Sacrifices and El Niño in Peru: Ancient Societies That Remained Peaceful

A Harsh Climatic Context

The Lambayeque Valley, on the arid coast of northern Peru, ordinarily receives very little rain. Its inhabitants depended mainly on the rivers flowing down from the Andes and on the rich marine resources of the Pacific Ocean. When El Niño, the periodic warming of Pacific waters, arrives, fish stocks disappear and torrential rains flood the valley, disrupting the entire food production system.

Faced with these crises, archaeologists seek to understand how societies responded. The large human sacrifice ceremonies, attested just before the collapse of the Moche and Sicán political systems, raised concerns about a rise in violence linked to scarce resources. The researchers wanted to verify whether these rituals reflected broader social tension.

A Study Based on the Skeleton

The researchers drew on the database of the Lambayeque Valley Biohistory Project, which brings together 30 sites and covers more than three thousand years of history. Within this corpus, 1,633 individuals have death dates precise enough to be placed within the paleoclimatic context reconstructed through modeling. This precision makes it possible to link the lives of these people to variations in rainfall and water availability.

The method compares the frequency of cranial trauma, generally associated with assaults, and post-cranial trauma, more often accidental, with changes in precipitation over time. This is an indirect indicator: not every injury reaches the bone, so some violence may escape analysis. The authors acknowledge this from the outset and take it into account in their interpretation.

Surprising Results

Only 25 individuals, or 1.6% of the sample, show signs of violence, and 77 (5.1%) show accidental trauma. No link appears between these injuries and ecological instability, nor between violence and the sex of the individuals. For the researchers, the finding is clear: interpersonal violence, outside ritual contexts, remained very marginal.

This result contrasts with other Andean sites, where cranial trauma is far more common, particularly during periods of climatic upheaval. The authors put forward two possibilities: access to the sea may have softened the scarcity of resources, or different forms of social organization deliberately limited violence. Their study does not settle between these hypotheses.

The Limits of the Method

The researchers stress that their figures remain estimates. Because it was not possible to radiocarbon-date each individual, they reason from long-term trends and may miss brief episodes, such as an isolated assault or a crisis lasting a few years. They therefore cannot rule out that isolated violence occurred without leaving a trace on the bones.

Systematic dating of the sample would, in their view, make it possible to detect past trends that have gone unnoticed. This remains a goal for future research in the valley.

A Lesson for Today

For the authors, the study shows that widespread violence is not an automatic response to climate change. As Thomas Snyder of the State University of New York explains, "we collectively choose how we respond to our own climate crisis". His co-author Haagen Klaus of George Mason University recalls that in Lambayeque, six millennia of adaptation to El Niño did not produce widespread violence.

Snyder emphasizes the value of archaeology for addressing current crises: studying past responses, he argues, helps us better understand how societies got through comparable ordeals.