Many Latvians and Lithuanians insists that their languages are among the oldest in Europe, and that they are ethnically not related to the surrounding Slavic and Germanic peoples. Indeed, I have read from linguists who seem to agree that the baltic languages branched off from Indo-European at a much different point.
Everything i learned about the great Migration period seemed to focus on population streams that went on farther south as well. So my question is: Was the baltic region affected by the Great Migration at all? If so, do we have a clue where these people came from? Or do we just not know, since they had no written documents?
My wife is from Latvia, so this topic fascinates me
Edit: I realized "Great Migration" is not an unambiguous term in English. I am of course referring to the Migration period starting around 375 AD.
For the longest time, it was believed that the Eastern Baltic region was unaffected by the migrations which took place in the 4th and 5th centuries - for reasons you have already stated in the opening post. The Baltic region did not see any noticeable movement in population (that we know of), and none of the tribes and migration waves into Europe brought Baltic language stratum with them (again, that we know of).
Nowadays, we are aware of raids and incursions into Baltic territory in these centuries from archeological evidence, such as iron arrowheads found in excavations in hillforts on the territory of modern Lithuania, as well as sources on contacts the Ostrogoths had with the Aestii (generally believed to be a name used throughout the Roman and Early Medieval period for the Western Balts, the ancestors of the Prussians and Curonians), more specifically the former having subjugated the latter.
The second phase of the "Great Migration", the Slavic migrations in particular, affected the Baltic region more directly as the north-eastward expansion of the Slavs moved straight into territory populated by the Balts. To my knowledge, there's still debate over how this expansion went, what Baltic tribes existed in modern day Belarus and western Russia and how far, but the general line of thought is that Baltic (or proto-Baltic) culture extended as far as modern day Moscow before being replaced by the Slavs over several centuries. This invasion aided in the dissolution of the Eastern Balts into separate tribes, including Lithuanians and Latgalians, who would later end up forming the Lithuanian and Latvian nations respectively.
And, in fact, Baltic peoples remained in East Slavic territory for several centuries after the Migration period - the Eastern Galindians, or the Goliads, who made up the easternmost extent of Baltic culture and remained surrounded by the East Slavs until well into the 11th century, if not longer, and their continued existence is attested in chronicles.