The first experiment in "melting pot" politics in North America appears to have emerged nearly 1,000 years ago in the bottom lands of the Mississippi River near today's St. Louis, according to archaeologists piecing together the story of the rise and fall of the native American urban complex known as Cahokia.
During its heyday, Cahokia's population reached an estimated 20,000 people – a level the continent north of the Rio Grande wouldn't see again until the eve of the American Revolution and the growth of New York and Philadelphia.
Cahokia's ceremonial center, seven miles northeast of St. Louis's Gateway Arch, boasted 120 earthen mounds, including a broad, tiered mound some 10 stories high. In East St. Louis, one of two major satellites hosts another 50 earthen mounds, as well as residences. St. Louis hosted another 26 mounds and associated dwellings.
These are three of the four largest native-American mound centers known, "all within spitting distance of one another," says Thomas Emerson, Illinois State Archaeologist and a member of a team testing the melting-pot idea. "That's some kind of large, integrated complex to some degree."
Where did all those people come from? Archaeologists have been debating that question for years, Dr. Emerson says. Unfortunately, the locals left no written record of the complex's history. Artifacts such as pottery, tools, or body ornaments give an ambiguous answer.
Artifacts from Cahokia have been found in other native-American centers from Arkansas and northern Louisiana to Oklahoma, Iowa, and Wisconsin, just as artifacts from these areas appear in digs at Cahokia.
"Archaeologists are always struggling with this: Are artifacts moving, or are people moving?" Emerson says.
Emerson and two colleagues at the University of Illinois at Urbana-Champaign tried to tackle the question using two radioactive forms of the element strontium found in human teeth. They discovered that throughout the 300 years that native Americans occupied Cahokia, the complex appeared to receive a steady stream of immigrants who stayed.
The two forms, or isotopes, of strontium occur naturally in rocks and soil and work their way up the food chain and accumulate in bones, including teeth. Different locations around the continent have different ratios of the two forms, strontium-87 to stronium-86.
The team first measured strontium ratios found in teeth from the remains of small animals found in the main complex's trash pits as well as in the pits of surrounding settlements. They also took samples from the teeth of modern squirrels in the area. This gave them the range of values one could expect to find in the teeth of people who were Cahokia residents from cradle to grave.
They analyzed small samples of enamel from 133 human teeth belonging to the remains of 87 people found in the complex's burial sites. The samples were taken from teeth that mature quickly, when a resident was five or six years old, as well as from teeth that mature later. Then the team looked for differences in the strontium ratios between the two types of teeth and compared them with the local ratios.
Some 30 percent of the individuals sampled had isotope-ratios in the fast-maturing teeth that were different from the isotope ratios for the Cahokia area. The ratios in the teeth that matured later fell within the local range. Taken together, these were clear signs that these individuals were immigrants, Emerson says.
The results of the strontium tests are set to appear in the April issue of the Journal of Archaeological Science. The paper's lead author is University of Illinois graduate student Philip Slater.
From the standpoint of demographic development, Cahokia falls into the same category as Rome, London, or any other population center, he says. "It puts Cahokia in the mainstream of how cities form."
Even so, he adds, the relatively high proportion of newcomers the team inferred from its results came as a surprise.
It's hard to imagine such an influx without thinking about the changes it must have brought, Emerson says.
"Cahokia had to develop new ways of thinking about political organization and religious organization, and social organization. The old ties of kinship no longer work if you're not related," he explains. “This suggests that Cahokia was innovative."
Researchers have noted that toward the end of Cahokia's run as a native-American metropolis, a 2-mile-long stockade some 15 feet high sprouted to surround the central portion of Cahokia's ceremonial center. The stockade had bastions at regular intervals, and the stockade cut through some residential areas.
This has led some researchers to conclude that the stockade was built to protect the central portion of the ceremonial center from civil unrest, rather than from an external invasion.
Climate likely had something to do with that unrest.
A team led by US Geological Survey scientist Larry Benson published a study five years ago that looked at climate trends in the region during Cahokia's rise and fall.
Tree-ring evidence suggests that the region experienced unusually heavy rainfall during the first 50 years of Cahokia's explosive growth, which began around AD 1050. From 1150 on, the region underwent a series of droughts, culminating in a severe 15-year drought that shut down farming in an area that was a key supplier of food to Cahokia. The palisades were built at about this time.
Whatever Cahokia's political or religious innovations might have been, they apparently were insufficient to hold the complex together, especially in the face of outside stresses.