Last updated January 11, 2018 at 10:29 am
A new study has turned our understanding of how the ancient Indus civilisation was organised on its head.
The Bronze Age civilisation was one of the most extensive urban cultures in ancient history, spanning back to 3,000 years BCE. At its peak around 2,500-2,000 BCE it was home to more than five million people, living in cities with brick houses and sophisticated water delivery and drainage systems. Where this water came from has been a source of debate among archaeologists.
The Indus has long been considered to be a river-based civilisation, with two of the five major “cities” located alongside Himalayan rivers. But the biggest concentration of settlements was far away from any major active rivers.
A now extinct river bed, or paleochannel, visible on satellite images, is thought to have been a source of water for these major settlements. The disappearance of this ancient river, known as the Ghaggar in India and the Hakra in Pakistan, was thought to have played a role in the collapse of the civilisation from 2,000 BCE on.
A new study set out to find out exactly when this river did dry up. Infra-red satellite mapping of the area was used to pick up the ancient river course. The river shows up as a sinuous blue line – the blue colour indicating a cooler and less reflective material which the researchers interpreted as ancient river sediments with higher moisture content.
How do you tell how long it’s been since those sediments were laid down by a river? In this case scientists used a technique called Optically Stimulated Luminescence. This method, developed in the 1980s, is used for long-range dating. It can tell how long it’s been since inorganic materials, such as quartz sand grains in river sediment, have been exposed to sunlight.
Sunlight effectively “bleaches” the quartz and resets it to a luminescence of zero. The longer it remains in the ground, the more it is exposed to natural ionizing radiation. This exposure to radiation excites nuclei in the quartz and it builds up a luminescence signal over time. To read this signal, scientists use luminescence readers to stimulate the quartz crystals with artificial light and measure the luminescence they give off.
Using this technique, Sanjeev Gupta from Imperial college London discovered that the river changed course around 8,000 years ago, long before human settlements.
This finding “resolves a question that has been debated for well over a hundred years”, says Gupta.
The ancient Indus people did not set up cities next to a river, but rather in a relatively recently abandoned river valley. This site probably offered much more long term stability, as Himalayan river belts regularly experience devastating floods or rapid changes in course. A recent example being the devastating flood of the Kosi river in 2008 which left three million homeless and led to the deaths of more than 2,000 people in India and Nepal.
Rather than relying on being located next to an unpredictable river, the authors suggest that the Indus cities were supplied with seasonal water flows from the monsoon.
Read the original paper in Nature Communications