The first physical evidence of individual migrants to Imperial Rome has been found in the teeth of some 2,000-year-old skeletons, says a study published in the open-access journal PLOS ONE.

The investigation took place in two Imperial-era cemeteries and showed that several individuals, mostly men and children, migrated to Rome, changing significantly their diet after their move.

It is believed that up to one million people lived in Imperial Rome, with voluntary immigrants accounting for about 5 percent of the population and slaves for up to 40 percent. However, these are just estimates, as there is no complete Imperial-era census for the city of Rome.

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"They say that all roads lead to Rome, but finding direct evidence of immigrants to the Eternal City has troubled archaeologists for decades," bioarchaeologists Kristina Killgrove of the University of West Florida, said.

To find physical evidence of human migration in Imperial Rome, Killgrove and colleague Janet Montgomery of Durham University, UK, examined skeletal remains buried at two Rome-area cemeteries during the 1st through 3rd centuries AD.

By analyzing the isotopes of the elements strontium and oxygen, it was possible to determine whether a person's tooth was formed during a time he or she was living in Rome.

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Killgrove analyzed 105 teeth for strontium isotopes, and Montgomery examined 55 of those for oxygen isotopes.

Aqueduct water and imported grain were also taken into account to assess the local isotope ranges.

"In the end, there was one female skeleton, several male skeletons, and several children whose sex could not be determined that likely were not born at Rome," Killgrove said.

At least eight individuals, five of them quite young, turned to be migrants from outside Rome, possibly from North Africa, the Alps and the Apennine Mountains in Italy.

The Colosseum in Rome. ThinkStock

The presence of many young individuals is particularly intriguing, as most known voluntary migration was by adult men.

"Children and adolescents could have come to Rome to be educated, to become apprentices, to be married, as part of a family that migrated, or even as slaves," Killgrove told Discovery News.

The scale of slavery within the Empire was quite large.

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"Unfortunately, we can't tell from isotopes or from the way they were buried what their status was," she added.

Carbon isotopes in the teeth, reflecting carbohydrate consumption, revealed that the immigrants' diet changed significantly when they moved to Rome. They possibly adopted and adapted to the local cuisine, which included mostly wheat, legumes, meat and fish.

"This case study demonstrates the importance of employing bioarchaeology to generate a deeper understanding of a complex ancient urban center," Killgrove and Montgomery wrote.

They stressed that further isotope and DNA analysis is needed to provide more context for their findings.