Lithuanian and Russian descend from one lost ancestral tongue, most linguists agree
Baltic languages such as Lithuanian and Latvian share quirks with the Slavic family that no other branch of Indo-European has. For over a century linguists fought over whether that meant a common parent or just long neighbourliness. Most now accept a single Balto-Slavic branch, though they still argue over how it split apart.
The evidence is a string of shared innovations. Winter's law lengthened vowels before certain voiced consonants, and because the later merger of two consonant types had to follow it, that merger counts as distinctly Balto-Slavic. Both families insert a high vowel before syllabic l, r, m and n, something only Germanic otherwise does, and both developed a contrast between rising and falling tones on long syllables, still heard as the broken tone in some Baltic speech. They also rebuilt word stress into fixed and mobile patterns bearing almost no resemblance to the ancient system preserved in Greek and Sanskrit.
Early comparativists Rasmus Rask and August Schleicher, writing in 1861, imagined a joint ancestor shared with Germanic, and Karl Brugmann later listed eight innovations supporting a Balto-Slavic branch. Opponents pushed back: the Latvian Janis Endzelins blamed intense contact, and the Frenchman Antoine Meillet argued the likenesses arose by parallel accident. The Pole Rozwadowski split the difference, crediting both inheritance and contact. The idea was also contested partly for political reasons, which have since faded from the scholarly debate.
How the branch divides remains open. The textbook view has two sub-branches, Baltic and Slavic. In the 1960s Vladimir Toporov and Vyacheslav Ivanov argued Slavic was simply an unusually innovative offshoot of an older Baltic-like continuum, and Frederik Kortlandt proposed three equally distant branches: East Baltic, West Baltic, represented by extinct Old Prussian, and Slavic. A 2003 computational tree by Gray and Atkinson dated the family's split to about 1400 BCE.
Slavic then spread with startling speed. By around 600 CE a nearly uniform Proto-Slavic was spoken from Thessaloniki to Novgorod, and some scholars think it served as the common language of the Avar Khaganate, whose Slavic troops joined the siege of Constantinople in 626. That surge swept away most intermediate dialects, and river names suggest Baltic speech once reached as far east as Moscow.
Source: Balto-Slavic languages
