Most of the samples are from cemeteries in and around the city of Rome, so on one hand they're not representative of the whole country, but on the other, Rome was the biggest, most traveled-to city in all of Italy, so if there wasn't much diversity or mixing going on there, that means there would have been even less in other cities and (especially) in the small towns and rural areas that made up most of the country.
As in all of Europe, there were major changes between the Mesolithic and Iron Age, but from then on, the samples all have the same 4 main components: Western Hunter-Gatherer, Anatolian Neolithic (farmers), Iran Neolithic and Steppe Eneolithic (the last two arriving mostly with Indo-Europeans), plus very low levels of Morocco Hunter-Gatherer. Even the "diverse" imperial samples have those same components, just in slightly different proportions, so the foreigners were not that foreign.
Iron Age & Republic (dark blue)
Most of these early Latin and Etruscan samples cluster between Northern Italians and Spaniards, and a few others cluster with Northern Italians, Central Italians, Southern Italians and off toward Sardinians. So the original, pre-empire Romans were heterogeneous but fully Southern European, just like modern Italians.
By 900 BCE at the latest [before the founding of Rome], the inhabitants of central Italy had begun to approximate the genetics of modern Mediterranean populations. [...] The Iron Age individuals exhibit highly variable ancestries, hinting at multiple sources of migration into the region during this period (Figs. 2A and 3B). [...] Together these results suggest substantial genetic heterogeneity within the Etruscan (n = 3 individuals) and Latin (n = 6) groups. [...] In contrast to prehistoric individuals, the Iron Age individuals genetically resemble modern European and Mediterranean individuals, and display diverse ancestries....
Imperial Rome (teal)
At this time there appears a "tail" toward the Near East, reflecting migrants to Rome from the Eastern Mediterranean. However, most of them cluster no farther than Cyprus, which seems to confirm that "Middle Eastern" migrants to Rome were of predominantly Greek ancestry.
During the Imperial period (n = 48 individuals), the most prominent trend is an ancestry shift toward the eastern Mediterranean and with very few individuals of primarily western European ancestry (Fig. 3C). The distribution of Imperial Romans in PCA largely overlaps with modern Mediterranean and Near Eastern populations, such as Greek, Maltese, Cypriot, and Syrian (Figs. 2A and 3C).
Late Antiquity (green)
Here the "tail" to the Middle East disappears and the samples shift back westward. After the capital of the empire moved east and Rome was sacked, the population declined to almost nothing, and the city would have been repopulated by Italians from other parts of the country, and possibly other Europeans too.
The average ancestry of the Late Antique individuals (n = 24) shifts away from the Near East and toward modern central European populations in PCA (Fig. 3D). [...] This ancestry shift is also reflected in ChromoPainter results by the drastic shrinkage of the Near Eastern cluster (C4), maintenance of the two Mediterranean clusters (C5 and C6), and marked expansion of the European cluster (C7) (Fig. 4C).
This shift may have arisen from reduced contacts with the eastern Mediterranean, increased gene flow from Europe, or both, facilitated by a drastic reduction in Rome's population in this period to less than 100,000 individuals, due to conflicts and epidemics (1, 3).
Medieval & Early Modern (yellow)
Here the samples are back to being similar to the Iron Age samples, and they're identical to modern Italians from North to South, except for a few outliers tending toward Spain and France. The latter may be related to the Holy Roman Empire and Norman conquests.
In the Medieval and early modern periods (n = 28 individuals), we observe an ancestry shift toward central and northern Europe in PCA (Fig. 3E), as well as a further increase in the European cluster (C7) and loss of the Near Eastern and eastern Mediterranean clusters (C4 and C5) in ChromoPainter (Fig. 4C). The Medieval population is roughly centered on modern-day central Italians (Fig. 3F).
[...]
This shift is consistent with the growing ties between Medieval Rome and mainland Europe. Rome was incorporated into the Holy Roman Empire (3), which spanned much of central and western Europe. The Normans expanded from northern France to a number of regions, including Sicily and the southern portion of the Italian Peninsula (and even sacked the city of Rome in 1084), where they established the Kingdom of Sicily (3, 36).
Recap
- By the time Rome was founded, its people (Latins and Etruscans) were already genetically Mediterranean, overlapping with modern Southern Europeans from Spain to Sicily.
- The empire brought migrants to Rome from the Eastern Mediterranean, but they were mostly of Greek ancestry and not that different than the Romans.
- When the western half of the empire fell, the migrants disappeared as Rome's population declined drastically. The city was later repopulated by Italians and other Europeans.
- By the Middle Ages, the inhabitants of Rome were back to resembling Iron Age people and identical to modern Italians from North to South, with a few outliers tending toward Western and Central Europe.
---------------
Antonio et al. "Ancient Rome: A genetic crossroads of Europe and the Mediterranean". Science, 2019.
