We find reciprocity in all species whose food supplies are subject to high fluctuations. Suppose you are a hunter-gatherer. One day you are lucky and kill a deer. You can’t possibly eat all of it in a day, and refrigerators are still a few centuries away. You decide to share the deer with the group, which ensures that you will benefit from others’ spoils when your haul is less impressive. The bellies of your buddies serve as your refrigerator.
