In order to assess migration to Rome within an updated contextual framework, strontium isotope analysis was performed on 105 individuals from two cemeteries associated with Imperial Rome—Casal Bertone and Castellaccio Europarco—and oxygen and carbon isotope analyses were performed on a subset of 55 individuals. Statistical analysis and comparisons with expected local ranges found several outliers who likely immigrated to Rome from elsewhere.
Who immigrated to Rome?
Of the nonlocal sample (n = 4), there are three adult males and one adolescent of unknown sex. Two of the males fall in the Middle Adult category (35-50) and one into the Older Adult category (50+), while the Adolescent is between 11-15 years old. The other four individuals whose isotope ratios were different from local Roman expectations, although not statistically conclusive, include two Older Children (7-12 years old), one probably male older Adolescent (11-15 years old), and one older Adolescent female (16-20 years old).
It is also impossible to answer from the present data whether these individuals were voluntary or compulsory migrants. The status of slave was multifaceted and mutable during the Empire , and there is no indication in the archaeological information from Casal Bertone and Castellaccio Europarco that any specific individual was a slave. There is, however, no evidence from isotopes that individuals buried in the mausoleum at Casal Bertone were nonlocal, whereas the necropoleis at Casal Bertone and Castellaccio Europarco both produced skeletons with nonlocal isotope ratios. Burial in a necropolis was customary for the lower classes, while burial in a mausoleum cost more . These isotope data may be showing a form of economic, status-related migration, with more lower class individuals and possibly slaves moving to Rome compared to wealthier individuals. Additional testing would be needed, though, to confirm this hypothesis.
Where did immigrants come from?
Because migrants often came to Rome in diasporic waves resulting from slavery, attempting to identify a general geographic origin can be instructive. The combination of strontium and oxygen isotope analyses is particularly useful for this in western Europe, although only general predictions of homeland can be made. Oxygen isotopes on the continent vary roughly east-to-west, while strontium isotopes are higher in the older rock of mountains such as the Alps and lower in the younger rock of volcanic areas like most of peninsular Italy. From the perspective of Rome, oxygen isotope ratios will decrease as one moves into the Apennine range running along the spine of Italy, and strontium isotope ratios will increase to the north and decrease to the south.
The four individuals with clearly anomalous isotope ratios—T15, ET38, T24, and T36—fall into three distinct strontium and oxygen isotope combinations. T15 and ET38 have oxygen isotope ratios within range of Rome, but strontium isotope ratios that are significantly higher, suggesting a possible origin in a place with older geology, such as the Alps or one of the islands in the Tyrrhenian Sea. As people arrived at Rome from all over the Empire, however, there are numerous locations in which these individuals could have been born.
Individual T24 has low strontium and low oxygen isotope ratios compared to Rome, suggesting an origin somewhere with a cool, wet climate and basalt or limestone substrate, such as the Apennines. Individual T36 has high oxygen and low strontium isotope ratios, suggesting an origin in a region of limestone or basalt with a hotter, drier climate than Rome, such as North Africa. For these individuals, however, a dietary explanation for the anomalous strontium isotope ratios, while much less likely owing to the concomitant δ18O values, cannot be completely ruled out. As Rome imported significant amounts of grain from north Africa during the Empire, and as human strontium isotope ratios from Egypt and the Nile Valley have been shown to be lower than those in Rome (around 0.707 to 0.708) , it is not impossible that T24 and T36 were consuming a significant amount of imported grain as children. Still, as shown further below, the dietary explanation is less likely than is an origin elsewhere.
The four additional individuals whose isotopes may indicate they were immigrants—T8, T70, T39, and ET76—fall into the categories above. T8 and ET76 have higher-than-expected strontium isotope ratios, showing up as outliers in the box plot in Fig 3. They may have arrived at Rome from a region of older geology such as northern Italy. Individuals T70 and T39, while not statistical outliers in the oxygen isotope box plot in Fig 5, are nevertheless 0.6-0.7‰ higher than the next closest local, suggesting they may also be immigrants. They could have arrived at Rome from a drier climate like North Africa. These four individuals highlight the challenge of identifying immigrants to Rome from a vast geographical expanse.
Finally, the fact that there is a large spread in both the strontium and oxygen isotope data compared to results obtained from other archaeological populations could indicate that people were arriving at Rome from places not too far removed, in a form of centripetal migration, as Prowse and colleagues  suggest for Portus. Both the strontium and the oxygen isotope ratios from Rome are diverse, and it is not unreasonable to assume that these may reflect the diversity of the population as well. It is also possible that even more individuals are essentially isotopically invisible migrants, if they came to Rome from homelands with similar strontium and/or oxygen isotope values. Further isotopic and DNA work will be necessary to better understand origins and homelands from skeletal remains.
