Tens of thousands of people once lived in Cahokia, the city at the heart of the mound-building Mississippian culture (which dominated the midwestern and southeastern United States from 700 to 1500 CE). And then, around 1450, they all left. Now, sediment cores from nearby Horseshoe Lake suggest that the area didn't stay deserted for long.
Wait, fecal chemicals last how long?
The study looked for the chemical signature of ancient human feces, which washed into nearby Horseshoe Lake over the centuries along with layers of soil, pollen, and other material. When bacteria in your gut break down cholesterol, they produce a chemical called coprostanol, which can survive in soil for hundreds or even thousands of years. More coprostanol in the soil means more people living (and pooping) in the area around the lake.
If you want to get really technical, archaeologist A.J. White of the University of California, Berkeley, and his colleagues actually measured the ratio of coprostanol to another chemical called cholestenol, which is formed when soil microbes break down cholesterol in the soil. A higher ratio means more human waste. That ratio can't measure population sizes, but it can tell researchers whether populations were increasing or decreasing—and how quickly.
"I do think that there's a lot of value in looking at human waste," White told Ars in a 2019 interview. "I think it's an area in archaeology that's kind of been somewhat of a novelty in the past, but I think that what we're showing is that it actually can produce pretty significant findings from something so simple and everyday as going to the bathroom."
Based on the amounts of coprostanol present in sediment layers dating to the centuries between the fall of Cahokia and the arrival of European colonists in the area, it turns out that indigenous groups moved back into the area around the abandoned city within a century or so after its collapse. That contradicts the popular idea that huge swaths of what is now the Midwest were basically empty when Europeans showed up.
"A lot of discussions around Cahokia stop around 1400 [CE], around when Cahokia is said to have been abandoned," White told Ars in a 2019 interview. "I think we're kind of adding sort of a new layer to the story in that area."