In the 1950s, Russian scientist Dmitri Belyaev ran a selective breeding project to, by artificial selection, breed (incredibly cute) domesticated silver foxes. The results of this multi-decade experiment were impressive, especially given that the foxes were selected solely for their amicability toward humans:
After only forty generations, the selected foxes began to display changes you (and Darwin, too) might think would take millions of years to evolve. As expected, they became incredibly friendly toward humans. Whenever they saw people, they barked, wagged their tails, sniffed the people, and licked their faces. But even stranger were the physical changes, which occurred at a higher frequency than in the control group. The ears of the selected foxes became floppy. Their tails turned curly. Their coats lost their camouflage and became spotty, with a star pattern appearing on the forehead. Their skulls became smaller. In short, they looked and behaved remarkably like their close relative the domestic dog.
The above quote comes from an article by Vanessa Woods and Brian Hare on Edge that looks at why our ancestors ‘came down from the trees’ and what separates us from other hominids, specifically chimpanzees (hint: it’s not the theory of mind, but tolerance and cooperation).
So what we have are chimps who cooperate but aren’t very tolerant, and bonobos who are very tolerant but don’t really cooperate in the wild. What probably happened six million years ago, when hominids split from the ancestor we share with chimpanzees and bonobos, is that we became very tolerant, and this allowed us to cooperate in entirely new ways. Without this heightened tolerance, we would not be the species we are today.