Climate change, violence and disease played a key role in the collapse of the Harappan civilization more than 3,000 years ago, according to a new study.

Harappan civilization, or the Indus Valley civilization, developed in the middle of the third millennium BC, at the same time as contemporaneous civilizations in Egypt and Mesopotamia. It stretched over a million square kilometers of what is now Pakistan and India.

The city of Harappa and the city of Mohenjo-Daro – the greatest achievements of this culture – are well known for their impressive, organized layout.

Recent excavations have demonstrated that the cities grew rapidly from 2200-1900 BC, when they were largely abandoned.

“The collapse of the Indus Civilization and the reorganization of its human population has been controversial for a long time,” said Dr Robbins Schug of Appalachian State University, who is the lead author of the study appearing in the open-access journal PLoS ONE.

Climate, economic, and social changes all played a role in the process of urbanization and collapse, but little was known about how these changes affected the human population.

Dr Schug and her colleagues examined evidence for trauma and infectious disease in the human skeletal remains from three burial areas at the city of Harappa. Their findings counter longstanding claims that the Harappan civilization developed as a peaceful, cooperative, and egalitarian state-level society, without social differentiation, hierarchy, or differences in access to basic resources.

The results suggest instead that some communities at Harappa faced more significant impacts than others from climate and socio-economic strains, particularly the socially disadvantaged or marginalized communities who are most vulnerable to violence and disease. This pattern is expected in strongly socially differentiated, hierarchical but weakly controlled societies facing resource stress.

The study adds to the growing body of research about the character of Harappan society and the nature of its collapse.

“Early research had proposed that ecological factors were the cause of the demise, but there wasn’t much paleo-environmental evidence to confirm those theories. In the past few decades, there have been refinements to the available techniques for reconstructing paleo-environments and burgeoning interest in this field,” Dr Schug said.

When paleoclimate, archaeology, and human skeletal biology approaches are combined, scientists can glean important insights from the past, addressing long-standing and socially relevant questions.

“Scientists cannot make assumptions that climate changes will always equate to violence and disease. However, in this case, it appears that the rapid urbanization process in Harappan cities, and the increasingly large amount of culture contact, brought new challenges to the human population. Infectious diseases like leprosy and tuberculosis were probably transmitted across an interaction sphere that spanned Middle and South Asia.”

The study shows that leprosy appeared at Harappa during the urban phase of the Harappan civilization, and its prevalence significantly increased through time.

New diseases, such as tuberculosis, also appear in the Late Harappan or post-urban phase burials.

Violent injury such as cranial trauma also increases through time, a finding that is remarkable, she said, given that evidence for violence is very rare in prehistoric South Asian sites generally.

“As the environment changed, the exchange network became increasingly incoherent. When you combine that with social changes and this particular cultural context, it all worked together to create a situation that became untenable,” Dr Schug said.

The results of the study are striking, because violence and disease increased through time, with the highest rates found as the human population was abandoning the cities. However, an even more interesting result is that individuals who were excluded from the city’s formal cemeteries had the highest rates of violence and disease. In a small ossuary southeast of the city, men, women, and children were interred in a small pit.

The rate of violence in this sample was 50 percent for the 10 crania preserved, and more than 20 percent of these individuals demonstrated evidence of infection with leprosy.

Dr Schug said lessons from the Harappan civilization are applicable to modern societies.

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Robbins Schug G et al. 2014. Infection, Disease, and Biosocial Processes at the End of the Indus Civilization. PLoS ONE 8 (12): e84814; doi: 10.1371/journal.pone.0084814