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Click the link in the description and sign up between now and December 11th to get a new pair of fun socks each month in 2021. {♫Intro♫}. They often don’t get as much attention, but North America had major cities long before.

European colonizers arrived. There was Tenochtitlan with the Aztec Empire, and Tikal with the Maya. But there should be another name on that list, too: Cahokia, built by the group known as the Middle Mississippians.

Cahokia was a major city that existed near what is now St. Louis until about 1350. At its height, it was the biggest and most politically-complex metropolitan area in the.

Americas north of Mexico, and it rivalled cities like London in size. It would have contained 200 earthen mounds, with open plazas, thousands of houses, temples, and public buildings, all organized into districts. Things weren’t always like this, though, and the path to Cahokia’s rise meant a huge shift in how people had lived in the area.

At least, that’s what we think. The residents of Cahokia left behind no written and essentially no oral history. So, to learn the story of this revolution that happened nearly 1000 years ago, we’ve needed some serious archeology.

Cahokia is found in an area called American Bottom. Situated between a number of major rivers, the region was a flood plain filled with diverse habitats, including open water, marshes and forests, and dry, upland areas. But before we get too far: It’s worth noting that “Cahokia” is a name given to this city much later by European settlers.

We don’t know what the people living there called it… and actually, we don’t know what they called themselves. Today, archaeologists just refer to them as members of the Middle Mississippian culture. But we do know that these people had been living in this area for a long time, practicing agriculture and shaping their environment even before they built Cahokia.

One of the ways scientists can figure out these kinds of things is by looking at pollen assemblages. pollen assemblages. Plants produce pollen every year, and some of it can settle into the ground or water. And pollen preserves really well, thanks to their tough outer shells, which protect their little packages of DNA from drying out or being destroyed.

So, by looking at places where pollen collected year after year, scientists can work their way backwards to guess what types of plants were around throughout history. Here, for instance, we see tree pollen disappear around 450 CE, while pollen from what were probably crop species becomes more common. Overall, that paints a picture of people clearing away the forest in the floodplains and uplands — followed by a rise in agriculture.

At the time, they were probably growing species like sunflower and squash. But then, around the year 900, we see the introduction of a new, important crop: corn. It was probably introduced to the area from the Southwest.

And there are a couple of forms of evidence for its arrival. For one, scientists can just straight up find the remains of corn plants buried in the earth. They’ve also found starch grains that look like corn stuck to the sides of pots.

But corn also left signatures in the environment. Notably, many plants use a type of photosynthesis called C-3, but maize plants use C-4. Exactly what’s happening in each type doesn’t matter that much here, but what’s important is that they make slightly different sugars.

Each kind has a slightly different ratio of light to heavy carbon isotopes — or, carbons with different numbers of neutrons. This all means that if you have a lot of C-4 plants, they will leave a specific signature behind. And it’s a signature scientists have found in lake sediments near Cahokia.

Overall, it looks like corn was introduced around 900 CE and, over the next few centuries, became increasingly important until it was the crop people ate. In fact, corn agriculture may have been part of what led to Cahokia becoming a booming city. Growing corn requires people to stay in an area more than the crops that had been growing previously.

And as people settled down more, they could grow more corn, and villages could get larger in a kind of recursive cycle. Excavations of the nearby area suggest that’s essentially what happened, maybe in just a few generations. The villages grew in size and complexity.

And this trend seemed to be holding steady until around 1050, when Cahokia experienced what’s sometimes called its “Big Bang”. Here, archaeologists see a number of really rapid changes — like a sudden population boom, what seems to be a new class structure, and massive construction projects. As for why this big bang happened, we’re not entirely sure.

The increased maize production probably played a role in at least supporting what was about to happen. But the actual change may have been due to some innovation or something about labor and politics. It’s not clear based on the evidence we have.

But we do know that the end result was impressive — especially the construction projects. Like Monks Mound, for instance. Rising up 30 meters, or the equivalent of a roughly seven-story building, the mound covers a huge area, features distinct terraces, and was topped with a long, tall central public building.

Details on how exactly Monks Mound was built can be hard to come by. But archaeologists can use excavations as well as sediment cores. That’s where you drill up columns of earth and basically make a cross-section of an area, to investigate how it changed over time.

Those cores and excavations have unearthed features like vertical wooden posts and paved limestone layers. And that all suggests Monks Mounds was probably built by layers of soil and clay, transported by hand in baskets. Meanwhile, the city itself also underwent a dramatic rearrangement.

Scientists can see that areas that had been residential were commandeered for city or elite use. Also, houses in Cahokia shifted away from being arranged around courtyards to what appears to be a planned grid system. They’ve determined this partly through excavations, but also with techniques like magnetometry, which allows them to peer underground, no digging required.

