A sprawling city in the heartland of the United States was a cultural melting pot hundreds of years before Europeans ever set foot in North America. A study of dozens of teeth found at Cahokia, an ancient metropolis near modern-day St. Louis, shows that immigrants moved to the city from across the Midwest and perhaps as far away as the Great Lakes and Gulf Coast regions. Cahokia rose to prominence around A.D. 1050, when it underwent what some archaeologists call a cultural Big Bang. "All of a sudden, there's a giant rise in the size of the site," said study...