They tell me that there is more genetic diversity among the chimpanzees on a single hillside in Gombe than there is in all the human race, worldwide - features and skin color notwithstanding!
I think that there is a semi-instinctive urge to divide people into "us" and "them" groups - it seems to have been done by every tribe and civilization from the dawn of time. In the case of people who exist in smaller, tribal communities, the name of the tribe often simply means "the people" in their language. As though the "other" group, if their existance is known, were something - less than people. Of course, this trait is capitalized upon by our leaders when they wish their people to make war or to compete in some way . . .
I should think that living on islands would make the "us" group seem tighter and more homogeneous, and the "them" group seem farther separated from "us", and perhaps by extension, less "human".
I do think that as the world grows smaller, and we travel and meet people of other cultures, their humanity and their sameness will become more evident - and as we meet and make friends with people around the world through the internet, the world's communities will change - for the better!
My $.02 - YMMV.