The linguistic assessment of The Tocharian Trek has made use of the following linguistic approaches: phylogeny and language contact.
In the phylogeny approach, the position of Tocharian in the Indo-European language family was investigated: when did the Tocharian branch split off the protolanguage, and are there any closer connections to other branches? A relatively early split-off of Tocharian seems needed in view of the archaeological evidence from southern Siberia, which starts off early compared to other archaeological cultures associated with Proto-Indo-European. The phylogenetic position of Tocharian within Indo-European was investigated by Louise Friis and Stefan Norbruis: with different approaches, both tested the "Tocharian Second" hypothesis, namely that Tocharian split off the protolanguage after the Anatolian branch, but before all other branches. Louise Friis focused on arguments for "Tocharian Second" based on verbal morphology, finding that these do not stand closer scrutiny in the light of revisions in the reconstruction of Tocharian verbal morphology. Stefan Norbruis focused on lexical evidence, finding that there is some, but little support for the "Tocharian Second" hypothesis, while there is also evidence for common innovations of Tocharian with Balto-Slavic and Indo-Iranian, which requires a more nuanced wave model of the disintegration of the protolanguage.
In the language contact approach, the prehistory of the Tocharian language was investigated on the basis of its contacts with Uralic, Turkic, Chinese and Niya Prakrit.
Contacts with Uralic appear to have taken place in southern Siberia, originally home to the Samoyedic branch of the Uralic language family, which provides the crucial link between the Tocharian language and southern Siberia. Impact of Uralic has most probably caused drastic changes in the sound system and nominal morphology of Tocharian. Contact between Tocharian and Uralic was investigated by Abel Warries, who found that some comparisons proposed earlier have to be discarded, while some in turn are strengthened, and some lexical comparisons could be added.
In the Tarim Basin, Tocharian has been claimed to have influenced the Middle Indian language Niya Prakrit or Niya Gāndhārī. This claim, and foreign influence on Niya Prakrit in general, was investigated by Niels Schoubben. He found that the elements attributed to Tocharian have to be explained otherwise, and some of these elements and many others are due to influence of Bactrian, which was much stronger than previously supposed.
Contacts between Tocharian and Chinese and Turkic were investigated by Michaël Peyrot (PI), partly in collaboration with Hans Nugteren and Jens Wilkens. It was found that there is little, but good evidence for prehistoric but relatively recent contacts between early Tocharian B and Turkic.
