Manusmriti, widely regarded to be the most important and authoritative book on Hindu law and dating back to at least 1,000 years before Christ was born, "acknowledges and justifies the caste system as the basis of order and regularity of society".
According to one long-held theory about the origins of South Asia's caste system, Aryans from central Asia invaded South Asia and introduced the caste system as a means of controlling the local populations. The Aryans defined key roles in society, then assigned groups of people to them.
