The mysterious fall of the largest of the world's earliest urban civilizations nearly 4,000 years ago in what is now India, Pakistan, Nepal and Bangladesh now appears to have a key culprit — ancient climate change, researchers say.
Ancient Egypt and Mesopotamia may be the best known of the first great urban cultures, but the largest was the Indus or Harappan civilization. This culture once extended over more than 386,000 square miles (1 million square kilometers) across the plains of the Indus River from the Arabian Sea to the Ganges, and at its peak may have accounted for 10 percent of the world population. The civilization developed about 5,200 years ago, and slowly disintegrated between 3,900 and 3,000 years ago — populations largely abandoned cities, migrating toward the east.
"Antiquity knew about Egypt and Mesopotamia, but 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 in Massachusetts. "There are still many things we don't know about them."
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Now Giosan and his colleagues have reconstructed the landscape of the plain and rivers where this long-forgotten civilization developed. Their findings now shed light on the enigmatic fate of this culture.
"Our research provides one of the clearest examples of climate change leading to the collapse of an entire civilization," Giosan said.
The researchers first analyzed satellite data of the landscape influenced by the Indus and neighboring rivers. From 2003 to 2008, the researchers then collected samples of sediment from the coast of the Arabian Sea into the fertile irrigated valleys of Punjab and the northern Thar Desert to determine the origins and ages of those sediments and develop a timeline of landscape changes.
"It was challenging working in the desert — temperatures were over 110 degrees Fahrenheit all day long (43 degrees C)," Giosan recalled.
After collecting data on geological history, "we could reexamine what we know about settlements, what crops people were planting and when, and how both agriculture and settlement patterns changed," said researcher Dorian Fuller, an archaeologist with University College London. "This brought new insights into the process of eastward population shift, the change towards many more small farming communities, and the decline of cities during late Harappan times."
Some had suggested that the Harappan heartland received its waters from a large glacier-fed Himalayan river, thought by some to be the Sarasvati, a sacred river of Hindu mythology. However, the researchers found that only rivers fed by monsoon rains flowed through the region.
Previous studies suggest the Ghaggar, an intermittent river that flows only during strong monsoons, may best approximate the location of the Sarasvati. Archaeological evidence suggested the river, which dissipates into the desert along the dried course of Hakra valley, was home to intensive settlement during Harappan times.
"We think we settled a long controversy about the mythic Sarasvati River," Giosan said.
Initially, the monsoon-drenched rivers the researchers identified were prone to devastating floods. Over time, monsoons weakened, enabling agriculture and civilization to flourish along flood-fed riverbanks for nearly 2,000 years.
"The insolation — the solar energy received by the Earth from the sun — varies in cycles, which can impact monsoons," Giosan said. "In the last 10,000 years, the Northern Hemisphere had the highest insolation from 7,000 to 5,000 years ago, and since then insolation there decreased. All climate on Earth is driven by the sun, and so the monsoons were affected by the lower insolation, decreasing in force. This meant less rain got into continental regions affected by monsoons over time."
Eventually, these monsoon-based rivers held too little water and dried, making them unfavorable for civilization.
"The Harappans were an enterprising people taking advantage of a window of opportunity — a kind of "Goldilocks civilization," Giosan said.
Eventually, over the course of centuries, Harappans apparently fled along an escape route to the east toward the Ganges basin, where monsoon rains remained reliable.
"We can envision that this eastern shift involved a change to more localized forms of economy — smaller communities supported by local rain-fed farming and dwindling streams," Fuller said. "This may have produced smaller surpluses, and would not have supported large cities, but would have been reliable."
This change would have spelled disaster for the cities of the Indus, which were built on the large surpluses seen during the earlier, wetter era. The dispersal of the population to the east would have meant there was no longer a concentrated workforce to support urbanism.
"Cities collapsed, but smaller agricultural communities were sustainable and flourished," Fuller said. "Many of the urban arts, such as writing, faded away, but agriculture continued and actually diversified."
I think it is a mistake to credit the collapse of civilizations to a single cause. In this case, there is also evidence of other factors including disease. In his book, Germs, Genes, & Civilization, David Clark notes that "[c]holera is thought to have origins in India and emerged onto the world stage only in the nineteenth century." (p. 72). However, it had been described by Hindu physicians as early as 400 B.C. and probably had been extant much earlier. (Id.) He theorizes that cholera may have appeared among the Indus Valley people. Clark writes:
Perhaps the major achievement of the Indus Valley culture was its water supply and drainage system. All major centers had sophisticated communal plumbing, with water supply channels and drains. Almost every house in major centers such as Mohenjo-Daro had its own baths. Drains took the dirty water to a communal underground sewage system. The Great Bath of Mahenjo-Daro was the earliest public bath in the world.With this background, he considers the theory that changes in the rivers led to the extinction of the civilization, but notes that other civilizations have suffered and survived similar difficulties. He also notes problems with theories that the Indus Valley people were overwhelmed by Aryan invaders. Most interestingly, he notes that "life in the countryside appears to have carried on with relatively little discontinuity during the collapse of the Indus Valley cities." In other words, what would have impacted all of the cities, but not the countryside.
Having discounted these other factors, Clark moves to his thesis: that it is the advanced water and sewage systems that led to the downfall of the Indus Valley culture. He relates that "[t]he latest archeological levels of Harappa and Mohenjo-Daro revealed large numbers of unburied skeletons" but which lacked signs of violence, such as warfare. The suggestion, then, is that they died of an epidemic. Specifically, he notes that the rims of the public wells were usually within a few inches of the ground, and the drainage system was only buried a foot to two feet underground. Blockages or flooding of the drainage system would have resulted in contamination of the drinking water supply. Moreover, access covers and other evidence shows that blockages were relatively common.
Today we might wonder how they got with this for a thousand years. The answer seems to be that highly virulent waterborne diseases had not yet evolved when the Indus Valley cities were first built. As we have already discussed, most virulent epidemic infections have emerged only in the last few thousand years. ... To put it rather unkindly, the evolution of cholera could well be the legacy of the Indus Valley culture. Cholera is perhaps the most likely agent, especially because it was known in India from early times. But because more than a thousand years passed between the fall of the Indus Valley culture and the earliest convincing descriptions of cholera, other diarrheal diseases, caused by related enteric bacteria such as Shigella or Salmonella, might have been the cause.