The Indus valley civilization saw its genesis in the holy land now known as India around 2500 BC. The people inhabiting the Indus River valley were thought to be Dravidians, whose descendants later migrated to the south of India. The deterioration of this civilization that developed a culture based on commerce and sustained by agricultural trade can be attributed to ecological changes. The second millennium BC was witness to the migration of the bucolic Aryan tribes from the North West frontier into the sub continent. These tribes gradually merged with their antecedent cultures to give birth to a new milieu.