-The Autochtonic theory, which says that the Slavs developed somewhere between the Caucasus and the Ural mountains around 1000 BC
- The Allochtonic theory, which says they only came there from Asia with the Volkswänderung
No, it's not certain, you are right. The theroy I was talking about is linguistic (my primary area of interest), it's called Proto-Balto-Slavic I think. Basicailly, it's that the Baltics and the Slavs where the same people until Volkswanderung, or very closely related.
Still, your Autochtonic theroy, which would actually make sense as Slavic takes a lot from the Northern Aryan group (think Ossetians, Scythians), still places them
west of the Urals.
Maybe by Europe your textbook means West of Poland.