By 1600BC the great cities of the Indus valley had been abandoned. Quite why the people left is uncertain. Perhaps repeated flooding or shifts in the river beds destroyed the harvests and wrecked the once buoyant economy. The gradual drying up of the great Saraswati River caused a serious decline in agricultural productivity. There is also evidence of malaria and probably cholera, which must have taken their toll. The arts of city life, such as writing, disappeared. Although some regions continued to prosper, notably Gujarat in the south west, many areas saw a decline in their population.
Eventually, indo-Europeans from central Asia invaded the valley and settled among the existing population. Many farmers and pastoralists moved into adjacent regions in the south and east, and, slowly, sand silt buried the ruins of the cities. But the Indus culture did not die.
Although the Indo Aryan languages of the new settlers from the west eventually prevailed, the indigenous Dravidian languages had a strong influence on their vocabulary grammar and pronunciation. The Hindu religion that crystallized in the 1st millennium BC combined elements introduced by the Indo Aryans with the earlier beliefs of the inhabitants of Indus Valley settlements such as Harappa. Today, many features of modern Indian life have their roots in the Indus civilization- the first great flowering of culture in the subcontinent.