The practice of human sacrifice in America’s largest prehistoric city was more subtle and complex than experts once thought, new research suggests.

Recent studies into the remains of sacrificial victims at the ancient city of Cahokia reveal that those who were killed were not captives taken from outlying regions, as many archaeologists had believed.

Instead, they may have been residents of the same community that killed them.

When Cahokia was at its peak 900 years ago, it was the largest city in what’s now the United States, a metropolis of about 15,000 people in southwestern Illinois, whose economic and cultural influence reached from the Great Lakes to the Gulf of Mexico.

But one of the many mysteries lingering among the city’s ruins, just outside modern-day St. Louis, is a burial mound excavated in the 1960s and found to contain more than 270 bodies — almost all of them young women killed as victims of human sacrifice.

Dated to between 1000 and 1100 CE, their remains were mostly buried in large pits, laid out in neat rows, and bearing few signs of physical trauma, perhaps killed by strangulation or blood-letting.

But the mound also contained a striking group of outliers: a separate deposit of some 39 men and women, ranging in age from 15 to 45, who — unlike the rest — had been subjected to all manner of physical violence: brutal fractures, shot with stone points still embedded in their bones, even decapitation. [Read about the recent discovery of men among the dead in Mound 72: “Infamous Mass Grave of Young Women in Ancient City of Cahokia Also Holds Men: Study“]

For more than 50 years, archaeologists have puzzled over the grisly scenes found in the mound, known as Mound 72.

“It is the significant site in this region and foundational to our understanding of Mississippian culture within this region and beyond,” said Dr. Phil Slater, an anthropologist at the Illinois State Archaeological Survey who took part in the new study.

“One of the big questions raised by the finds at Mound 72 focused on the mass burial events. Those appear to be unique, certainly unique to have so many such features within a single small mound.”

For decades, the prevailing theory has been that its victims were forcibly brought to Cahokia from regions under the city’s control, and sacrificed as offerings to its rulers, its dead, or its deities.

“The initial interpretation of the burials of young women suggested they represented ‘tribute’ from outlying communities,” Slater said.

“Our analysis provide[s] … evidence that suggests the young women may have come from within the region, if not from Cahokia itself.”

What’s more, the research done by Slater and his colleagues finds that the same is true for the 39 victims of the more violent, traumatic deaths in the mound.

They’re not only local, the results show; they also turn out to be the most biologically different from the rest of the dead found in the mound. And yet, they’re also the most similar to each other, suggesting that they may have been members of a unique, and perhaps isolated, population within Cahokia.

Slater and colleagues Dr. Kristin Hedman and Dr. Andrew Thompson revisited Mound 72 with a focus on the victims’ teeth — studying both their chemistry and their physical structure.

The team analyzed 203 teeth from 109 of the people found in 3 separate burials in the mound.

Among them: a grave known as Feature 214, thought to be one of the earliest mass burials in the mound, featuring 24 bodies arranged in two layers and dated to around the year 1000;

and also Feature 105, dated to around 1050, where more than 50 were buried in two layers of two rows, aligned shoulder to shoulder;

and finally Feature 229, which included two layers of human remains — an upper layer of 15 men and women, whose remains were gently laid to rest on cedar litters, and 229-lower — the mass grave of 39 men and women whose mutilated bodies appear to have been dumped, rather than peacefully interred.

“They appear,” Slated said, “to have been lined up and pushed in.”

“We now have the opportunity to take another look at some of these [features] and challenge some of the earlier interpretations,” he added.

“Who were the individuals buried in Mound 72 and what do their deaths and burials signify?”

In their search for answers, the team looked, in part, for chemical clues in the victims’ teeth.

Specifically, they wanted to measure the levels of the element strontium that they contained. Strontium occurs naturally in groundwater as it leaches in from local rock formations. But different regions have different levels of various strontium isotopes, depending on their geology.

So as humans eat and drink, the concentrations of strontium that are specific to the local food and water supply become bound in their teeth.

