A thousand years after the Native American city known as Cahokia sprouted on a floodplain east of modern-day St. Louis, Missouri, the story of its explosive birth and precipitous decline remains one of America's great mysteries. (Read "Cahokia: America's Forgotten City" in National Geographic magazine.)
But a new study published in the Journal of Archaeological Science may shed fresh light on how this city of thousands—perhaps 20,000 or more—formed in a matter of just 50 years.
By examining the strontium content of teeth from the remains of 87 ancient Cahokians and comparing it to the strontium signatures of local fauna, a team led by Thomas Emerson of the University of Illinois at Urbana-Champaign has concluded that at least one-third of Cahokia's residents immigrated from areas outside the floodplain known as the American Bottom.
How did they reach this conclusion, and what might this mean for our understanding of Cahokia? In an interview earlier this week, writer Glenn Hodges posed these questions to Emerson, who is also the Illinois State Archaeologist.
First of all, how do strontium signatures work?
Essentially, it works under the principle that you are what you eat. Strontium is in the bedrock across the world. It dissolves into the water supply, the water is absorbed by animals and plants, humans consume the animals and plants, and the strontium moves into their bones and teeth.
Strontium varies by the kind of bedrock, and that's what allows us to use strontium to tell where somebody was raised. To determine the signature for Cahokia, we used small mammals that probably never moved more than a mile from where they were born—squirrels, rabbits, etc.
So you take that animal data as a baseline, and compare it to the strontium in people's teeth.
Yes. There's one set of teeth that matures when you're about five or six, and there's a second set that matures between six and sixteen. So we're looking at people whose infant teeth indicate they were living in a different place, but by the time they became teenagers they were living in Cahokia. Once you move beyond that 16-18 age group, we can't recognize immigrants anymore.
How did you extrapolate from 87 individuals to one-third of the population?
You have to assume that your sample is representative. We accessed almost all the individual remains that are held in institutions. The population available for testing is just that small.
So if one-third of your sample immigrated in their youth, the actual proportion of immigrants might be higher? Because you've got to figure that these kids came with parents who already had all their adult teeth.
Yes. So the numbers are probably higher. We just have no way to get at that right now.
Do you have any idea where the immigrants were from, or do you only know that they're not from the Cahokia area?
Our level of research right now is confined to identifying immigrants. To understand where they came from, we have to greatly expand the database of strontium throughout the mid-continent, and that research has not been done yet.
Is that research anywhere on the horizon?
Yes, it is. We started it in 2009 with preliminary work, and now we're looking to expand that database.
How long do you think it will take to get a useable database?
Probably two to three years. It's basically getting the right kind of samples and getting them processed. Of course, we have to get the money to do it, but the research is fairly simple.
So how does this change the picture we have of Cahokia? Wasn't it already thought that immigration was a factor in its explosive growth?
It depends on who you talk to. There's still a group of archaeologists thinking in pretty traditional terms—yeah, there were some immigrants, but it was pretty much a homogeneous population getting larger.
But when you start thinking about Cahokia as multiethnic and probably multilinguistic, with massive population growth and nucleation [forming around a central area], you have to ask: What sort of social, religious, and political organization do you need to actually make that function? When people don't have anything in common, how do you create unity?
In terms of research, it's a sea change. Now we can look at Cahokia in comparison with the growth of cities all over the world. From other areas we know that's how cities grow, by immigration. I don't care if you're looking at Roman London, or Delhi, or some of the big Chinese cities, they're actually nucleations of dissimilar people.
So this kind of takes Cahokia out of this romantic mythology of the Indian past, and shows how these people were facing the same kind of problems as people all over the world do when you begin to urbanize. It allows you to do cross-cultural comparisons with a lot more validity.
So you're really talking about expanding the scope of Cahokia research, and connecting it with this larger body of scholarship?
There are basically two schools of thought. One is based on very localized interpretations and perspectives of Cahokia, and the other sees Cahokia as a player in international research.
And you think the balance is shifting to the latter?
I think ultimately it will be the dominant perspective on Cahokia.