It has been an open question for historians of the fall of the Roman Empire the extent to which ethno-tribal migrant caused the transformation toward the post-Roman order. In Britain, for example, there has long been debate as to whether the shift from a predominantly Celtic population with a cosmopolitan Latin-speaking patina (at least demographically in terms of origins; I understand there are those who argue that late-Roman Britain was predominantly Latin-speaking), was due to a mass migration of Germans, or more a matter of institutional and cultural defection and conversion. In the early 20th century the model in vogue was predicated on migration. The Welsh and English were perceived by many to be distinct races, with the latter having affinities with the Germanic peoples in blood as well as speech. In the late 20th century the pendulum swung in the other direction. I recall reading Norman Davies’ The Isles in 2000, and he relayed the conventional view of historians of the period that the post-Roman world of Britain saw the conversion through elite emulation of Britons into Saxons (and Angles and Jutes) based on documentary evidence of co-existence and subordination of a Celtic population in early Anglo-Saxon England.
Ten years later Peter Heather wrote Empires and Barbarians to resurrect a moderate migrationism for the post-Roman world. What he was rebutting was the perception that the idea of German folk migrations, which included the movement of women and children along with men, was a post hoc myth. Though even the most extreme cultural constructionist would assent to the proposition that some Germans did migrate into the late Roman world and capture the post-Roman successor states, they usually emphasized that tribal identities were ad hoc, novel and newly constructed, and German identity was highly malleable easily co-opted by aspirant non-Germans. In other words, the Goths, Vandals, and Anglo-Saxons were motley coalitions of opportunists, whose ethnic self-identity was a matter of recent myth.
Some of this is certainly true. Going back to Anglo-Saxon England, Alfred the Great’s early genealogy is littered with names that seem to exhibit a British, not German, provenance. It is not unreasonable that British warlords would on occasion switch sides to maintain their position at the top of the status hierarchy, just as some Visigothic nobles in Spain after the Muslim conquest converted to the new religion and became progenitors of the local Islamic aristocracy.
But we shouldn’t go too far. Last year PoBI finally published their paper, The fine-scale genetic structure of the British population, and confirmed the suggestions of earlier genetic work that a substantial proportion of the ancestry of the contemporary English population derives from Germans. Not the majority, but a substantial minority. In other words, Peter Heather was correct in England. Cultural change was catalyzed by substantial demographic change. There is more and more evidence that in two areas of the post-Roman world where Romanitas faded, with the local decline or extinction of Christianity and Roman speech (whether Latin or Greek), Britain and the Balkans, there was substantial demographic change induced by a migration into Roman territory of Germans and Slavs respectively.
With that, I submit two open access papers on ancient genomes from Britain: Genomic signals of migration and continuity in Britain before the Anglo-Saxons and Iron Age and Anglo-Saxon genomes from East England reveal British migration history. The plot at the top of this posts shows a striking result: most of the Roman era individuals are genetically least differentiated from the Welsh, and modern East Anglians are the most shifted toward the Dutch. This is exactly the pattern we would expect from archaeology, as the pale of German settlement was along the Saxon Shore.
So genetics tells us that extreme positions of total replacement or (near) total continuity are both false. Rather, the genetic landscape of modern England is a synthesis, with structure contingent upon geography. But, it also shows us that substantial demographic change which produces a genetic synthesis can result in a total cultural shift. Though we may think of elements of culture as entirely modular, with human ability to mix and match components as one might see fit, the reality is that often cultural identities and markers are given and taken as package deals. But, it probably took the transplantation of a total German culture through a mass folk movement to give the Saxons enough insulation from the local British substrate to allow them to expand so aggressively and become genetically assimilative and culturally transformative.