Our hominid ancestors lived for more than a million years before the advent of agriculture. The enormous changes that came with agriculture in the last 10,000 years have hardly begun to affect our gene pool. Early humans would most likely have lived in nomadic bands of around fifty people. And they would have done almost everything in the company of others. They would have almost never been alone.

First agriculture, and then industry, changed two fundamental things about the human experience. The accumulation of personal property allowed people to make more and more individualistic choices about their lives, and those choices unavoidably diminished group efforts toward a common good. And as society modernized, people found themselves able to live independently from any communal group. A person living in a modern city or a suburb can, for the first time in history, go through an entire day – or an entire life – mostly encountering complete strangers. They can be surrounded by others and yet feel deeply, dangerously alone.

The evidence that this is hard on us is overwhelming. Modern society – despite its nearly miraculous advances in medicine, science, and technology – is afflicted with some of the highest rates of depression, schizophrenia, poor health, anxiety, and chronic loneliness in human history.

A wealthy person who has never had to rely on help and resources from his community is leading a privileged life that falls way outside more than a million years of human experience.

Communities that have been devastated by natural or man-made disasters almost never lapse into chaos and disorder. If anything, social bonds are reinforced during disasters, and people overwhelmingly devote their energies toward the good of the community rather than just themselves. What catastrophes seem to do – sometimes in the course of a few minutes – is turn back the clock on ten thousand years of social evolution. Self-interest gets subsumed into group interest because there is no survival outside group survival, and that creates a social bond that many people sorely miss.

A modern soldier returning from combat – or a survivor of Sarajevo – goes from the kind of close-knit group that humans evolved for, back into a society where most people work outside the home, children are educated by strangers, families are isolated from wider communities, and personal gain almost completely eclipses collective good. Whatever the technological advances of modern society – and they're nearly miraculous – the individualized lifestyles that those technologies spawn seem to be deeply brutalizing to the human spirit.

Soldiers experience this tribal way of thinking at war, but when they come home they realize that the tribe they were actually fighting for wasn't their country, it was their unit.