Lithuanian still retains many of the original features of the nominal morphology found in the common ancestors of the Indo-European languages, and has therefore been the focus of much study in the area of Indo-European linguistics. Studies in the field of comparative linguistics have shown it to be the most conservative living Indo-European language (1)
Lithuanian retains cognates to many words found in classical languages, such as Sanskrit and Latin. These words are descended from Proto-Indo-European (1)
What is unique about the Lithuanian Language, the spoken language, is that it survived as a spoken language uninterrupted despite adverse circumstances at times. This longevity is what makes the language so unique. Considered to be the oldest "Spoken Language" - Living Language.
The Lithuanian language did not become a written language until much later .
The Grand Dictionary of the Lithuanian language,
consisting of 20 tomes containing more than half a million
First Lithuanian book (1547) The Simple Words of Catechism by Martynas Mažvydas
The earliest surviving written Lithuanian text is a translation dating from about 1503–1525 of the Lord's Prayer, the Hail Mary and the Nicene Creed written in the Southern Aukštaitijan dialect. Printed books existed after 1547, but the level of literacy among Lithuanians was low through the 18th century and books were not commonly available. (1)
R. Balta "LITHUANIA"