It's not a "proto-indo-european"! It did change, and change massively. But it seems that because of how the number of native speakers was always low, and thr populayion was concentrated in a relatively isolated part of Europe, this evolution was more self-contained and reflective.

The language seems to have more archaic features and forms than, say, closely related slavic languages, and its vocabulary has more similarities to old sanscrit than one's average european language.

For curios language learners this means that the grammar is harder than even (already hard enough) slavic grammars.