Darwin’s “Survival of the fittest’, doesn’t mean superiority of the strongest, biggest and meanest. Originally, "fitness" only refers to the offspring brought in to the world. It was never meant to mean more than that. Using the intended definition of fitness, research has repeatedly shown that friendliness is the key to life’s biggest successes, but it's not all rainbows and unicorns.
Humans are not the top of the food chain, so our ancestors clearly could not rely on overpowering the lions, tigers and bears which confronted them. They had to be clever to survive, compassionate and FRIENDLY, which could also look like "sharing" and "helping." This is how humans have lasted so long.
The flip side, though, is that our friendliness creates powerful bonds with certain people. Eventually our friends feel more like family, our ultimate in-group, and that creates a sense of aggression if they are threatened in some way - or even if a threat is perceived. Where there is an in-group, there must therefore be an out-group.
Psychologist Jonathon Haidt, talked about this is his 2008 Ted Talk, The Moral Roots of Liberals and Conservatives. Generally there are diametrically opposed viewpoints to nearly every problem. Even though people may interpret things differently because of "moral diversity," nobody is wrong. It's because of both viewpoints that humans have survived for so long. It's kind of like Mother Nature hedging her bets.
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