The mechanisms might differ but not the outcomes

An interesting couple of articles and research that are tangentially related to Cultural achievements have an ephemeral genetic trace discussing an article by Razib Khan.  

It is a complex story but basically his argument from the genetics is that while we focus on the culture of ancient Rome (and Greece, and others), this was largely a product of cultural forces arising in urban centers and not from genetics.  At their peak these urban centers are usually wonderful concoctions of people from many genetic lineages across imperial domains.  However, the urban centers are not representative of their nations and the genetic story usually reverts to the mean of the region, no matter how cosmopolitan it might appear.

When the urban centers collapse, the genetic heterogeneity disappears and is ultimately and eventually replaced by the genetic patterns from the countryside.  Which makes sense given that all urban centers are demographic sinks.  They do not produce enough natural reproduction (children) to sustain the city population.  Cities always grow from immigration from the countryside or from elsewhere.  Growth is almost always substantially fueled by inbound migrants from the countryside.  As they peak, they attract peoples from far and wide.  But those heterodox genetic lineages are all washed away with an urban collapse and the people from the countryside repopulate the shrunken city.

The two new articles I came across are both interesting in themselves as well as what they suggest in terms of the above thesis.  

The first is Brutal massacre sheds light on migration during Viking Age by Andrew Curry.  The subheading is Slaughter of men, women, and infants paints a “before” picture of population movements into Scandinavia.

The men who died aboard the Kronan closely resembled modern Swedes and showed strong similarities to the people of Sandby Borg, suggesting Viking Age immigration from the British Isles and eastern Baltic didn’t leave much of a mark on Scandinavians’ genetic profile. “We didn’t expect that previously,” Götherström says. “We’re getting people coming from the west and east in the Viking Age, but for some reason they do not have as many children.”

The results sent the research team searching for possible explanations. One possibility is that the Viking-era immigrants were not the types to settle down and have kids, says Ricardo Rodriguez Varela, a co-author and geneticist at Stockholm University. They might have come as traders or diplomats, for example, or as members of social groups that tended not to have large families: “Christian monks or slaves produce less offspring.”

The second is The genetic history of Scandinavia from the Roman Iron Age to the present Ricardo Rodríguez-Varela, et al.   From the Abstract:

We investigate a 2,000-year genetic transect through Scandinavia spanning the Iron Age to the present, based on 48 new and 249 published ancient genomes and genotypes from 16,638 modern individuals. We find regional variation in the timing and magnitude of gene flow from three sources: the eastern Baltic, the British-Irish Isles, and southern Europe. British-Irish ancestry was widespread in Scandinavia from the Viking period, whereas eastern Baltic ancestry is more localized to Gotland and central Sweden. In some regions, a drop in current levels of external ancestry suggests that ancient immigrants contributed proportionately less to the modern Scandinavian gene pool than indicated by the ancestry of genomes from the Viking and Medieval periods. Finally, we show that a north-south genetic cline that characterizes modern Scandinavians is mainly due to the differential levels of Uralic ancestry and that this cline existed in the Viking Age and possibly earlier.

Both are observing that there is a base regional genetic profile that continues across time but that in both instances, the cosmopolitan age (Imperial Rome and Vikings) both have much greater genetic heterogeneity than either before or after.

In the case of Rome, the hypothesis is that the genetic heterogeneity was concentrated in cities and when cities (demographic sinkholes anyway) collapsed, in eliminated the variety and was replaced with the latent rural base.

In Scandinavia there must be something else as there was never the degree of urbanism as on the Italian peninsula.  Most likely, I would guess, is that the Viking era genetic heterogeneity was a function of settled trade-routes and that when those dried up, the foreign trading individuals returned home.  There is a possibility that they remained but they were in some way constrained in family size or formation (for example if many of the foreigners were celibate priests.)

An interesting parallel with an interesting twist.