Immigrants have apparently been part of the North American experience from very early on, says a new study out of the University of Illinois.

New evidence reveals Cahokia, a large, pre-Columbian city planted where the Missouri and Mississippi rivers come together, maintained a significant population of reginal outsiders -- in contrast to the long-held belief by scientists that Cahokia was a generally stable, homogeneous enclave that drew its inhabitants from the immediate area.

The ancient community, which Thomas Emerson, a state archaeologist and director of the Illinois State Archaeological Survey at the University of Illinois, said included members of an array of Native cultures, was an experiment in urban life.

"Archaeologists are realizing that Cahokia at AD 1100 was very likely an urban center with as many as 20,000 inhabitants," Emerson said in a news release. "Such early centers around the world grow by immigration, not by birthrate."

The new analysis, published in the Journal of Archaeological Research, tested the chemical composition of 133 teeth from 87 people buried at Cahokia during the height of its development.

The study team specifically examined the ratios of strontium isotope, or, the variations of naturally-occurring metals, in the teeth and the remains of small animals also found in the general vicinity.

"Strontium isotope ratios in rock, soil, groundwater and vegetation vary according to the underlying geology of a region," the researchers wrote in the study. "As an animal eats and drinks, the local strontium isotope composition of the water, plants and animals consumed is recorded in its skeletal tissues."

The chemical residuals of strontium are not necessarily unique to any one location, Emerson said, but the ratios in one's teeth can be compared to those existing in the plants and animals of the immediate environment.

"Teeth retain the isotopic signature of an individual's diet at various periods of life depending on the tooth type sampled, ranging from in utero to approximately 16 years of age," the study continued.

In other words, the strontium signature in a person's teeth can be compared to that found in their place of burial, to conclude whether or not the person in question only lived in that specific area.

Differing strontium signatures in early and later teeth is an indicator a person has immigrated.

Emerson, state archaeological survey bioarchaeologist Kristin Hedman and graduate student Philip Slater carried out such an analysis of the teeth found at Cahokia and found immigrants formed one-third of the population of the city throughout its history, which stretched from approximately AD 1050 through the early 1300s.

"This indicates that Cahokia as a political, social and religious center was extremely fluid and dynamic, with a constantly fluctuating composition," Emerson said. "Cahokia, because it was multiethnic and perhaps even multilingual, must have been a virtual 'melting pot' that fostered new ways of living, new political and social patterns and perhaps even new religious beliefs," he said.