Bound: Africans versus African Americans

The film opens with a folk tale about two loving brothers, born in the land of warm waters, Africa. Suddenly one brother is torn from the other never to be seen again. The one brother waits at the water's edge all his life then charges his son to do the same. This goes on for generations until several hundred years later, in the land of cold winters, America, when the daughter of one brother walks towards the daughter of the other brother and with every step they get closer until they walk past each other, one never noticing the other. Who did these descendants of two loving brothers become so isolated?