When did Proto-Slavic begin developing as a separate linguistic group?

Proto-Slavic began to develop as a separate linguistic entity in the 2nd millennium BCE. During this period, there was close contact between the Baltic and Slavic protolanguages, and the Proto-Slavic area—located south of the Pripyat River—is believed to have functioned as a specific peripheral part of the broader Proto-Baltic area.1

While Proto-Slavic emerged as an independent group during the 2nd millennium BCE, it remained quite unified for a long time thereafter. By the 6th century CE, although the Slavs had expanded across a vast territory, the Slavic language maintained a uniform phonological and grammatical structure, with significant dialectal variations primarily limited to vocabulary. The eventual dissolution of this linguistic unity is evidenced by the distinct development of the name of the emperor Charlemagne (747?–814) across different Slavic dialects, indicating that the language had begun to diverge into the various Slavic languages by the late 8th or early 9th century CE.12