Migrations and language shifts as components of the Slavic spread

The rapid spread of the Proto-Slavic language in the second half of the first millennium CE was long explained by the migration of its speakers out of their small primary habitat in all directions. Starting from the 1980s, alternative theories have been proposed that present language shift as the main scenario of the Slavic spread, emphasizing the presumed role of Slavic as the lingua franca of the Avar Khaganate. Both the migration and the language shift scenarios in their extreme forms suffer from factual and chronological inaccuracy. On the basis of some key facts about human population genetics (the relatively recent common ancestry of the East European populations), palaeoclimatology (the Late Antique Little Ice Age from 536 to around 660 CE), and historical epidemiology (the Justinianic Plague), we propose a scenario that includes a primary rapid demographic spread of the Slavs followed by population mixing and language shifts to and from Slavic in different regions of Europe. There was no single reason for the Slavic spread that would apply to all of the area that became Slavic-speaking. The northern West Slavic area, the East Slavic area, and the Avar sphere and South-Eastern Europe exhibit different kinds of spread: mainly migration to a sparsely populated area in the northwest, migration and language shift in the east, and a more complicated scenario in the southeast. The remarkable homogeneity of Slavic up to the jer shift was not attributable to a lingua-franca function in a great area, as is often surmised. It was a founder effect: Proto-Slavic was originally a small Baltic dialect with little internal variation, and it took time for the individual Slavic languages to develop in different directions.