It is well known that in the 5th and 6th centuries of the Common Era the social complexity and economic productivity of the Roman cultural zone underwent a regression. There were two areas in particular where massive transformation occurred. The interior Balkans and Britain were ethnically and religiously changed in totality from their Roman-era state.
Romanian and Vlach dialects today are a testament to the strength of Latin in the interior Balkans, while Albanian attests to indigenous linguistic diversity. But the dominant languages today are Slavic, due to the putative mass migration of tribes under the leadership of the Avars in the 6th and 7th century. These regions also had to be re-Christianized. Something similar happened in Britain, where mass migrations of Germans transformed the language and religion of the whole region.
But we know that genetically the Balkan Slavs and English are actually genetically more like the earlier populations than descendants of migrants. This is not to say that the exogenous genetic material is trivial. It is significant. There was a mass migration of Slavs and Germans. It was just that these did not contribute to the preponderance of the genes.* And yet the language shifted, and the Christian religion faded.
How did this happen?
Late Roman society was defined by specialized economic functions on the production frontier. This was the ancient world’s equivalent of the “just-in-time” economy. In contrast, the Slavic tribes beyond the frontier were arguably even less influenced by Rome than the Germans. These were deeply rustic people. And that was their cultural advantage.
Using a biological analogy, the Late Roman society was like an asexual lineage maximizing short-term gains at the expense of long-term resilience. The “shock” of barbarian incursions in the 5th to 6th centuries totally unraveled the Roman system.
In contrast, small-scaling farming societies organized around clans and tribes, which is how the Slavs were organized, could maintain themselves. Often the Slavs were ruled by non-Slavic groups with origins in the steppes (Avars, Bulgars, etc.), but in the end, the Slavic identity swallowed up their rulers, more or less.
This new setup was successful enough to attract converts from local populations. There is circumstantial evidence that the Anglo-Saxon House of Wessex was actually originally British (look at the early names in the genealogy). They may have been post-Roman British aristocrats who “barbarized.” In Merovingian Francia, Gallo-Roman elites were taking to trousers and aping German Frankish style, but, on the whole the cultural balance was tilted toward the “Romans” rather than the “Germans.” Not so in Britain, it seems.
With all this outlined, it not so surprising that a complex urban society could be culturally assimilated in some ways by a simpler agro-pastoralist lifestyle. The further back you go into the past, the more likely it happened, because the less “robust” the cultural technologies of the urban society were.
* The gene flow into the Balkans was greater proportionally than into Britain. In some cases, more than 50% of the ancestry might be attributable to migrants in the northern and western regions.
Hope there is an exhaustive book of population genetics and culture in the offing.
The problem of urban centers is supply, transport and trade.
For all of this you usually need a higher form of social organisation and military protection.
Take intensive crop farming, the base of Roman food production as an example.
How are your peasants supposed to do their job if foreign forces fight them, burn and plunder crops?
This can only work if the invading force wants to keep the production up and the trade going, while just placing themselves on top of this “complex society”.
If they are more simple tribal people without a good connection to the inhabitants and their culture, but take the country for their clans and glory, like heathen Germanics and Slavs, to keep up the foreign culture is not their interest.
That was one of the big differences between them and the Frankish, Lombard or Gothic rule.
Without the legions protection from external forces, the system instantly collapsed when tribal people conquered the helpless, military amateurish countries.
Thats not as much a question of economy but political = in the end military power.
Economically and intelkectualky specialised people need protection from specialists in defending = professional police and military.
If those are gone almost completely, like in the Balkans and Britain, they are so weak.
To make this even more clear, the Slavic and Germanic settlement in Britain and Balkans was not even a big conquest, but more of a settlement indeed of different sized groups taking their share of the country, unlike the more organised conquests.
Its this lack of central, higher authority which made the economic collapse even worse. Higher culture and economy needs to be protected and wanted, it doesnt grow without central power.
The conquered had little choice but to adapt and religion is just part of it.
