India's caste system is among the world's oldest forms of surviving social stratification. The BBC explains its complexities. The system which divides Hindus into rigid hierarchical groups based on their karma (work) and dharma (the Hindi word for religion, but here it means duty) is generally accepted to be more than 3,000 years old. How did caste come about? 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"....