3.4.3 The Era of Decline

Beginning around 1800 BCE, the centuries of trade between the Indus valley and Mesopotamia came to an end. Over the next four centuries, the cities of the Indus River valley were slowly depopulated, and the civilization declined, likely in stages. Why and how this decline occurred remains unknown. One common view is that it was related to regional climate change. Around 2000 BCE, the floodplain of the Indus River shifted dramatically, creating dry river beds where cities had been and water once flowed. Changes in the pattern of seasonal wind and rainfall, known as the monsoon in South Asia, may have caused these environmental effects. Without a secure source of water for drinking and irrigation, the cities would have suffered declines in population. Another theory suggests that centuries of environmental degradation caused by urbanization and large population growth made the land unsuitable to human populations. Still other views point to the possibility of tectonic activity that changed the course of the rivers, or even epidemic disease that decimated the population.

Before these environmental factors began to be considered, the view for many years was that the Indus valley civilization was violently destroyed in a conquest by nomadic Indo-European speakers calling themselves Aryans, a Sanskrit-speaking group of nomadic pastoralists who raised cattle and horses. Some Aryans began migrating from the Eurasian Steppe north of the Black and Caspian Seas around 3500 BCE. Over time, different groups spread into Europe, Anatolia, Iran, and eventually Pakistan and India.

The Aryan invasion theory of decline depends heavily on Indo-European works of religious literature like the Rigveda, produced sometime between 1500 and 1000 BCE in northwestern India. This work includes a great number of hymns, rituals, descriptions of deities, and other largely religious topics geared toward understanding the origin of the universe and the nature of the divine. But in parts it also discusses the arrival of the Aryans and describes them attacking the walled cities and forts of the indigenous population. While some of these descriptions were likely developed centuries after the fall of the Indus valley civilization and may be unrelated to it, some scholars continue to hold that these passages describe the conquest of Mohenjo-Daro. Archaeological evidence attesting to the fact that Mohenjo-Daro was attacked around 1500 BCE and mostly destroyed lends some credibility to these claims.

We may never know what best explains the collapse of the Indus valley civilization. A few or all these causes may have played a role. For example, environmental degradation caused by years of resource exploitation and high population density certainly had an effect. These issues could have been compounded by climate change and disease. And in a weakened state, the people of the region would have been far more vulnerable to attack by raiding groups like the Aryans.

However it happened, by around 1500 BCE, the social and political systems had broken down, and the sophisticated culture of the Indus valley civilization had collapsed. The architectural styles that characterized the cities at their height were abandoned, as was the writing system, the sophisticated metalworking, and other artisanal crafts. The Indus valley civilization had come to an end.