It has been generally thought that the Harappan Civilisation (the Indus Valley Civilisation or, more correctly, the Indus-Saraswati Valley Civilisation) lasted about 2,000 years from about 3,300 BCE to about 1,300 BCE with a peak period from about 2,600 BCE to about 1,600 BCE. It was known that settlements in region date back almost 9,000 years ago (7,000 BCE) but it was thought that these were small, isolated settlements and not part of a cohesive society.
But a new paper reports on studies based on “a high resolution oxygen isotope (δ18O) record of animal teeth-bone phosphates from an archaeological trench itself at Bhirrana, NW India, preserving all cultural levels of this civilization. Bhirrana was part of a high concentration of settlements along the dried up mythical Vedic river valley ‘Saraswati’, an extension of Ghaggar river in the Thar desert. Isotope and archaeological data suggest that the pre-Harappans started inhabiting this area along the mighty Ghaggar-Hakra rivers fed by intensified monsoon from 9 to 7 ka BP. The monsoon monotonically declined after 7 ka yet the settlements continued to survive from early to mature Harappan time. Our study suggests that other cause like change in subsistence strategy by shifting crop patterns rather than climate change was responsible for Harappan collapse”.
The “Harappan civilisation” lasted, it seems for 4 to 5 millenia, and climate was probably just one of many factors causing its decline. But the decline was nothing very sudden. It may well have been a gradual depopulation of the region as inhabitants moved east and south. Some may have moved back to the north-west as the Thar Desert established itself.
What is striking is how the two great rivers, the Indus and the Saraswati (Ghaggar or Hakra), totally dominated the landscape. It is not surprising that an advanced water management “technology” developed. The growth of managed agriculture and trade followed. The period of intensified monsoon from 7,000 BCE to about 5,000 BCE could have been a time of plenty which promoted growth and settlement.
What is still remarkable is that there is no evidence of war-like events. The decline was not due to an invasion or a war. But the society was structured and had many building standards and techniques which were applied across many of the towns. Some form of central administration – if not rule – must have been in place.
It seems to be that the “settled”, peaceful society that seems to have existed may well have been due to the “invention” of professional guilds and the classification of people by function (and not primarily of hierarchy). Classification were perhaps established during the times of plenty. While it may have been a “horizontal” classification which was so successful, it was also the classification which lead eventually to the castes and the perversions of castes which are visible today. It is not impossible that it was societal pressures as a horizontal classification gave way to a vertical stratification which contributed to the decline. But that is all just my speculation.