The great anthropologist, Claude Levi-Strauss, theorized that there are three basic 'oppositions' that operate at the base of any culture. An opposition was a pair of extremes within which anything was fixed in the cognition. The three basic 'needs' that make the human mind human are hunger, sex and aggression. So, Levi-Strauss said, the basic oppositions that operate in any culture need to clearly address these.
The opposition that deals with hunger is the food/non-food opposition. It classifies everything between two extremes - what may be eaten and may not. The opposition that deals with sex is the sister/wife opposition. This classifies all females (in patriarchal societies; in matriarchal socieities, this could be called the brother/husband opposition) into either 'sisters' - those with whom sexual relations are not permitted, and 'wives' - those with whom sexual relations are permitted.
The third opposition is perhaps the most powerful one - the us/others opposition. In the beginning, this opposition was quite basic - it was very simple as it meant on human/non-human. Even then, it seems to have fomented trouble as there is believed to have been endless trouble between neanderthal man and early Homo sapiens, with the former finally being hunted to extinction. Since then, as human societies evolved, they harnessed the power of this opposition to form tight, well-organized units that had a high degree of survivla skills. The us/others opposition kept operating on different levels - friends/foes, better/worse, stronger/weaker, bigger/smaller - in each of these, the ones you aligned with fell into the 'us' bracket while those with whom you competed fell within the 'others.'
It is this opposition that lead to genocides, as for each society to survive, it had to wipe out all threats, perceived and real. The biggest genocide in history - the wiping out of Native Americans, and the second biggest genocide in history - the Holocaust, are both examples of the extremes to which this opposition can take us.
History is to be learnt from, not to be repeated.