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Human Fecal Particles Show That Extreme Weather May Have Led To Ancient Cahokia's Demise

Washington University archaeologist John Kelly works with a student mapping research at Cahokia Mounds.
Chris King
Researchers have wondered for years what led to the decline of the largest ancient civilization in the present-day United States. New evidence from fecal particles suggests a major flood and drought could have pushed people to leave the city.

After analyzing microscopic particles of human feces, scientists have found more evidence to support the theory that extreme-weather events may have contributed to the demise of an ancient city that built the Cahokia Mounds.

In the Proceedings of the National Academy of Sciences, researchers based in California, Wisconsin and Massachusetts reported that drought and flooding that occurred around 1150 A.D. could have caused people to leave Cahokia.

It’s likely that ancient Cahokians defecated outside and rain would wash their feces into in a nearby lake, which accumulated over time, said A.J. White, a graduate student studying archaeology at the University of California, Berkeley. White and his colleagues made conclusions about population levels based on the amount of fecal particles they saw in sediment collected from Horseshoe Lake in Madison County.

“The more people there, the more poop was left behind,” White said.

Archaeologists have long debated what drove people to leave one of the largest ancient cities in North America. Some have found evidence to suggest that floods and droughts could have played a role, but researchers have found it difficult to draw direct connections between changes in climate and the population decline.

“People are quite resilient,” White said. “I think one sort of disaster is likely not going to be that significant in the long run, but when you start adding up these multiple environmental events, in addition what could be going on in society that could be bad, I think you get this much more nuanced and complicated picture of why people would go away from Cahokia.”

Other researchers argue that political instability played a bigger role in Cahokia’s demise than environmental challenges. There isn’t convincing enough evidence that a flood occurred around 1150 A.D., and the timing is too early to be linked to the city’s population decline, said John Kelly, an archaeologist at Washington University in St. Louis. Researchers believe people departed from Cahokia during the 13th century.

“Their dates are too old,” Kelly said, regarding White’s study. “They may be right there is a flood at that point, but I don’t see from the archaeological evidence anything that really jumps out at you the way it does a century later.”

The fecal particles tell a limited part of Cahokia’s story, and more evidence is needed to understand why people abandoned the ancient city, Kelly said.

“As we get more data, as we look at what we’re interpreting, things are going to change — that’s how science works,” Kelly said. “For the time being, I don’t think we can be necessarily conclusive about it. I’m glad that the article is out, that they’ve presented their results and we can discuss it and figure out how we can go from there.”

White, the U.C. Berkeley researcher, is interested in using old fecal particles in sediment to learn about other ancient civilizations, such as a 20,000-year-old site in Jordan. Regarding Cahokia, there still remains much work to be done to connect what the fecal particles reveal with what other pieces of evidence, like old pollen and cultural artifacts, have shown about the city.

“I hope that in the future, we can make even more links between what’s happening in the archaeology and what’s happening in the environmental datasets,” White said. “I think the closer we bring those together, the closer we’ll get to the truth.”

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Eli is the science and environment reporter at St. Louis Public Radio.