Proto-Tocharian language

Proto-Tocharian, also spelled Proto-Tokharian (/təˈkɛəriən/ or /təˈkɑːriən/), is the reconstructed proto-language of the extinct Tocharian branch of the Indo-European languages.

Proto-Tocharian is the unattested reconstructed ancestor of an Indo-European eponymous extinct branch, known from manuscripts dating from the 5th to the 8th century AD, which were on the northern edge of the Tarim Basin and the Lop Desert. The discovery of this language family in the early 20th century contradicted the formerly prevalent idea of an east–west division of the Indo-European language family on the centum–satem isogloss, and prompted reinvigorated study of the family.

The documents record two closely related languages, called Tocharian A (also East Tocharian, Agnean or Turfanian) and Tocharian B (West Tocharian or Kuchean). The subject matter of the texts suggests that Tocharian A was more archaic and used as a Buddhist liturgical language, while Tocharian B was more actively spoken in the entire area from Turfan in the east to Tumshuq in the west. A body of loanwords and names found in Prakrit documents from the Lop Nor basin have been dubbed Tocharian C (Kroränian). A claimed find of ten Tocharian C texts written in Kharoṣṭhī script has been discredited.

Tocharian A and Tocharian B, the two major languages descendant of Proto-Tocharian, are mutually unintelligible, which led linguists to think that the split of Proto-Tocharian in several branches was several millennia ago. As part of the same language family, the Tocharian languages and their common ancestor are studied together by scholars.

Proto-Tocharian shows radical changes in its vowels from Proto-Indo-European (PIE). Length distinctions eventually disappeared, but prior to that all pairs of long and short vowels had become distinct in quality, and thus have different outcomes. Many pairs of PIE vowels are distinguished in Tocharian only by the occurrence or non-occurrence of palatalization. For example, PIE o and ē both evolved into Proto-Tocharian ë (possibly /ə/), but PIE ē palatalized the preceding consonant, and left a y when no consonant preceded, while neither of these occurs with PIE o.

Reconstructing the changes between PIE and Proto-Tocharian vowels is fraught with difficulty, and as a result there are a large number of disagreements among different researchers. The basic problems are:

Historically, the evolution of the Tocharian vowels was the last part of the diachronic phonology to be understood. In 1938, George S. Lane remarked of Tocharian that "the vocalism so far has defied almost every attempt that has been made to bring it to order", and as late as 1945 still asserted: "That the subject [of palatalization] is a confused and difficult one is generally recognized—but so are most of the problems of Tocharian phonology." However, rapid progress towards understanding the evolution of the vocalic system, and with it the phonology as a whole, occurred during the period of approximately 1948–1960, beginning with Sieg and Siegling (1949). By 1960, the system was well-enough understood that Krause and Thomas's seminal work of that year is still considered one of the most important Tocharian grammatical handbooks.

Despite the apparent equivalence between the Tocharian A and B vowel systems, in fact a number of vowels are not cognate between the two varieties, and Proto-Tocharian had a different vowel system from both. For example, Tocharian A a reflects a merger of two Proto-Tocharian vowels that are distinguished in Tocharian B as e and o, while Tocharian B a reflects a stress-based variant of either Proto-Tocharian ā or ä, while Tocharian A preserves original ā and ä regardless of the position of stress.