For thousands of years, civilizations flourished beside rivers in northern Peru’s Lambayeque Valley, even as the surrounding coastline remained desert. New research suggests daily life there was remarkably free of deadly interpersonal violence, despite episodes of mass human sacrifice linked with environmental upheaval.

El Niño periodically warms the central and eastern tropical Pacific, disrupting established weather patterns. In Lambayeque, the rains associated with these unusually warm waters can send catastrophic floods racing down from the Andes.

Previous excavations of temples belonging to the Sicán culture uncovered evidence of human sacrifice. Among the discoveries were six young women positioned with their heads facing the mountains from which floodwaters arrived.

The Sicán culture dominated the valley for roughly 600 years, from approximately 750 to 1375 CE. Researchers questioned whether ritual killings during environmental disasters reflected a wider surge of violence across society.

The evidence produced a strikingly different picture. “The main thing that jumped out at us was just how few cases of interpersonal violence we observed outside ritualized contexts,” Dr Thomas Snyder of New York University said in a statement.

The Sicán culture emerged around 1,300 years ago, but civilization in the Lambayeque Valley stretched back for more than twice that length of time. The immediate predecessors of the Sicán were the Moche Civilization.

Because the region’s dry climate preserved human remains, archaeologists assembled a sample of 1,633 skeletons dating to thousands of years before European arrival. Snyder and his coauthors compared the timing of those deaths with climatic events and examined cranial trauma as evidence of violent death.

El Niño brought more than flooding. The phenomenon was first identified through the collapse of fishing stocks around Christmas during El Niño years, while abundant offshore fish supported much of the valley’s wealth during other periods.

Together, floods and reduced fishing stocks had devastating effects. The evidence suggests leaders responded with mass human sacrifice in an effort to prevent or reverse disasters they believed were connected to the natural world.

“Leaders of the Moche and Sicán appear to have held power partly by their claims to be intercessors with the natural world,” said Dr Haagen Klaus of George Mason University in the same statement.

Klaus said large scale sacrifices preceding the collapse of the Moche and Sicán political systems appeared to represent attempts to restore balance by offering precious lives to supernatural forces. These ritual settings contained the valley’s most extensive evidence of violence.

Researchers expected that such dramatic sacrifices might accompany broader turmoil in everyday life. Instead, among remains not associated with sacrifices, only 25 people, or 1.6%, showed evidence of death by violence.

More than three times as many people apparently died in accidents. The low level of violent death also contrasted with findings from some other parts of the Andes, where evidence of such deaths was more common.

Neither violent nor accidental injuries increased to a detectable degree during periods of hostile climate conditions. The researchers therefore found no clear sign that environmental disruption produced a widespread escalation of lethal violence in the valley.

The authors acknowledged substantial uncertainty because of the age and condition of the remains. Injuries affecting vital organs without damaging bones would not appear in the analysis, while poisoning could also escape detection.

Researchers also could not match most deaths with individual years. Instead, they studied longer periods when El Niños and other forms of climate chaos occurred more frequently, including two “Mega Niños” that each lasted around 50 years.

Mass sacrifices could be connected with deposits of mud carried by floods, but individual murder victims could rarely be dated through the same evidence. Radiocarbon dating every person in such a large archaeological sample was not considered feasible.

Snyder said finer dating might reveal an increase during individual El Niño years, although the authors doubted that any such effect would be large. Their results were consistent with findings farther south, where climate conditions were associated with violence only at the highest inhabited altitudes.

The findings challenge the idea that climatic stress inevitably unleashes widespread violence. “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,” Snyder said.

The study was published with open access in Frontiers in Environmental Archaeology. Its findings portray a society where ritual killings could reach horrifying proportions while lethal violence outside those ceremonies remained surprisingly uncommon.

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