LIMA—Karina Venegas Gutiérrez was digging between the zebra and the hippo pens at the Parque de las Leyendas zoo here when she uncovered something strange. Like much of Peru’s capital city, the zoo is built atop layers of settlements that stretch back millennia. Venegas Gutiérrez and her colleagues in the Division of Archaeology at the Parque de las Leyendas are charged with excavating that history. She has found scores of buildings and artifacts, and even a few human mummies. But in 2012, she unearthed something she still can’t quite explain: the remains of more than 100 dogs, resting alongside a similar number of humans. The unusual discovery may help archaeologists understand the roles of dogs in pre-Columbian Peru—including their potentially important role as sacrificial victims.

During the World Congress on Mummy Studies here last week, Venegas Gutiérrez led a tour of the site for several scientists (and one curious journalist). As a particularly angry-sounding zebra bellowed nearby, she discussed her still puzzling find. Over the course of their excavation, she and her team unearthed the skulls of 126 humans and 128 dogs. They found small dogs, large dogs, and everything in between. Most belonged to three types of street dogs that still roam Peru’s towns and cities today. One skeleton showed a distinct underbite reminiscent of a tiny bulldog. Some of the dogs still had skin and hair; a few were so well-preserved that they still had noses and ears. All of them had been arranged in peaceful postures, as if they had fallen asleep, and were wrapped in textiles for burial, just as most humans were at that time. Nearby ceramics and other artifacts suggest both dogs and humans were buried around 1000 C.E. Venegas Gutiérrez plans to radiocarbon date the remains next year.

Archaeologists have found a couple ancient dog cemeteries in Peru, where many canines are buried with offerings. But this is different. “In those places, there are only dogs,” Venegas Gutiérrez said. “But here we have dogs and humans.” Some pre-Columbian Peruvian cultures believed that dogs escorted humans to the afterlife, and archaeologists have found a few human tombs there that contain companion dogs. That doesn’t seem to be the case at zoo site either, Venegas Gutiérrez said. Here, only two humans were buried with dogs next to them; one was a young boy, who was curled up to a pregnant dog. The rest of the human and canine remains seemed to be more or less randomly buried in the same general area and at the same time. “Why? What happened here?” Venegas Gutiérrez wondered.

The human skeletons offer a tantalizing clue. They represent both men and women, mostly between the ages of 20 and 40—an unlikely age to die of natural causes. Many of them show signs of violent injury, including skull fractures, cracked ribs, and broken limbs. These injuries show no sign of having healed, meaning they were sustained immediately before death and were likely fatal. The dogs, however, show no signs of fatal wounds on their skeletons, leading Venegas Gutiérrez to suspect they were probably strangled.

Her best guess is that the dogs were part of a ritual sacrifice, perhaps carried out hastily after a traumatic event in the community. “Maybe the dogs were offered to the humans after a mass death of some kind,” she said. As for who may have killed these people and why, she still can’t say.

The burials coincide with a transitional time for Peru’s coastal societies. People from the Lima culture, who made their living by fishing and built temples and communal spaces from narrow adobe bricks, occupied the area between 300 and 800 C.E. Their buildings were abandoned around 1000 C.E., and the agricultural society of Ychsma (pronounced EESH-ma) moved in. They tore down many of the older Lima buildings and reused their bricks for their own constructions.

Venegas Gutiérrez has found clues that perhaps the arrival of the Ychsma wasn’t entirely peaceful. At the burial site, the Lima buildings had been partially dismantled and then buried under more than a meter of large river rocks that had apparently been hauled in for that purpose. Above them rose the new Ychsma constructions. Venegas Gutiérrez found the dogs and the humans in the layer of river rocks.

“It’s amazing work,” said Salima Ikram, an archaeologist at the American University in Cairo who studies Ancient Egyptian animal mummies. In addition to the cultural insights, tracing the evolution and history of the local dog breeds could reveal how dogs interacted with humans and the environment, and how those interactions may have changed over time. “[Studying animals] is a way not only to understand a culture and people’s interactions with the natural world, but to learn about climate, environment, and climate change,” Ikram said.

Venegas Gutiérrez is hoping more clues about the dogs and their humans will come from a second site in the zoo that she has recently started excavating. Just past the tiger pen, she has found another area where dogs and humans seem to have been buried together, also around 1000 C.E. In the last month alone, she has uncovered eight particularly well-preserved dogs along with a naturally mummified guinea pig—the zoo’s first. So whereas most of her colleagues at the zoo focus on living animals, Venegas Gutiérrez will continue to see what she can learn from these long dead ones.

*Update, 22 August, 11:25 a.m.: The article has been updated to more accurately represent the involvement of the zoo's division of archaeology.