Despite sacrificing people after catastrophic El Niño flooding, ancient societies were generally peaceful

To understand how societies might react to climate change, archaeologists look at social responses to extreme climate events in the past — and in some places, that included measures as drastic as human sacrifice. Mass ritual sacrifices along the desert coast of northern Peru have been linked to catastrophic flooding caused by El Niño, which made researchers wonder if such sacrifices were part of a rise in violence driven by resource scarcity. But a new survey of the Lambayeque Valley suggests that violence was extremely rare, even during periods of ecological upheaval.  

“The main thing that jumped out at us was just how few cases of interpersonal violence we observed outside ritualized contexts,” said Dr Thomas Snyder of New York University, co-author of the article in Frontiers in Environmental Archaeology. “This really highlights that widespread violence isn’t a predetermined response to climate change. As a society, we both get to and have to choose how we respond to our own climate crisis.” 

“In Lambayeque there’s 6,000 years of creative adaptations to El Niño and longer-term changes in climate, and widespread violence wasn’t one of them,” said Dr Haagen Klaus of George Mason University, co-author. “Violence and social unraveling are not destined outcomes, ever.” 

A vulnerable valley 

The researchers examined the Lambayeque Valley Biohistory Project’s skeletal record, covering 30 sites and more than 3,000 years of the valley’s history. 1,633 individuals had relatively clear dates of death, allowing the scientists to put their lives in context with paleoclimatic modelling. The researchers tracked the incidence of cranial trauma (usually caused by violence) and postcranial trauma (usually accidental) over time, and compared it to contemporary changes in precipitation and water availability. 

“The Lambayeque Valley was home to a sequence of creative, innovative, and resilient cultures,” said Klaus. “The people of ancient Lambayeque were probably very different from us. On the other hand, archaeological evidence shows that in so many ways they were mirrors of us — in how they lived their daily lives, interacted with their world, ran their economies, and confronted (and sometimes contributed to) environmental crises.” 

The valley normally had very little rainfall, but it benefited from rivers and seas bursting with life. El Niño events killed off fish stocks and flooded the valley with torrential rain, throwing local food production systems into crisis.  

“Leaders of the Moche and Sicán appear to have held power partly by their claims to be intercessors with the natural world,” explained Klaus. “Large-scale events of human sacrifice just before the collapse of the Moche and Sicán political systems appear like attempts at bringing the forces of nature back into balance by offering their most precious lives to the supernatural. It’s here where the most extensive evidence of violence is found in Lambayeque.” 

“If these big ritual sacrifices were indicative of some underlying crisis, we hypothesized that these events might have been extending into people’s everyday experience of the world,” added Snyder. 

No case to answer 

But surprisingly, only 25 individuals (1.6% of the sample) showed evidence of violence and only 77 (5.1% of the sample) showed evidence of accidental trauma. There was no link between either violence or accidents and ecological instability. There was also no link between a person’s sex and violence.  

This is very different to other sites in the Andes, where cranial trauma is much more common, especially during periods of climatic upheaval. It’s possible that Lambayeque Valley’s access to the sea helped alleviate resource scarcity, or that differing ways of organizing society intentionally reduced violence.  

However, Snyder and Klaus point out that their results are estimates. They used proxies for accidents and violence, which could conceal some violence: not all wounds affect the skeleton. Additionally, they emphasize that their findings reflect long-term trends and might not capture short-term incidents.  

“One of the challenges ended up being temporal resolution,” explained Snyder. “With an archaeological sample size this large, it just wouldn’t be feasible to radiocarbon date every individual. So we’re really looking at long-term associations — or their absence. In an ideal world I would like to radiocarbon date the entire sample, to see if any trends appear that we might have missed.”  

“A lot of people think that archaeological research is interesting, but we really want to emphasize that it is also useful,” concluded Snyder. “As the saying goes, ‘there is nothing new under the sun’. By looking to the past, we can answer questions about how people have dealt with similar crises to those we’re facing now and apply that knowledge to the present.”