Cahokian Collapse: Re-evaluating Archaeological Assumptions and Evidence Underpinning the Ecocide Hypothesis

Written by: Jon Zhang ‘24
​Edited by: Owen Wogmon ‘23

Imagine if your hometown were the site of a majestic ancient civilization. As a proud native of St. Louis, Missouri, I never needed to.

Cahokia Mounds, situated just across the Mississippi River from St. Louis, was the largest pre-Columbian settlement north of Mexico, home of the Aztecs. The settlement lasted from roughly 800 to 1400 CE, and, at its peak from 1050 to 1150 CE, its population ranged up to 20,000 inhabitants, surpassing that of Paris at the time [1,2]. Originally consisting of around 120 earthen mounds spread over 1,600 hectares, the settlement represented both the center of Mississippian culture and an engineering marvel. To this day, at its heart lies Monks Mound, the largest prehistoric earthwork in all of North America, standing 30 meters tall. Considering such marvels, it’s no wonder that Cahokia has been designated a UNESCO World Heritage Site [3].

I first learned about the Cahokian civilization in fourth grade. Since I had only read about the illustrious Aztec and Incan empires of Meso and South America, as well as the grandeur of cities like Tenochtitlan and Cusco, I was surprised to uncover the existence of another major indigenous civilization right in my backyard. While I suspect it remains hardly known outside the Midwest and anthropological and historical circles, Cahokia left an impression on me at ten years old.

So how did this impressive society fall? One leading hypothesis purports that overpopulation and overutilization of natural resources played a major role. As the community came to rely more heavily on wood for fuel and construction, trees were cleared, particularly upstream from the settlement. Tree root systems support soil, which absorbs rainwater, so without this added stability, precipitation likely caused sediment runoff that impacted the Mississippi floodplain. Without ecological systems in place to absorb rainwater, more of it was likely carried downstream to Cahokia, with floodwaters eventually wiping out fields of crops. With threats to their food supply, residents of the great city may have departed [4]. This theory implies that indigenous peoples, at least in part, inadvertently contributed to their doom. A grand civilization ironically brought to its knees by its own inhabitants. 

As plausible as this might sound, a recent study challenges this hypothesis. While at Washington University in St. Louis, geoarchaeologist Caitlin Rankin led a team in the excavation of three sites near the central precinct of Cahokia Mounds, an area with the lowest local elevation. If flooding had occurred, these sites would have borne the brunt of it. Digging trenches and collecting sediment samples, the researchers analyzed the strata, or layers, of sedimentary rock below the surface [5]. Their work was published this past summer.

Using stratigraphy and radiocarbon dating, the team reconstructed the landscape from nearly a millennium ago. Although they found evidence of flooding by identifying fluvial deposits – sediments deposited by floodwater – the researchers concluded that major flooding could not have occurred throughout the height of Cahokian civilization. At two of the excavation sites, the archaeologists uncovered a waste product from mining and burning coal called coal clinker within the fluvial deposits, suggesting that significant flooding did not actually occur until the industrial era, long after the settlement was abandoned. In fact, the rock layers remained stable. Specifically, the stratigraphy revealed the presence of an Ab horizon, mineral layers lying just beneath the surface. If notable flooding had occurred, this horizon would have been washed or buried away by fluvial deposits [5]. 

So, if it wasn’t flooding, then what did lead to the decline of Cahokian civilization? While it is still unclear, anthropologists speculate that societal unrest, such as political instability or war, may have compelled residents to seek new homes [6]. Similar to the flood theory, another explanation approaches the question from an environmental perspective: in 2019, researchers from the University of Wisconsin concluded that climate change may have facilitated Cahokia’s ruination. Using a novel approach that combined archaeological and environmental data collection, the scientists examined fecal stanols and lake core sediments from Horseshoe Lake, a lake situated within Cahokia’s grounds. Using the fecal samples as a proxy for population size (discovering more samples indicates that more people may have lived nearby) and assessing the lake cores for climate indicators, the researchers determined that population decline coincided with a reduction in precipitation. The lack of rain could have impaired crop growth, offering an alternative ecological explanation for Cahokia’s downfall (7). 

