From a Phys.org online article:
There was a massive shift in Roman residents’ ancestry, the researchers found, but that ancestry came primarily from the Eastern Mediterranean and Near East, possibly because of denser populations there relative to the Roman Empire’s western reaches in Europe and Africa.
The next several centuries were full of turmoil: the empire split in two, diseases decimated Rome’s population and a series of invasions befell the city. Those events left a mark on the city’s population, which shifted toward western European ancestry. Later, the rise and reign of the Holy Roman Empire brought an influx of central and northern European ancestry.
Scholars have been studying Rome for hundreds of years, but it still holds some secrets—for instance, relatively little is known about the ancestral origins of the city’s denizens. Now, an international team led by researchers from Stanford University, the University of Vienna and Sapienza University of Rome is filling in the gaps with a genetic history that shows just how much the Eternal City’s populace mirrored its sometimes tumultuous history.
To read more: https://phys.org/news/2019-11-genetic-history-rome.html