In this way, teeth can be analyzed to reveal their owner’s geographical history, allowing scientists to determine whether people were born in the same region where they were buried, or whether they immigrated from elsewhere.

This method holds particular promise for researching Cahokia, noted Dr. Thomas Emerson, director of the Illinois State Archaeological Survey, since the city’s rapid growth had been widely attributed to immigration from outlying areas.

“With the development of strontium analysis, there became a way to actually test the immigration hypothesis by looking at the bodies of the people buried at Cahokia,” he said.

And the results showed that, based on their chemical traces, the dead were largely, but not exclusively, local to the floodplain where Cahokia was built, known as the American Bottom.

Of those buried in Feature 105, for instance, about 7 percent of the teeth revealed levels of strontium that were not consistent with the immediate Cahokia area.

In Feature 214, 36 percent of the teeth turned out to have non-local signatures.

And for those who had been carefully buried in the upper layer of Feature 229, 17 percent of the teeth contained non-local levels of strontium.

But for the people buried below them, who had been violently killed and dumped, their teeth were all exclusively within the range of Cahokia’s strontium ratios — and they also displayed the smallest degree of variation in their chemistry.

What’s more, Slater said, the physical measurements of their teeth revealed even more surprising results.

The shapes and sizes of the mutilated victims’ teeth were uniquely similar to each other, and were also different from the morphologies of the teeth of the other victims.

“The 229-lower feature stood out among the four [features], because it seems to have been composed of a distinct sub-population within Cahokia,” he said.

“Both the strontium and dental data of the group show that they separate themselves from the rest of Mound 72.”

Taken together, Slater said, the data suggest that these mutilated victims were all local to the Cahokia region, but also physically different in some ways from the general population.

“It is possible that Feature 229-lower represents a small group that lived in the region — as is suggested by the strontium data — but was isolated from others for a long enough period of time to acquire similar dental characteristics that we tested for,” he said.

Beyond the scope of those 39 individuals, the findings obtained throughout Mound 72 suggest that the 270 people buried there were not all immigrants, or captives from raids into distant territories, as some experts had suggested. [Learn more about the makeup of Cahokia’s dead: “Sacrificial and Common Graves Alike Reveal Diversity in Ancient City of Cahokia“]

Instead, it seems that the sacrificed were themselves Cahokians — or at least a relatively consistent mixture of immigrants and locals, with native Cahokians forming the majority of the group, a mixture found even in common, non-sacrificial graves in the city.

It’s a prospect that opens up a new set of questions, Slater said.

“On the surface, these results refute interpretations that these people were [killed as] tribute from outside communities and suggest that they were actually local people from within the American Bottom and part of the Cahokia population in some way,” he said.

“However, their unique burial contexts indicate special — either good or bad — treatment in death.

“This actually raises just as many questions as it answers: Why were these groups of people all buried at the same times?

“Why were some being killed and buried, as in Feature 229-lower?

“How were these groups integrated to the rest of Cahokia’s population socially or politically?

“These are things we still don’t know.”

As part of ongoing research called the Cahokia Collapse Project, the team is now planning to study strontium levels in other regions that interacted with Cahokia, in an effort to clarify where some of its immigrants might have come from. [More on theories about Cahokia’s collapse: “Epic Fire Marked ‘Beginning of the End’ for Ancient Culture of Cahokia, New Digs Suggest”

For now, Slater says, their research offers valuable new insights into the nature of life and death in what was once America’s largest city.

“An important value of this specific [study] lies in providing multiple lines of evidence … to try and understand who these people were – and Cahokia at large,” Slater said.

“It is also important to realize that just because someone has ‘answered’ a question previously, that doesn’t mean you shouldn’t revisit it with a new perspective or new, improved methodology.

“That’s part of the collective advancement of science.”

The team reports their findings in the American Journal of Physical Anthropology.

Thompson AR, Hedman KM, & Slater PA (2015). New dental and isotope evidence of biological distance and place of origin for mass burial groups at Cahokia’s mound 72. American journal of physical anthropology PMID: 26173443