If you stayed in the tribe you were born in, no one would expect more from you than to contribute a bit of your time and effort for the well-being of the social group. Every pair of arms is useful, and on average there are enough that you can deal with the random invalid tribesperson; in fact even the invalid tribesperson would have a legitimate 'usefulness' to the tribe.
Given that this was the state of things for million of years, we shouldn't taint our assumptions by basing it on our modern utilitarian view of society. For most of our time on Earth, for every animal except the human, only the individual has determine their own usefulness, even if it is just to eat, reproduce and die. There's nothing wrong with that.