Witzel quotes the surveys made by the Pakistani archaeologist Muhammad Rafiq Mughal which showed there were settlements on the Pakistani side of the … - Tony Joseph

The millennium or so that followed the dimming of the Harappan Civilization would have been the most tumultuous and turbulent period in the history of the modern human in south Asia. But we have very little record of this and hence very little understanding of it. Look at all that happened: a long-standing civilization, the largest of its kind at the time, fell apart due to the ravages of a long drought, and its most visible symbols of power and prestige slowly disappeared even as urbanism itself did; people migrated to the east and the south in search of a new life; a new set of migrants came in from the north-west, bringing new languages and a different culture that put emphasis on sacrificial rituals and prioritized pastoralism and cattle breeding over urban settlements; another set of migrants came in from the north-east, bringing new languages, new domesticated plants and perhaps wetland farming techniques and a new variety of rice... and thus the pot of Indian culture was put on the boil. Four thousand years later, it is still simmering, with new ingredients getting added once in a while, from the Jews to the Syrians to the Parsis.

The most exhaustive, multi-year geological study on the the possible reasons for the decline of the Harappan Civilization was published in a 2012 paper titled 'Fluvial Landscapes of the Harappan Civilization' which identified a clear cause: a prolonged drought that ultimately made monsoonal rivers go dry or become seasonal, affecting habitability along their courses. To quote: 'Hydroclimatic stress increased the vulnerability of agricultural production supporting Harappan urbanism, leading to settlement downsizing, diversification of crops and a drastic increase in settlements in the moister monsoon regions of the upper Punjab, Haryana and Uttar Pradesh.'

The residents of Mehrgarh who raised the first mud-brick homes of two or three rooms may not have realized it then, but they were laying the foundation for the first efflorescence of civilization in South Asia, called the Harappan Civilization, or the Indus Valley Civilization. It took about 4500 years or over 150 generations, for those humble mud-brick abodes to turn into the urban structures of a Harappa or a Mohenjo-daro or a Dholavira and there must have been many twists and turns along the way. But once agriculture took root, and modern humans started creating a surplus that they could save and invest, the wheels of history started spinning fast - which would, of course, lead to the invention of the wheel itself!