Pre-IE and alternating thematic vowels

I like to ponder about Pre-Indo-European (Pre-IE) from time to time but strangely, despite popular interest, it's a topic that doesn't often get explored seriously by academics, leaving that topic to kooks. (Oh dear Aunt Martha, I hope I'm not one.) Despite the unsaid stigma, long-range comparative linguistics deserves a respected place in academia because questions about where the world's languages ultimately come from are never going to go away on their own.

One thing I've been curious about is the nature of the thematic vowel in PIE and what that says about earlier stages of this reconstructed language. This vowel is everywhere in PIE grammar - in nouns, in verbs, in adjectives. It's everywhere. This vowel often is an unchanging *o (as in the paradigm of *eḱwo- "horse": genitive *eḱwosyo) but every once in a while it throws a curveball and alternates between *e and *o without any immediately apparent pattern, as with thematic verbs. It's a rebel vowel without a cause.

Quirks like this in a language aren't unexpected. In fact, every natural language has some bizarre twist, like English and its irregular conjugation of "to be" (e.g. "am", "is", "are", "was", "were", "been"). All these flaws in this grammatical persian rug can tell us a lot about the older stage of a language. PIE is no different. This is my daring explanation of what took place in Pre-IE to cause this mess in the language. Sit back and pass the popcorn.

Modern thinking now says that the presentive thematic paradigm of *bʰer- (as illustrated above) was in fact only a subjunctive in PIE proper because afterall Anatolian has none of the most basic thematic verbs that are found in abundance in other branches at all. So the famous present thematic paradigm as in the above picture is a slight inaccuracy since this had only developped after the dissolution of the core PIE speech community circa 4000 BCE. Nonetheless, even with updates to the IE model, alternating thematic vowels are here to stay and still show up in PIE presents through the use of suffixes like *-sḱe- (1pp *-sḱo-mes / 3ps *-sḱe-ti) and *-ye- (1pp *-yo-mes / 3ps *-ye-ti). Here too, there is vowel alternation so it must be quite ancient.

Aside from the 1ps, *o is placed before voiced consonants and *e before voiceless ones. I've already explained how the 1ps ending here originates from the perfect ending *-h₂e (see Thoughts on the early Indo-European subjunctive 1ps ending). However, this doesn't in itself resolve why *o is found here unless we surmise that in some stage of Pre-IE the 1ps ending in the subjunctive was once *-o-m, comprised of thematic vowel and 1ps non-indicative *-m which, rather conveniently for me, is used in the post-IE 1ps thematic preterite (e.g. *bʰérom "I carried"). The prior existence of voiced *-m then has produced the *o in *-oh₂ and now the above-mentioned relationship between vocalism and voicing is now consistent and predictable.

However, it begs the question: What caused the change of *e to *o before voiced consonants in the first place? Are there parallels in other languages? It turns out that Modern English can give us an answer. In the English minimal pair sack / sag, the vowel is typically pronounced slightly longer in sag [sæˑg] than in sack [sæk]. It is referred to as half-length and native speakers generally don't realize they're doing it. Even if one were to pronounce final -g as [k] (without any voicing at all), an English speaker will normally still perceive a final voiced stop if the vowel is pronounced with sufficient length. Vowel length alone in these instances has become the perceptual cue to distinguish historical final -t from final -d in English. (See here [pdf] and here for abstracts relating to this voicing effect on English vowels.)

Now we can apply this same phenomenon to Pre-IE by theorizing that there is a single vowel underlying *e/*o alternation. We can write this underlying vowel as *ə (a schwa). At an earlier stage, vowel length first develops on the phonetic level to produce ever-widening variations. The variations then become phonemicized and the bifurcated vowel begins to drift in two different directions of quality depending on the initial length. In the final stage, we obtain the *e/*o pattern via rounding of former *a.

However, there may be a third alternation that this earlier schwa produced, that of *i. Stems in *-o- sometimes show up as *-i- stems, particularly in compounds, without discernible difference in meaning. In reduplicated verb forms, we find reduplication with *e (*bʰe-bʰor-) but also reduplication in *i (*di-deh₃-). This may be the effects of schwa when positioned behind word accent, rather than after it. In these instances, short schwa would become *e while longer schwa rose to *i instead of becoming *a (> *o) as we find when positioned after the accent. So *dideh₃- would be from earlier *[dəˑ'dehʷ]- while *bʰebʰor- from *[bʰə'bʰar]-. In compounds, accent would have originally been on the last component of the word and the "thematic schwa" of preceding stems would be in an open syllable, making it longer. Thus *-o- is reduced to *-i- in these circumstances. Over a few generations, it would have become fashionable to treat these compounding *i-stems as independent stems, as an alternative to *o-stems without difference in meaning.

If someone can think of a better explanation for all this, I'm all ears. My head hurts and I'm taking two ibuprofen.