How did Proto-Baltic split into different Baltic languages?

Proto-Baltic, which emerged from Proto-Indo-European dialects, began to develop as an independent linguistic unit in the 2nd millennium BCE. Its evolution into distinct languages was driven by the vast size of the Proto-Baltic territory, which limited contact between its various dialects.12

By the middle of the 1st millennium BCE, the area was already sharply divided into dialects. These eventually diverged into two primary groups:3

• Western Balts: This group included Old Prussian and Yotvingian. They are characterized by the retention of certain archaisms, such as the neuter gender and specific diphthongs, which were lost in Eastern Baltic languages.45
• Eastern Balts: This group includes Lithuanian, Latvian, and the extinct Selonian, Semigallian, and Curonian languages. The protolanguage of the Eastern Balts split into Lithuanian and Latvian around the 7th century CE.563

The Baltic language area began to shrink significantly after the 1st millennium CE as Slavic-speaking populations migrated into the region, leading to the gradual assimilation of many Baltic tribes by the 14th century.3