Regardless of the driving factors behind Cahokia’s decay, dispelling the hypothesis that indigenous peoples contributed to the collapse of their civilizations presents a crucial step towards understanding. While overexploitation of resources leading to ecological damage certainly sounds like a plausible theory, new evidence posits that this simply was not the case. 
This prevailing narrative posits that the Cahokians inadvertently sabotaged their own success, carrying pernicious undertones that “primitive” American Indians wrecked their society. 
“There’s just no indication that Cahokian farmers caused any sort of environmental trauma,” asserts Professor Jane Mt. Pleasant, agricultural scientist and associate professor emerita at Cornell University. Professor Mt. Pleasant rejects the insidious presumption underpinning the ecocide hypothesis, that “indigenous peoples did everything wrong” [2]. 

Perhaps buying into narratives of great civilizations collapsing from ecological hubris actually reflects our own anxieties. It is tempting to adopt an industrialized, Western lens and assume that humanity instinctively exploits nature for its own purposes, inevitably causing widespread deforestation, pollution, and climate change in its wake. But according to Dr. Rankin, this line of thinking is flawed. 

“We shouldn’t project our own problems onto the past,” Dr. Rankin reminds. “Just because this is how we are, doesn’t mean this is how everyone was or is” [2]. 

If we perpetuate assumptions about indigenous peoples that no longer appear archaeologically substantiated, we inadvertently uphold views of Native American primitiveness. Yes, the full answer to Cahokia’s decline remains a mystery. Nevertheless, doing away with ethnocentric tendencies undermining the achievements and capabilities of indigenous peoples is a path we can follow now. Instead, we can choose to marvel at their legacies, just as my ten-year-old self did a decade ago. 

References
[1] Cahokia Mounds | archaeological site, Illinois, United States | Britannica [Internet]. [cited 2021 Nov 29]. Available from: https://www.britannica.com/place/Cahokia-Mounds
[2] Elbein A. What Doomed a Sprawling City Near St. Louis 1,000 Years Ago? The New York Times [Internet]. 2021 Apr 24 [cited 2021 Dec 5]; Available from: https://www.nytimes.com/2021/04/24/science/cahokia-mounds-floods.html 
[3] Centre UWH. Cahokia Mounds State Historic Site [Internet]. UNESCO World Heritage Centre. [cited 2021 Nov 29]. Available from: https://whc.unesco.org/en/list/198/
[4] Woods WI. Population nucleation, intensive agriculture, and environmental degradation: The Cahokia example. Agric Hum Values. 2004 Jun 1;21(2):255–61. doi: 10.1023/B:AHUM.0000029398.01906.5e
[5] Rankin CG, Barrier CR, Horsley TJ. Evaluating narratives of ecocide with the stratigraphic record at Cahokia Mounds State Historic Site, Illinois, USA. Geoarchaeology. 2021;36(3):369–87. doi: 10.1002/gea.21848 
[6] New insights into the curious disappearance of the Cahokia Mounds builders [Internet]. STLPR. 2015 [cited 2021 Dec 5]. Available from: https://news.stlpublicradio.org/health-science-environment/2015-05-04/new-insights-into-the-curious-disappearance-of-the-cahokia-mounds-builders
[7] White AJ, Stevens LR, Lorenzi V, Munoz SE, Schroeder S, Cao A, et al. Fecal stanols show simultaneous flooding and seasonal precipitation change correlate with Cahokia’s population decline. Proc Natl Acad Sci. 2019 Mar 19;116(12):5461–6. doi: 10.1073/pnas.1809400116
[Image] Hampshire M. Cahokia as it may have appeared c. 1150 CE [Internet]. [cited 2021 Dec 12]. Available from: https://www.britannica.com/place/Cahokia-Mounds