Was there an [a] sound in PIE?

Question

I have come across a controversy about the [a] vowel sound in PIE. I have noticed in most reconstructed PIE words the [a] sound is not present, even when it seems to be present in most of the daughter languages, and it only seems to be reconstructed for some rare words. I have looked it up and it appears there is a debate among scholars on whether there was an [a] sound or at all. What are the reasons for this?

Also, are there any historically attested examples of [e] and [o] becoming [a] in IE languages?

Answer 1

The evidence for a phonemic *a in earliest PIE (before Anatolian and Tocharian broke off) is extremely lacking, it only seeming to be present in a small number of words that are likely borrowings, often present in only a small number of branches.

That said, a-colouring from *h2 is attested in all branches, and so *a ought to be reconstructed for earliest PIE, just only as an allophone of *e rather than as a phoneme in its own right.

Late PIE (after Anatolian and Tocharian broke off) seems to have for the most part either lost or merged the laryngeals, meaning this earlier *a allophone had become phonemic.

For your second point, it's worth remembering that in reconstructions, the labels *e, *o, *a (and indeed any other symbols at all that are in use) are purely for convenience and should not be taken too literally as representing the sounds represented by those symbols in the IPA, and their exact realisation is very much open to debate (in particular, there is debate over whether *o was even rounded in most instances).

So, we shouldn't be looking for instances of [e] & [o] becoming [a], but front & back non-high vowels becoming low unrounded vowels.

Spontaneous rounding and unrounding of non-high back vowels (especially lower vowels) is extremely common (as an example in the historic era, look at the American unrounding of the LOT vowel), so *o becoming *a (and vice versa) are completely unsurprising.

Lowering of mid front vowels is also not unusual (as a recent example, the DRESS vowel /ɛ/ in English has become variously [æ] & [ɐ] in California & Northern Cities English respectively). Schwas (another candidate for the sound of PIE *e) are also very frequently lowered. Likewise, non-high front vowels often become non-high back vowels which can then unround for the same reasons mentioned above. As you can see, *e becoming *a also isn't especially odd, even if it is less expected than it is for *o.

From a PoV of looking at the vowel system as a whole, 5-vowel systems are pretty stable, and 3-vowel systems are both pretty stable, but 4-vowel systems are not, so once *o and *a merge, that may provide enough pressure to encourage the *e to merge too (or else for there to be another split, returning back to a 5-vowel system).

Answer 2

The general consensus is that, phonetically, early PIE had a low vowel of some sort (which could broadly be called [a]), but phonemically, it did not have a phoneme /a/ (or it had a marginal one at best).

The reason for assuming a phonetic [a] is that various words have a consistent low vowel across several daughter languages, like in "before, fore, front" (Latin ante, Greek anti, Sanskrit anti).

The reason for not assuming a phonemic /a/ is that, for the most part, it doesn't seem to be necessary. When we see those consistent low vowels, the laryngeal theory explains them as an underlying /e/ adjacent to /h₂/. And indeed, Hittite shows a direct reflex of /h₂/ in the "front" word: ḫant- "face, fore". (Reconstructed PIE phonemes are generally written like *this rather than /this/, and that's what I'll use for the rest of the answer.)

Why not assume a phonemic /a/? Because this "laryngeal-coloring" theory very nicely explains certain inconsistencies in ablaut (the way the main vowel within a PIE root changes in different forms). For most roots, there are three common ablaut grades: ∅, e, and o. But certain roots seem to replace the e-grade with something else: sometimes ō, sometimes ā, sometimes ē. And these same roots do something weird with the ∅-grade, which varies by language; in Greek, the ∅-grade of those roots show o, a, and e, respectively (Greek didōmi "give", dotos "given"; histāmi "stand", statos "stood"; tithēmi "place", thetos "placed"); in Latin, the ∅-grade is always a (datus "given", status "stood", factus "done").

If we assume laryngeals, these inconsistencies completely disappear. Those roots are now reconstructed as *deh₃, *steh₂, *dheh₁; the weird long vowels are the result of a laryngeal after *e; the weird short vowels are the result of a laryngeal on its own without a vowel next to it. And the ablaut is back to the classic ∅ vs e vs o, same as any other root.

And once we've added the laryngeals to the theory, well, it turns out they can explain basically all instances of phonemic *a. Some linguists still reconstruct *a as a marginal phoneme in words like *aból "apple", while others explain even these instances away as *h₂eból. At this point, adding *a to the theory as a full-on phoneme makes it more complex without adding much explanatory power, so Occam's razor suggests we should leave it out.

Answer 3

To your last point: In Indo-Iranian IE *e and *o regularly become “a”, *ē and *ō regularly become “ā”. The only notable exception is that *o in open syllables becomes long “ā”.

For example: Greek hepta = Vedic sapta- = Persian haft. Greek oktō = Vedic aṣṭā = Persian hašt.