After the fall of Dravidian civilization, the Aryans (non Indian invaders) took over the Indus valley. Eventually they introduced and strenghthened the Caste System (falsely justified by religion) and thus strengthened their position by manipulating the true Indians to oppress their fellow Indians and made a fortune for themselves! Even today, Brahmins refuse to abandon the caste system as it superficially and wrongly gives Brahmins and their assosciates a false social and economic standing and even Mahatma Gandhi could not remove it either!
http://www.geocities.com/taxiacademy/ ).