Killgrove and Montgomery. "All Roads Lead to Rome: Exploring Human Migration to the Eternal City through Biochemistry of Skeletons from Two Imperial-Era Cemeteries (1st-3rd c AD)". PLOS One, 2016.
Oxygen stable isotope ratios (δ18O) have been determined in carbonate in paired first and third molar teeth from individuals (N = 61) who lived in the town of Portus Romae ("Portus") and who were buried in the necropolis of Isola Sacra (First to Third centuries AD) near Rome, Italy. We compare these analyses with data for deciduous teeth of modern Roman children. Approximately one-third of the archaeological sample has first molar (M1) values outside the modern range, implying a large rate of population turnover at that time, consistent with historical data. Delta 18Oap values suggest that a group within the sample migrated to the area before the third molar (M3) crown had completely formed (i.e., between 10 and 17.5 years of age). This is the first quantitative assessment of population mobility in Classical antiquity. This study demonstrates that migration was not limited to predominantly single adult males, as suggested by historical sources, but rather a complex phenomenon involving families. We hypothesize that migrants most likely came from higher elevations to the East and North of Rome. One individual with a higher δ18O value may have come (as a child) from an area isotopically similar to North Africa.
Origins of the immigrants to Imperial Rome
There are several possible origins for the outsiders (at birth) buried in the cemetery of Isola Sacra who must have come from regions where δ18O of local precipitation is lower than in Rome by up to 2.6%. One possible region of origin is the Roman Imperial provinces lying to the North of the Italian landmass. In general, the δ18O of modern precipitation decreases northward, reaching values up to 3% less than found in Rome (Bowen and Wilkinson, 2002; Longinelli and Selmo, 2003), within the geographic range of the Roman Empire of 100 CE. Isotope ratios of average annual precipitation to the North of Italy would generally be lower than the values required to account for the outsiders’ values. Furthermore, seasonal variation in δ18O of precipitation in these more northern regions is larger than in Rome because the range between winter and summer temperature increases as average annual temperature decreases, and because δ18O varies linearly with temperature. Thus, an individual whose M1 teeth happened to mineralize through the winter months while consuming water largely derived from precipitation would display significantly lower δ18Oap values than "local" Romans.
As another possible locus for the outsiders, we note that δ18O values of modern meteoric water vary continuously to values up to 4% lower than those encountered in Rome at distances as close as 100 km in the foothills and heights of the Apennine Mountains (Longinelli and Selmo, 2003). Derivation of the outsiders principally from this region seems to be the most likely scenario. Other possible regions of origin of the outsiders might be the Iberian Peninsula or Greece, both of which were under Roman control at this time. Rain falling in these regions also displays δ18O values lower than those corresponding to the outsiders’ inferred drinking water, although coastal regions in both these provinces might have included such values.
The continuous gradation of δ18Oap between local and outsider δ18Oap values suggests that these individuals came from locations at gradually farther distances and gradually higher elevations than Rome. If the outsiders were from as far away as southern Gaul (where δ18O of rain is about 2% lighter than in Rome), we would expect to see a cluster of analyses at discretely lower δ18Oap values, rather than the continuum that we actually observe. However, we can not exclude the possibility that some of the outsiders came from further away to the north.
The δ18Oap values show that approximately one-third of the individuals in our sample were not born in the region around Rome, but migrated to this area from regions where local drinking water has somewhat lower δ18Oap values. It has further been shown that a significant minority of the sample as a whole were individuals who migrated as children, so that migration to Portus was not a predominantly single adult male activity. Migrants to Portus were families, most obviously as children accompanying the parents.
The data support historical demographic estimates of high mortality rates in the Roman urban region, and the consequent need for high rates of population replacement to maintain the size of the Roman population in this era. Although such isotopic data suggest a method of quantitative assessment of this steady population replacement, a detailed numerical estimate is beyond the scope of this paper. The individuals with low δ18Oap values could have been from as close as 100 km to Rome, in the hills surrounding the Apennine Mountains. It is also possible that they came from the transalpine provinces of the Roman Empire much further to the north, where low δ18Oap precipitation falls even at low elevations. However, due to the observed scatter of the δ18Oap data around the "local" Roman range, we conclude that this latter explanation seems less likely. Only one individual was found to have a conspicuously high δ18Oap value; the observed value is consistent with an origin in a region with higher δ18Oap in drinking water, like the Nile Delta, although it is impossible to exclude possible origins in southern Italy. Further Sr isotopic analyses of these teeth might help to resolve this issue. Analyses of δ18Oap in bones of these same people may also show further evidence of population movement during adulthood.
Prowse et al. "Isotopic Evidence for Age-Related Immigration to Imperial Rome". Am J Phys Anthropol, 2007.