In addition, the existence of a Dnieper-Oka Baltic [ru] branch is hypothesized, with the extinct Golyad language being the only known member.

The range of the East Baltic linguistic influence once possibly reached as far as the Ural Mountains, but this hypothesis has been questioned.

[4][5][6] Old Prussian, a Western Baltic language that became extinct in the 18th century, had possibly conserved the greatest number of properties from Proto-Baltic.

The substantial number of false friends and various uses and sources of loanwords from their surrounding languages are considered to be the major reasons for poor mutual intelligibility today.

[15] One reason for the late attestation is that the Baltic peoples resisted Christianization longer than any other Europeans, which delayed the introduction of writing and isolated their languages from outside influence.

[27] In regards to the same geographical location, Asko Parpola, in a 2013 article, suggested that the Baltic presence in this area, dated to c. 200–600 CE, is due to an "elite superstratum".

[33] In the same vein, Kallio argues for the existence of a lost "North Baltic language" that would account for loanwords during the evolution of the Finnic branch.

These two factors combined with others have obscured the history of the Baltic languages, leading to a number of theories regarding their position in the Indo-European family.

[43] Finally, a minority of scholars argue that Baltic descended directly from Proto-Indo-European, without an intermediate common Balto-Slavic stage.

As in the case of the Celtic languages of Western Europe, they were reduced by invasion, extermination and assimilation[citation needed].

Studies in comparative linguistics point to genetic relationship between the languages of the Baltic family and the following extinct languages: The Baltic classification of Dacian and Thracian has been proposed by the Lithuanian scientist Jonas Basanavičius, who insisted this is the most important work of his life and listed 600 identical words of Balts and Thracians.

"[54] Of about 200 reconstructed Thracian words by Duridanov most cognates (138) appear in the Baltic languages, mostly in Lithuanian, followed by Germanic (61), Indo-Aryan (41), Greek (36), Bulgarian (23), Latin (10) and Albanian (8).