If we look at even animals in the wild, they too have the tendency to have a specific territory and another being entering it is considered hostile. So boundaries do exist partially even in the animal kingdom. Now taking in to account the world even during ancient times, conquest and capturing of territories was common even then, man developed specific boundaries and areas which can be considered his/her own in accordance with their race or ethnicity, which gave the innate nature or feeling that they need to protect it form outsiders(just like the animal kingdom).