To be honest I am guilty of "wiki-citing." There ought to be a term for copying and pasting from Wikipedia. However, I have an evolutionary psychology textbook that cites about 1000 sources, many of which are peer-reviewed studies, & I have read enough about the subject to know those scientists most likely meant what those quotes implied they meant.
Even if your tribe was not competing with another tribe it is still battling the forces of nature for survival. A tribe that cooperates well together is likely to weather the inevitable storms, and survive long enough to reproduce and spread on those altruistic traits. If the men cooperate and coordinate their hunting activities they will be more likely catch their prey. I will summarize what my textbook says about reciprocal altruism (in more detail later), and I think you will see why it is quite inevitable why RA would evolve. If you share your food with a friend when he strikes out hunting, than he is likely to share with you when your hunt is unsuccessful. If neither of you share, you will both likely die.
Plus, when you see things like cooperation and sharing between groups of apes & bats, that tends to lend credence to the hypothesis that general altruism is a trait that evolved a long time ago over eons.
But, behavior is very complex. We have many examples of authoritarian governments, cults etc. Obviously, human beings have evolved psychological traits that make them susceptible to a cruel and charismatic leader. However, even under authoritarian govs the adherents are still cooperating with each other.