poor sleep, high cortisol) were not the ones with too few acquaintances, but the ones who were unhappy about not having made close friends. These students also had higher than normal vascular resistance, which is caused by the arteries narrowing as their tissue becomes inflamed. High vascular resistance contributes to high blood pressure; it makes the heart work harder to pump blood and wears out the blood vessels. If it goes on for a long time, it can morph into heart disease. While Cole discovered that loneliness could hasten death in sick people, Cacioppo showed that it could make well people sick—and through the same method: by putting the body in fight-or-flight mode. A famous experiment helps explain why rejection makes us flinch. It was conducted more than a decade ago by Naomi Eisenberger, a social psychologist at UCLA, along with her colleagues. People were brought one-by-one into the lab to play a multiplayer online game called “Cyberball” that involved tossing a ball back and forth with two other “people,” who weren’t actually people at all, but a computer program. “They” played nicely with the real person for a while, then proceeded to ignore her, throwing the ball only to each other. Functional magnetic resonance imaging scans showed that the experience of being snubbed lit up a part of the subjects’ brains (the dorsal anterior cingulate cortex) that also lights up when the body feels physical pain.