The basic ev-bio necessity behind the psychological unity of human brains is not widely understood.
“Necessity” may be over-stating it. Humans are not very diverse, due to recent genetic bottlenecks. We do have enormous psychological differences between us—due to neotony-induced developmental plasticity—and because there are many ways to break a human. We haven’t yet lived in the human hive long enough for the developmental plasticity to result in clearly-discrete queen/worker/warrior castes, with associated mental adaptations, though.
Go back in time, and often proto-humanity was divided into multiple tribes: the neanderthals, the cro-magnons, etc—with consequently reduced psychological unity. Travel and the modern gene pool expansion and stirring has obliterated the competeting tribes.
“Necessity” may be over-stating it. Humans are not very diverse, due to recent genetic bottlenecks. We do have enormous psychological differences between us—due to neotony-induced developmental plasticity—and because there are many ways to break a human. We haven’t yet lived in the human hive long enough for the developmental plasticity to result in clearly-discrete queen/worker/warrior castes, with associated mental adaptations, though.
Go back in time, and often proto-humanity was divided into multiple tribes: the neanderthals, the cro-magnons, etc—with consequently reduced psychological unity. Travel and the modern gene pool expansion and stirring has obliterated the competeting tribes.