Pack animals like dogs naturally discourage fighting amongst themselves [1]. Strife weakens the pack. Consider the effort it takes 1 person to build a house, or the amount of effort it would take to build an mp3 player alone from scratch. Game theory would have you believe the optimal way to win at poker is to booby trap the card table and rob your competition. Another theory suggests altruism ultimately trumps selfi…
The aberration is that we live in densely populated areas where selfishness is a possible behaviour.
Our social-limit is estimated at around 500 people. Surnames developed around the 13th century in Britain, before that it was patronyms meaning we could recognise people via association. This is still common in rural areas, my wife's grandfather likes to travel to car shows, he's told people down in the US to just ask for him by name. He lives in a rural part of Canada and sure enough, people will reach him because everyone knows him.
Altruism works, because for the majority of human history the people you chose to help or not to help were also the people who would sooner or later face the same decision. One instance of uncooperative behaviour would render you persona non grata. Your neighbour needs help thatching his roof after a storm, do you help? Game Theory keeps saying "no, because you could steal his land!" reality says "yes, because if he doesn't die from exposure, which he most likely won't, then he won't help you round up your goats when your fence breaks and they'll eat all your crops and you'll likely starve to death in the winter".
The simple fact is most people are willing to do "favours" with no questions asked, and with no expectation of payment except "being owed one". The classic is helping a friend move. There's absolutely no reason to help someone move, until you need to so you earn the help in advance, and when someone breaks the trust the response is normally "I'm never helping that asshole again."