The Bronze Age Indus Valley civilisation, which began around 5500 BCE and peaked 2000 years later, is recognised as one of the three great cradles of urban development, along with Egypt and Mesopotamia.
And although the Indus communities stretched from northeast Afghanistan to Pakistan and northwest India, one matter has long puzzled archaeologists.
The largest concentration of settlements was located in an area bordered, at a considerable distance, by the Indus River on one side and the Ganges-Yamuna river system on the other.
This is puzzling, because it is an article of faith – not to mention abundant evidence – that urbanised Bronze Age settlements always grew near permanent rivers. In Mesopotamian and Egyptian cultures, as well as some other Indus Valley areas, abundant fresh river water fuelled agriculture as well as enabling migration and the transport of goods.
Yet the thriving ancient active centre of Indus Valley life is situated nowhere near a major water source.
Now, however, a team of researchers led by earth scientist Ajit Singh, of the Indian Institute of Technology Kanpur, has solved the mystery – and challenged several basic assumptions in the process.
The first of these is the idea that a change to the route of a major river led inevitably to the decline of settlements along its original course. This position also assumes that the history of any major river runs in chronological lockstep with the history of nearby towns and cities.
Using a variety of techniques, including analysing satellite images and deciphering uranium-to-lead decay data for zircons retrieved from different sand strata in in the region, Singh’s team determined that indeed there was once a mighty river running through the heart of the Indus Valley settlements – but thousands of years before they were built.
The river has survived. It rises in the Himalayas and is today known as the Sutlej. Sediment analysis showed that 8000 years ago it ran through the settlement areas but then suddenly changed course.
The reason for the course change is unknown – Singh’s team speculate that it might have been prompted by a long decline in the strength of the Indian summer monsoon – but it developed into a watercourse running through a steep valley, and it remains there today.
As it departed it left a scar across the landscape – a paleochannel – which, centuries later, presented an enticing settlement prospect for early Bronze Age farmers.
“We suggest that this abandoned incised valley was an ideal site for urban development because of its relative stability compared to Himalayan river channel belts that regularly experience devastating floods and lateral channel migration,” the researchers write in the journal Nature Communications.
The departure of the Sutlej, however, did not leave the Bronze Age settlers high and dry.
Singh and his colleagues report a partial infill of the paleochannel with very fine-grain sediments. They interpret these as silt deposits from smaller, transient river systems that developed to funnel monsoon waters draining out the Himalayas.
The sediment strata over millennia reflect the relative strengthening and weakening of monsoons in the area.
The result, for thousands of years, was a stable area of land in which urban enclaves could develop with very little risk of damage and crop loss caused by major floods. The finding, conclude the scientists, tips a central tenet of ancient history on its head.
“We find that river dynamics controlled the distribution of Indus sites in the region, but in the opposite sense to that usually assumed,” they write. “It was the departure of the river, rather than its arrival, that triggered the growth of Indus urban settlements here.”