People don't watch each others backs and they never did. People act in their own self interest, and it was in one's interest to help the tribe survive.
People do watch each others backs - sometimes for selfish reasons, sometimes just because they've been brought up that way and it's automatic by the time they are adults. But whatever the reason, we are strongly tribal.
Here's a small anecdote for you: I was a small child when the IRA tried to assassinate Thatcher (and they managed to murder some of her ministers in that attempt). Now my family was anti-Thatcher - but my God they turned anti-IRA as a result of that and other bombings. Because Thatcher like or hate her, was part of our tribe.
Every now, when I heard that the Boston bombs were placed in litter bins, my first thought was "IRA", because that was their signature, and if you look at British boards, I wasn't the only one thinking that. When Americans heard the same thing, they immediately thought "Saudi". That was pure tribalism in action there, on the part of both - and it's not rational.
You keep talking about "self-interest", but that is part of our rational selves - you actually have to calculate what your self-interest is before you can act selfishly, and it's done by the conscious mind.
Tribal instincts work on a deep subconscious level. Many people try to pretend these instincts don't exist, but loyalty to the tribe is pretty much one of the strongest forces on earth, second only to loyalty to family. In fact traditionally tribes were extended family.