In his travels around the world since the publication of his bestseller Start with Why, Simon Sinek noticed that some teams were able to trust each other so deeply that they would literally put their lives on the line for each other. Other teams, no matter what incentives were offered, were doomed to infighting, fragmentation and failure. Why?

The answer became clear during a conversation with a Marine Corps general.

“Officers eat last,” he said.

Sinek watched as the most junior Marines ate first while the most senior Marines took their place at the back of the line. What’s symbolic in the chow hall is deadly serious on the battlefield: great leaders sacrifice their own comfort—even their own survival—for the good of those in their care.

This principle has been true since the earliest tribes of hunters and gatherers. It’s not a management theory; it’s biology. Our brains and bodies evolved to help us find food, shelter, mates and especially safety. We’ve always lived in a dangerous world, facing predators and enemies at every turn. We thrived only when we felt safe among our group.

Our biology hasn’t changed in fifty thousand years, but our environment certainly has. Today’s workplaces tend to be full of cynicism, paranoia and self-interest. But the best organizations foster trust and cooperation because their leaders build what Sinek calls a Circle of Safety that separates the security inside the team from the challenges outside.

The Circle of Safety leads to stable, adaptive, confident teams, where everyone feels they belong and all energies are devoted to facing the common enemy and seizing big opportunities.

As he did in Start with Why, Sinek illustrates his ideas with fascinating true stories from a wide range of examples, from the military to manufacturing, from government to investment banking.

The biology is clear: when it matters most, leaders who are willing to eat last are rewarded with deeply loyal colleagues who will stop at nothing to advance their leader’s vision and their organization’s interests. It’s amazing how well it works.