What features led linguists to reconstruct a Proto-Kartvelian language?

Linguists have reconstructed the Proto-Kartvelian language by identifying significant correspondences in the phonology, vocabulary, and grammatical structures—including systems of word inflection, derivation, and syntax—across the South Caucasian languages (Georgian, Svan, Mingrelian, and Laz).1

Key features supporting this reconstruction include:

• Ablaut Patterns: A functional vowel alternation (ablaut) where word roots appear in full, zero, or reduced grades. These patterns are strikingly parallel to those found in Indo-European languages, suggesting early contact.2
• Lexical Evidence: The presence of shared vocabulary and specific Indo-European loanwords, such as terms for "warm" and "breast," further indicates historical interaction.2
• Archaic Structural Features: Svan retains archaic features, such as the inclusive–exclusive pronominal system (distinguishing between "we" including or excluding the hearer), which have been lost in other Kartvelian languages.2

These commonalities allow specialists to outline the parent language's structure, while the divergence of Svan suggests it separated from the rest of the family at an early stage.12