How did East Germanic become separated from Northwest Germanic?

The first major linguistic division within the Germanic languages occurred between East Germanic and Northwest Germanic, a split that can be dated roughly to the 1st–3rd centuries CE.1

This separation was characterized by distinct linguistic innovations that diverged between the two groups:

• Verb Tense: In Northwest Germanic, the reduplicated forms found in the past tense of certain strong verbs were eliminated and replaced by new ablaut alternations (often involving the vowel ē²). Conversely, East Germanic (such as Gothic) maintained the original reduplicated forms.1
• Consonant Development: The Proto-Germanic sound z was generally maintained as z (or s in final position) in East Germanic. In contrast, this sound generally became r in Northwest Germanic.1

While Northwest Germanic encompassed a vast territory from Scandinavia across much of Germany and the Low Countries, East Germanic speakers were located between the middle Oder and Vistula rivers before migrating southeast toward the Carpathian Mountains and beyond. This geographic separation, combined with differing linguistic developments, solidified the divide between the two branches.12