Along I-55 heading from Illinois into Missouri, just before St. Louis’s iconic Gateway Arch becomes visible, another monument rises into view—Cahokia Mounds, the remnants of the largest pre-Hispanic settlement north of the Mexican border. The city dates to around the year 600, and during its height in the early 12th century, 20,000 people from the Mississippian Mound Building culture inhabited the six square-mile settlement. But by 1400, the community was abandoned. Researchers have searched for clues and debated the causes for Cahokia’s fall for decades, pointing the finger at various culprits including drought, disease and political unrest. Now, they’ve discovered new clues in lake sediments and the layers of poo its citizens left behind.
As Matthew Taub at Atlas Obscura reports, the archaeological record shows the once vast city began to lose population beginning around 1200, and by 1400 it was almost completely abandoned. In search of answers, archaeologists set out to combine archaeological evidence with the environmental record. To do that, they looked at sediment cores drilled from Horseshoe Lake, which is also on the site.
Specifically, they looked for “fecal stanols,” molecules from human poop created in the gut during digestion that would have washed into the lake. By researching the concentration of the stanols in the core samples, they could make a timeline of the rising and decreasing human population in the area, a procedure they outlined in a previous paper published last year.
For this study, the team was also looking for environmental information contained within the cores, including evidence of flooding from the nearby Mississippi River and wet or dry conditions, which can be assessed by looking at the ratios of two different isotopes of oxygen. The research appears in the Proceedings of the National Academy of Sciences.
The fecal stanols more or less conformed to what archaeologists had deduced about the rise and fall of the population of Cahokia. But the environmental data sheds some new light on the last few centuries of the settlement. In 1150, the data shows that a major flood occurred in Cahokia, which is the same time that the settlement began changing, with fewer and less densely packed houses in the area. Craft production also changed during this period.
The lake cores reveal that the precipitation in the area also decreased in this period, which may have made it more difficult to grow the maize and other crops that supported such large settlement. The evidence suggests “some kind of socio-political or economic stressors that stimulated a reorganization of some sort,” occurred in this period, says co-author and University of Wisconsin–Madison anthropologist Sissel Schroeder in a press release. “When we see correlations with climate, some archaeologists don’t think climate has anything to do with it, but it’s difficult to sustain that argument when the evidence of significant changes in the climate shows people are facing new challenges.”
This is not the only project that is using the relatively new science of fecal stanols to look at ancient cultures. For Smithsonian.com, Lorraine Boissoneault recently reported on similar work being conducted in the basin of Lake Titicaca in Peru’s Andes Mountains. The hope is the fecal biomarkers can help chart the population of hunter-gatherer cultures and nomadic groups around the lake—cultures that are difficult to get population estimates on because they have no concrete settlement record. The environmental data, including indications of climate change, also shows how cultures in the past have adapted their societies to cope with an altering world, which may offer pertinent lessons for our modern civilization.