The number of human skulls that emerged from the sand stunned Prieto. They were in an "excellent state of preservation," he said.
The site was less than a kilometre - about a 15-minute walk - from the ancient Chimu metropolis of Chan Chan, the largest city in pre-Colombian South America.
That the Chimu sacrificed children here, and in such numbers, came as a surprise to researchers. Archaeologists knew the Inca people, who conquered the Chimu at the end of the 15th century, killed children in mountaintop rituals. But before this research, no similar accounts existed for the Chimu.
"It is an unknown chapter that we can add to the big book on ancient sacrifice in world societies," said John Verano, an archaeologist at Tulane University, who, with Prieto and their colleagues, is an author of a PLOS One study published today. The sacrificial site, covering 700 sq m, is named Huanchaquito-Las Llamas for a nearby coastal town and the llamas.
Prieto and his colleagues excavated the site between 2011 and 2016. Boys and girls were killed, the scientists say, citing anatomical details and DNA extracted from teeth. The study authors estimate that the children were between 5 and 14 years old. Radiocarbon dating placed the mass sacrifice around the year 1450.
Many world religions refer to child sacrifice, Verano said, such as the binding of Isaac in the Bible. But archaeological evidence is rare, and attributing sacrifice as the cause of death for human remains is often difficult. Not so in this case.
"What we've got is no ambiguity at all - all of these kids have their chests cut open," Verano said.
Horizontal marks, similar to incisions made in some thoracic surgeries, cut across their chests. This was probably a way to remove the children's hearts.
"This site really represents something remarkable," said Haagen Klaus, a bioarcheologist at George Mason University who was not involved with this research.
"It is disturbing and disquieting to see the sacrifice of children on any scale," he said. "We study sacrifice not for the gruesome detail, but as anthropologists and bioarcheologists, our reasoning is to reconstruct a larger living world."
Human sacrifice was rarely a simple transaction, said Klaus, who cautioned against too "simplistic and robotic" theories. Children, to long-ago South Americans, had a "different kind of personhood" than what we understand, he said.
Children were said to come from mountain spirits who were old and recycled ancestors. Infants were supposedly untamed and wild. Children existed in the space between the supernatural and human, and as they grew, they became "a bit more human every day." Sacrifice was a way to influence ancestors - whom Klaus described as the "most powerful entities" in these peoples' cosmos - using something partly supernatural and wholly precious.
"Around 1450, that was right at the peak of Chimu power, at their greatest moment," Verano said. The mass sacrifice "is something that was directed by a state-level society."
The Chimu civilisation was a powerful empire along the Peruvian north, with millions of inhabitants. They fished along the coast and raised herds of llamas for meat and alpacas for wool.
A mega-El Nino event would have struck these people "like a punch in the stomach," Verano said.
The region is arid and receives about a tenth of an inch of rain a year. Klaus agreed "very strongly" with the interpretation that this sacrifice was a response extreme weather. Heavy rains could have led to flash floods, agricultural collapse and vanished fishing stocks. At least one empire preceding the Chimu crumbled after the heavy, months-long rains of a severe El Nino.
The site contains prints of dogs and other animals preserved in what had been wet mud. In places, heavy foot traffic, by adults in sandals and barefoot children, were visible in the mucky surface. Sacrificial burials were dug through the mud. "The thick layer of mud, right on top of the clean sand, with evidence of footprints, shows the connection between the rains and the sacrificial event," Prieto said.
Excavations continue in the area, Verano said. The researchers found a second sacrificial site, which may be as massive as the first, about a thousand metres away. Most recently, they found what may be a third location. "The story's not over yet."
