this is kinda of unique because of the height this civilization reached, humans naturally form an ethnic group first and foremost around what they feel, feelings are pure instinct, so if your parents look like you, are are related genetically to, your instincts won't stress you out, because you're safe, this feeling evolved to try to dominate to be not be dominated as a group
is this fact used by the powerful for ends they want, sure, is that a reason to consider a nigger or a white lib my neighbor, no