Magnetometry is based on the fact that different soils and materials interact with the Earth’s magnetic field in subtle, but detectable ways. For instance, certain objects, like metal tools, fire pits, or volcanic rock, interact with the Earth’s magnetic field more, while building foundations or limestone interact less. So, by walking over an area with the appropriate tools and then making a map of our results, we can see signatures of what may be old walls, ditches, and other construction.

One of the most astounding things about Cahokia at its height, though, was just the sheer number of people living there. It looks like the population multiplied by a factor of five to ten really fast, over the course of about fifty years. Estimates of its max population range from about 10,000 to 20,000 people.

And if you’re thinking that doesn’t sound like that many people, remember that having megacities just about everywhere is a pretty recent occurrence. Cahokia wasn’t the biggest city in the world — places like Constantinople and the Chinese city of Kaifeng had populations in the hundreds of thousands. But it still rivalled other major cities.

Like, London’s population around the year 1100 was about 14 to 18,000. And Rome was only at about 30 to 40,000. One of the ways that scientists have been able to get at these numbers without, you know, census records, is through something called fecal stanols.

Fecal stanols are organic molecules from the gut that are passed out through feces. So yeah, they’re about as lovely as you imagine. Like pollen, they can stick around for centuries in things like lake sediments, and the amount we see closely relates to population size.

Amazingly, scientists can also tell that quite a lot of the people who’d moved to Cahokia came from a fair ways away. Like, we know the people living here had connections with folks living as far away as the Gulf. Coast and the Great Lakes.

We can see evidence of this by the fact that people had exotic goods that wouldn’t be easily found in the area, like shell beads. There are also a number of distinctive burial customs present around Cahokia, which implies people were bringing their cultural practices with them from afar. We can also find evidence of migration by looking at the ratio of strontium isotopes in people’s bones and teeth.

Strontium is an element naturally found in rocks, soil, and groundwater, and it comes in different isotopes. The exact ratio differs from place to place based on the geology of the region. When an animal ingests it, the strontium sometimes chemically acts like calcium and integrates itself into bones and teeth.

Unfortunately, signatures aren’t always unique to one area, so we can’t use them to say for certain where people were coming from originally. But since scientists can define a strontium signature for “local” people, we can say, at least, that they weren’t from here. And as many as 20 to 30% of residents had distinctive “not-from-Cahokia” signatures.

What may be most fascinating, though, is that besides the city becoming much bigger, it also appears to have become much more stratified, with evidence of work specialization and what you would call an elite class. Some of this experts can infer from other evidence. The planned nature of the buildings means we’d need logistics and a large labor force, for instance.

But we also do see archaeological evidence — like, how people ate. Which we can tell by looking at animal remains people left behind as they prepared food. As time goes by, some groups consistently had more access to the best cuts of meat, like the hindquarters, while others ate more low-quality parts.

Our most dramatic evidence, though, probably comes from what we’ve found of burials. Certain graves have included indications of status, such as grave goods — like thousands of shell beads or exotic artifacts. Other burials included what appears to be mass burials of people, likely enslaved people, who were sacrificed and buried underneath the elite individual.

Overall, this paints a picture of a society that fairly suddenly grew in size, complexity, and stratification. But Cahokia would not last. Its population peaked around the year 1200 before going into a fairly rapid decline.

And by about 1350, the area appears to have been abandoned. The reasons for this are speculative. We don’t know, but people have suggested a number of options.

One is that flooding might have increased. Cahokia was built in a floodplain, after all. And we can tell there was more flooding thanks, one last time, to those sediments.

We can see the kinds of deposits we normally see during intense flooding show up, like clay and fine silt. Some have even suggested this flooding was due to the deforestation we mentioned way back at the beginning of this whole saga. Without trees, erosion increased leading to more runoff, causing larger floods.

Alternatively, it might have been due to changes in the climate as a relatively dry spell ended and more rain started to fall. Other people have blamed some sort of social unrest. Scientists have unearthed what looks like defensive palisades popping up around here — which is a kind of defensive wall.

In the end, it is not clear exactly what happened, and there’s still a lot to understand — including how exactly these features were built and what they were for. And that’s not even including all the questions we have about the people who lived there. But through careful analysis of things like sediment cores, magnetometry, and isotopic signatures, scientists can at least piece together parts of the dramatic shift that happened here nearly 1000 years ago.

Alright, hard transition because we don’t have a great link to socks from this episode—but this episode is sponsored by Awesome Socks Club, which is a charity sock subscription that my brother and I have just started! So here’s the situation: you’re going to have to buy socks sometime in the next year and that’s, like, going to be a bit of a pain so we want to make this easy for you. We have created a sock subscription, you can sign up, you will get a different pair of fun socks every month for all of 2021.

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So you solve a problem for yourself, you also make the world a little bit better, you can check the link in the description to learn more, or just go to awesomesocks.club/SciShow. {♫Outro♫}.