@browncoat - That is one theory, but I think they have difficulty explaining what happens when someone takes advantage of that kind of deal. Wouldn't the human who didn't help out but still reaped the benefits end up reproducing more?
Also, this isn't really that unique. Most animals do some kind of grouping arrangement to protect from predators or in order to hunt. We might do it in a slightly different way, but I'd say compared to, say, ants, we aren't all that good at it.
I mean, if nothing else, look at what happens when people group together for cycling. It's good in theory, but as soon as one of them clips another one, the whole group goes down. You'd never see a flock of birds behave in such a clumsy way.