Wait, fecal chemicals last how long?

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.

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."

Prehistoric and post-apocalyptic

From 1150 CE onwards, the amount of coprostanol in the lake bottom gradually declined, which suggests that the local population was doing the same. Things bottomed out (sorry; not sorry) around 1400. Archaeological excavations at Cahokia had suggested the same timing but not the cause of the city's collapse.

It turned out that the sharpest decreases in coprostanol coincided with evidence of catastrophic floods and severe droughts, a one-two punch that eventually spelled doom for a city whose survival depended on nearby maize farming. Oxygen isotope ratios in the soil suggested a shift toward dry summers and wet winters around 1150 (bad news if you're a maize farmer), and in 1150 a cluster of larger sediment grains suggests a large flood of the nearby Mississippi River.

Cahokia held on as a major regional power for a few more centuries even as the climate got worse for farming, but by 1400, it was all over. And there's no archaeological evidence to suggest that people kept living in the area once the city fell—just crumbling earthen mounts and the post-apocalyptic landscape of the Vacant Quarter. The mud at the bottom of Horseshoe Lake tells a different story, however; coprostanols, along with pollen and bits of charcoal, suggest that people were living (and yes, pooping) in and around Cahokia a century after the city was abandoned.

Repopulating a ghost town

For about a century after the city's demise, the surrounding region looked like a ghost town, according to the sediment cores. Then, around 1500 CE, more coprostanol started washing into the lake again—people were back. But they were living in a very different landscape than the old Cahokians; these layers of sediment contain more pollen from trees and grasses, suggesting that woods and prairie had started to reclaim the former maize fields.

The grasslands were good news for the newcomers, because they offered perfect habitat for bison. Charcoal particles in the lake sediment may suggest that local people burned the grass as a form of game management. Burning prairie grass in the early spring helps new, fresh grass grow in its place—and that tasty new growth is like a magnet for hungry bison.

Overall, that way of life sounds remarkably similar to an indigenous group called the Illinois Confederation, who lived in the area by 1700. The Illinois settled in villages during the spring and fall, where they worked small farms and gardens. Most of the village packed up to follow the bison herds during the summer, and in winter everyone split up into smaller camps to weather the leaner months. There's not enough evidence to say whether the Illinois were the first people to repopulate the land around Cahokia, but White and his colleagues say that whoever did probably had a pretty similar lifestyle.

It's no surprise that this phase of life in the area hasn't yielded much archaeological evidence; smaller, more mobile groups of people would have left fewer obvious traces behind, and the sites that have been found don't often contain artifacts clearly linking them to the post-Cahokia period.

A survival story, not a lost civilization

The population around Cahokia kept increasing until around 1650—which is surprising given what was happening elsewhere in North America. Historical records describe devastating epidemics in Canada, the Caribbean, Mexico, and the eastern United States around the same time that populations in the so-called Vacant Quarter were thriving. That could mean that the interior of the US was a bit more isolated from the spread of European disease—at least temporarily.

Around 1700—just a year after the first French religious mission showed up in the area—the population around Horseshoe Lake started to shrink. That's eerily apparent in the sudden decrease in coprostanol in the sediment core. Grass pollen also starts to get a bit scarcer around 1700, which could suggest that the prairie, which once supported great bison hunts, was also shrinking. And historical records describe smallpox and measles outbreaks in the area, along with war with neighboring Iroquois groups (who were also facing pressure from encroaching Europeans).

A small Illinois population was still holding on by the early 1800s, when the US government forcibly moved many of them to Oklahoma. Today, the survivors of that relocation are officially known as the Peoria Tribe of Indians of Oklahoma: a sovereign political entity with nearly 3,000 people—including some who still live in Illinois.

White says the coprostanol study provides an important example of indigenous people's resilience and persistence in the face of social upheaval and environmental challenges. "Throughout the history of archaeological research, Native American disappearance has been emphasized more than Native American persistence," he and his colleagues wrote. That's especially true at Cahokia, where so much attention has focused on the city's abandonment.

American Antiquity, 2020 DOI: https://doi.org/10.1071/aaq.2019.103; (About DOIs).

If you want to read more about Cahokia, check out Finding North America's Lost Medieval City.