Does enyone on this board have a view about the origin of IE thematic

With that I mean the ending of the stem in the thematic declination
o/e declension of nouns, and the wovel between root and ending in
forms of many verbs (present/imperfect tense, some aorists).

Is there some connection back into the steppe-three or is this an IE

In the texts I have read, the general view is that this is a late
innovation. But I have never seen any good explanation for this view.