Reading both Bryan Ward-Perkins’ 2000 paper Why did the Anglo-Saxons not become more British? and The fine scale genetic structure of the British population, published in 2015, is interesting. To date, this second paper is probably the “best of breed” when it comes to estimating Anglo-Saxon admixture into the British population in the 5th to 7th centuries (confirmed with a smaller sample ancient DNA publication). The authors conclude that:
Two separate analyses (ancestry profiles and GLOBETROTTER) show clear evidence in modern England of the Saxon migration, but each limits the proportion of Saxon ancestry, clearly excluding the possibility of long-term Saxon replacement. We estimate the proportion of Saxon ancestry in C./S England as very likely to be under 50%, and most likely in the range 10%-40%.
The ancient DNA paper gives an estimate of ~38% Anglo-Saxon (German) for the “East English.” So the two seem roughly in line. The C./S. England cluster refers to the genealogical network of the lowlands of central and eastern England.
There are several ways we can look at this. First, the majority of the ancestors of the modern English were British. That is, Brythonic people of various levels of Romanization. They became Anglo-Saxon. Even on the “Saxon Shore” in the far east of England it is likely that the majority of the ancestors of the natives derive from post-Roman Britons (if barely).
A second way to look at it this that this validates Peter Heather’s model in Empires and Barbarians: The Fall of Rome and the Birth of Europe. The model being that the post-Roman barbarian migrations were coherent “folk wanderings,” and large numbers of Germans moved into the collapsing Roman Empire. In post-Roman Britain, a large number of Germans clearly arrived and demographically marginalized many Britons. To be sure, it is unlikely that in the year 550 AD the census size of Germans to Britons in East Anglia was ever 38 to 62 in ratio. Rather, I suspect that in the centuries after the rise of the Anglo-Saxon kingdoms German elites had higher reproductive rates than the Britons due to their superior access to resources. Over time this resulted in their contribution being more prominent in the genealogies of people alive today.
Ward-Perkins’ paper is an interesting complement because he comes to roughly similar demographic conclusions well before the DNA evidence was available (though he does allude to ancient DNA!). He reiterates that the English seem to have subjected their British (i.e., Brythonic) heritage to the dustbin. English notoriously has very few loanwords from Celtic languages, and the Latinate that it does have seems to have arrived later. Not from sub-Roman Britons (who likely exhibited some level of bilingualism in Latin, which their elites still patronized for a period). Second, the Christian religion, which seems to have been well established when the Empire abandoned Britain, went into sharp regression in Anglo-Saxon areas. Third, despite attested instances of intermarriage between British and Anglo-Saxon elites, the royal genealogies of the English and the Welsh remained very distinct. Even the House of Wessex, which has clearly Celtic names early in the genealogy (they may have been Britons who assimilated to a Saxon identity), claims descent from Woden!
The conclusion from Ward-Perkins’ paper, and confirmed now by DNA, seems to be that the post-Roman Christian Britons assimilated to a German pagan identity. Through comparative methods, he makes suggestions for why this might have happened, and how it happened. Ward-Perkins observes that the Germanization of Slavic peoples during the medieval period exhibited analogs to the Germanization of the British: there are very few Slavic loanwords in eastern German dialects. He also suggests that the Islamicization of the Near East and North Africa may present an analogy, as subjugated peoples escaped their subordinate positions through religious conversion. A milder version of this also occurred in Frankish Gaul, where Gallo-Roman aristocrats donned trousers and adopted German forenames.
In short, Ward-Perkins seems to be arguing that the strongly subordinate position of the Britons due to their differences in culture from the German ruling class motivated them to shift their identities. The reality is that in the Late Antique period much of Europe was superficially Christianized in the first place, so adopting paganism may not have been that difficult for semi-literate warlords, let alone the peasantry. The spread of Slavic identity in the Balkans (among with migration), the Magyarization of much of Pannonia, which had previously been Slavic and Latinate, are examples within Europe of this pattern.
But, there is another dynamic between Briton and Anglo-Saxon that Ward-Perkins thinks is salient: the centuries of vicious battles between the two groups. With hindsight, the slow defeat of the Britons and their marginalization to the “Celtic Fringe” seems inevitable, but as late as the first half of the 7th century A.D. this was not clear. Cadwallon ap Cadfan, king of the northwest Welsh kingdom of Gwynedd, conquered Northumbria in 633, so that rule by Britons all the way to the North Sea was a reality for a short period. While in Gaul the Franks quickly conquered the whole region and began integrating the Gallo-Roman aristocracy into their own power structure, in Britain, the native population retained independent geopolitical power for centuries, and so were perceived as a clear and present threat. Which they were deep into the early medieval period. To be a Briton in Anglo-Saxon territory then was to be behind “enemy lines.”
Because we have good genetics for this region we can quantify the likely genetic contribution of the new culture that came to dominate the land of Britain. What became England. But also due to the interest in archaeology in England, we know that there was a wholesale human geographic change in this territory. The manner in which peasants tilled the land transformed with the arrival of the Germans. In other words, a wholesale transformation of folkways, and the adoption of an alternative ancestral framework. The nominally Christian Britons having taken up tribal German paganism with the shift in language was the least of it.
There is a narrow and specific insight here. The unwinding of the complex Roman state, which was pacified and economically complex, and centered around cities, and the switch to a more militarized and small-scale, “primitive”, the social system almost certainly allowed for the adoption of German folkways by sub-Roman Britons. As the villas of yore decayed and collapsed the semi-Romanized inhabitants of Britain discarded their Classical raiments and moved into the misty forests of their primordial tribal past. To survive the shock of the German folk migration the Britons reverted to a way of life more akin to their pre-Roman ancestors, including the resurrection of hillfort culture. But in the process, the ability of the post-Roman Britons to absorb the Germans into an aspirational “high culture,” the fragments of Romanitas, disappeared. The Anglo-Saxons saw in their British enemies just another coalition of tribes as themselves. And so it reverted to a Hobbesian scenario. And here the British lost.
There is also a general insight. How can a people “forget” their past origins? How can they create entirely fictive genealogies? As economists would say: incentives matter. Many human populations emerge through a process of genetic amalgamation, but cultural identity is not governed by the laws of segregation. Cultural identity and memory can rupture and shift far more rapidly because the laws of cultural inheritance are more plastic and protean. There was a clear folk migration of massive numbers of Germans into what became England, but their history and folkways were adopted wholesale by the native peoples whom they conquered. This sort of process likely has occurred many times across human history.