I take it you only listen to anarchists about government? And christians... A minority? Right.

Tribes are a little different, they don't have a government so much as a leader... But there are no tribes without a leader. Why? Because there needs to be someone to settle disputes and make decisions. (Another function of government.) You see this even on small teams, if no leader is assigned someone will become that leader, sometimes even if they don't want it.

Back to the weakness thing. Some tribes do kill off "deformed" people but that isn't necessarily about weakness. Do you know of any tribes where the leader has his thugs kill the elderly? Or kids, since they are weak? Or allows the murder of women, since they are weaker than men? Hell, even the weakest hunter can't be summarily murdered simply because the best hunter is stronger... Because their society, along with pretty much all societies, don't allow murder. (Except when they do, but that's usually due to a spiritual belief. For instance those African albinos are believed to be cursed.)