Theories regarding the sudden disappearance of the Indus Valley, or Harappan, civilization close to 4,000 years ago is quite consistent across sources. This was the world’s earliest urban civilization, which began approximately 5,200 years ago. The population of over 5,000,000 lived along the Indus River in what is now India, Pakistan, Nepal, Afghanistan, and Bangladesh – spanning over 386,000 square miles. While the land along the river valley was fertile, those just beyond were arid and unable to be cultivated, preventing the civilization from expanding further.
The loss of the Indus Valley Civilization is most baffling mainly due to their advanced culture. While the civilizations of Egypt and Mesopotamia are well known by the population at large, “the Indus civilization, which was bigger than these two, was completely forgotten until the 1920s,” said researcher Liviu Giosan, a geologist at Woods Hole Oceanographic Institution. It was at this time that evidence was unearthed indicating a civilization using advanced cities using grid systems, plumbing and sewage systems, mapped trade routes and sea routes to Mesopotamia, and as yet un-deciphered system of written language. Findings indicate use of domesticated animals (though not horses), as well as crops including cotton, peas and barley, though not on a large scale due to lack of extensive irrigation systems. Experts say that such an advanced culture was found again for many centuries. Construction was done using stone and bronze. There is no evidence of armies, slaves, conflicts, or other aspects of ancient societies. They were believed to have been a society similar to democracy, with no signs of monuments, sacrificial altars, etc. common to civilizations long after their time.
So how did such an advanced civilization disappear? Two theories attempt to explain, Aryan Invasion and Climate Change.
THE ARYAN INVASION THEORY claims that Aryans, a nomadic, Indo-European tribe conquered the Indus Valley Civilization. As described above, the Indus Valley people were peaceful, non-militarized. They did not have the use of horses or effective weapons to hold off an attack by invaders on horseback with advanced weaponry. Initial attacks began approximately 2,000 BC. They would have been easy prey for the Aryans. However, elements of Indus culture are found in later civilizations, which puts this theory in question. This would seem to indicate that they were not wiped out, but perhaps driven from their home.
CLIMATE CHANGE THEORY seems to be the prevailing school of thought. As with most river valleys, the Indus region was prone to at times destructive flooding. Likewise, this part of the world had and still has monsoon season. Over time, however, studies show that the monsoons lessened in intensity. Giosan states that solar energy varies in cycles, which impacts monsoons and their intensity. As monsoons weakened, the valley became quite fertile. This is how the Indus Valley people were able to cultivate crops, as indicated above. However, over time the increasing solar insolation caused the monsoon-based river to dry, leaving insufficient water supply for drinking, the bath and plumbing systems of the Indus people, as well as their domesticated animals and crops. The monsoons migrated to the east due to the climate change, and so then did the Indus civilization. They had to move eastward as well, to the more fertile ground along the Ganges River, Giosan speculates. Here monsoons were still reliable. In the Ganges region, this large civilization is believed to have broken into smaller villages and farms, as larger cities could not be supported there. Since they were unable to produce sellable goods, trade with the Mesopotamians lessened as well.
Other scholars agree with the climate change theory as well as the migration toward the Ganges, but believe that other natural disasters: drought, deforestation, flooding, and changes in the course of the Indus River may have also contributed to the collapse of the Indus Valley Civilization. Around 1800 BCE, the Indus valley climate grew cooler and drier and a tectonic event may have diverted the river system toward the Ganges Plain. Excavations in the Ganges Plain show that settlement began around 1200 BCE. By around 1700 BCE, most of the Indus Valley Civilization cities had been abandoned by this time.