Like the Lithuanian language, Latvian has its roots in the Proto-East-Baltic languages. The Proto-East and Proto-West Baltic languages are descended from Proto-Indo European. However, Latvian language has progressed more rapidly than its more conservative sibling, Lithuanian. While some similarities may exist between the two, but the two languages split somewhere between the 5th and 7th centuries AD, so Lithuanians and Latvians are not able to communicate with each other using their native tongues.
As with Lithuanian, the oldest surviving documents in written or printed Latvian come from church sources. The Bible was translated into Latvian in the late 17th century, and this was foundational to the survival and development of the Latvian language. This translation was the basis for other translations and written discourses about the Latvian language, even up to the time when nationalism took over and poets and writers sought to use their national language in a literary manner.
There are three Latvian dialects: the “middle,” “central,” or “standard” dialect; the Livonian (and Tamian) dialects; Livonian or High Livonian, which consists of Selonian and Latgalian dialects. These dialects have their basis in a Latvian that was adopted by other peoples and adjusted accordingly or are regional variants of Latvian.
Selected References
Schmalstieg, William R., “The Origin of Lithuanian Language.” Lituanas, Vol. 28, No. 1 (1982).