The strongest ancient feature of the Balkan linguistics isn’t even the vestiges of the Romance (tidied up by the nationalist revivalists in the XIX c.) but the pervasive Sprachbund of murky, possibly Proto-Illyrian, origin ( https://en.wikipedia.org/wiki/Balkan_sprachbund#Origins ). Whatever was there, it’s clear that the ethnicities and traditions mixed in the melting pot for a very long time. The Danube corridor being a revolving door of migrations, they were incessant waves of new conquerors and new refugees cramming into the Balkans. The Byzantines had a history of granting refuges South of Danube to some groups, Gothic, Turkic, Slavic, in exchange for help protecting the border from the other groups. And the diverse names of the generals and chieftains make it clear that the border-settlers were already mixed. Quite possibly, so were the Slavs even before they crossed to the South (there are all the unresolved questions about the homelands and the genesis of the Slavic Antes and Sklavenes … by the way the former attacked Byzantium even before the Avars).
My point is that it is very hard, at present, to estimate the extent of genetic displacement in the Balkans and it may be higher than projected. But the linguistic imprint of the Roman area is pervasive all the same,
“Late Roman society was defined by specialized economic functions on the production frontier. This was the ancient world’s equivalent of the “just-in-time” economy. In contrast, the Slavic tribes beyond the frontier were arguably even less influenced by Rome than the Germans. These were deeply rustic people. And that was their cultural advantage.”
What is meant above by “production frontier”. This is a well defined term in economics. It means the efficient use of all inputs. A firm or an individual is operating at the production frontier if, given the quantity used of each input (typically some combination of land, labor and capital, i.e., machinery or other equipment), the greatest possible level of output (production) is obtained.
In this meaning, any Slavic tribes beyond the frontier are either producing more than is actually possible with the quantities of each input they are using, or perhaps, in a loose use of the term, they are using a more advanced technology than Late Roman society. This strikes me as unlikely (warning: attempt at British understatement).
In the italicized passage above, the term appears to mean merely “frontier”; either “production” is entirely superfluous or I am misundertanding something.
It’s worth to note that even if Germanics and Slavs were minorities for the whole of Britain or the Balkans, they were most likely numerous in the strategic places that mattered.
Especially when the more elaborated technical and intellectual means, state, Christianity and urban centers came back and the more fertile soils supported larger populations.
Both Slavic and Germanics expanded on Medieval times, then based on the population and civilisational bonus they had to the degraded, smaller units of Celtic and Romance speakers.
Its also worthwhile to not that Southern Slavs assimilated then a lot of the small units of Vlach herders or dominated them.
The only exception being the Vlachs of Romania in which fewer centers existed and those available could be taken, resulting in Slavs of significant numbers being assimilated instead.
Bottom line: It might suffice to have a big impact on a strategic and cultural-economic center from which the culture can expand on even with a constantly decreasing ancestral influence.
I’d say the same happened with Indo-Europeans in SEE and Anatolia.
The lower complexity and greater egalitarianism of Slavic societies meant that many of the large scale political units that did evolve in the Slavic world were created by non-Slavs. You already mentioned the Avars, but also Russians and Bulgarians owe their ethnogenesis to Nordic and Turkic elites respectively (later of course the elites adopted Slavic speech and Roman religion). A perhaps abortive ethnogenesis was the Kingdom of Samo, a Frankish merchant who created a Western Slavic Kingdom along the eastern rim of Germany.
were most likely numerous in the strategic places that mattered
OTOH the epigraphics from “strategic places” tends to show Christian names, and participation in the Byzantine bureaucracy. Like most of the Danube Bulgar inscriptions use Greek letters rather than Orkhon runes, and the best known text is by a Christian tax collector.
Bulgarien are a special case and that the literate Byzantine administration was Greek speaking comes as no surprise. Turks used Christian administrators too, but they were in control of the apparatus.
But who really controlled the countries central, strong and most productive points?
The Vlachs had to flee to the mountains and isolated valleys. Came up with a very basic, small scale pastoralist way of life to stay independent and avoid the new masters of the fertile ground.