There seems to be some sort of misconception about humans, we are animals, tool using, clothes wearing, highly evolved animals so we breed, excrete, etc like any other. That being accepted our behaviour has parallels in nature, in territorialism, in competition, and in conflict with rivals and perceived threats. On this basis all humans are the same species, it is only socialization, and training the things that create cultures that makes us seem to differ.
Our barriers are our tribal territories on a larger scale. Had we all remained in our own places eventually we ,like Darwin's finches would become different species unable to interbreed, this has not happened. I am not saying that there aren't those within the world's sub groups who for whatever reason don't conform but like reports of rogue creatures, these are very rare, but the act of reporting increases the seriousness of how the situation is seen. The British Empire was not all Tiffin and tea, or pink gin and children banished to the nursery it was a time of suppression,repression and appalling abuses.