Even before tribes there were bands, very small groups of people who banded together for survival and companionship.
Freud says that civilization began when the first person threw an insult rather than a rock. Once upon a time, two bands encountered one another. Likely adrenalin levels spiked. They prepared for a conflict. Someone, though, made a gesture of conciliation. Maybe they offered food. Or a gift.
We are fiercely reciprocal creatures and this reality is a foundation upon which any social construct rests. Salespeople know that our feelings of reciprocity are so strong that if they offer you a free meal and night's lodging, you'll feel obliged to buy one week per year of a timeshare for life. That gesture of giving triggered the other band to offer something in exchange just as surely as landing a blow would have resulted in a fight. From that day on, these two little bands would periodically meet and make exchanges.
One may have had a unique fruit from a tree in their area. Another may have had some fibers they'd learned to weave into a garment. As they began to trade things ideas came along with them. "No," they would say in words or demonstration. "You don't eat the fruit like that. You first open it like this." Or they laughed and said, "No, this is not a hat. It is a loincloth." Or maybe in that moment of confusion they realized it could be a hat and that could solve the problem of cold heads. And the exchange of goods became the exchange of ideas that work like genes, mingling to become something and someone new.
The two bands learned from each other and traded the excess of one for the excess of the other (the 100th orange from my tree has less value to me than my first apple from your tree; I will happily trade you my 100th orange for your 100th apple). They also began to exchange ideas and goods that could be combined to create new ideas and goods.
Progress comes from more us and less them. "They" are a group who may steal our apples. "We" are a group who may trade apples for oranges. Trade networks didn't just enrich lives by exchanging goods and ideas. It also made their world safer. If something happens to the tree in my little patch of the world, I don't automatically starve. After trade comes finance: you can loan me oranges now until my apple tree is again producing. And then comes investment: I will set aside some grapes to turn into wine that we won't drink for months or years and eat your apples in the interim.
Arrangements break down. Businesses dissolve. Groups go to war. But progress is the steady expansion of us, a richer group with whom to exchange and to be inspired by.
More us generally means more options and more prosperity. Less them means fewer enemies, fewer people excluded from the process of negotiating and defining our shared world. This is the route to peace and prosperity however unwieldly it may be.
By contrast, more them means more enemies, more people from whom to feel estranged, to see as vile or unworthy. Fewer us means fewer people with whom we can trade, inspire, borrow or lend. This is the route to conflict and impoverishment.
More us and less them. It has been a useful guide for peace and prosperity for thousands of years. It still is.