Why can verbal roots in PIE only contain the vowel e?

Question

Verbal roots of PIE are generally reconstructed as (C₅) (C₃) C₁ e C₂ (C₄) (C₆); with certain phonetical restrictions, especially on the outmost consonants.

I wonder why only "e" should be allowed as a root vowel and why it is generally attempted to "explain away" cases of other root vowels (like "a"), as stated in Tichy's "Indogermanistisches Grundwissen" (p. 35; my approximate translation):

Next to verbal roots with the ie. base vowel e, some roots with a are also documented, e.g. * Hiaǵ 'to worship'. In spite of all efforts, there has been no success in plausibly explaining away these cases.

So: Why should all roots have only the root vowel e and what evidence would a theory postulating that in all PIE roots the only vowel is e be based on?

Answer 1

It's not that PIE roots always contain the vowel e, it's that PIE roots don't contain vowels. This is a common misconception, unfortunately aided by the traditions of IE lexicography.

Take a root like lei̯k^w- 'leave'. This root is found in:

• e-grade, e.g. Gk. pres. leip-ō
• o-grade, e.g. Gk. pf. le-loip-a
• zero-grade, e.g. Gk. aor. e-lip-on

What this shows is that the vowel (or lack thereof) depends on the grammatical category, not the root. The e in leip-ō is due to the fact that this class of present stems are formed with e, not to anything about the root's lexical entry. The tradition in IE studies is to cite all roots in the e-grade, but it could just as well have been the o-grade (in which case you might now be asking why all PIE roots contain the vowel o). It would be less misleading to cite roots as e.g. l-i̯k^w, with no vowels at all, but for historical reasons this isn't how it's done.

The explanation for the existence of some roots with a, at least those which can't plausibly be ascribed to the combination of e with h2, is probably that in late PIE this system was beginning to break down, as it does in all the daughter languages, with consonantal roots starting to give way to vowel-containing stems. But there are very few of these a-roots, i.e. on the whole the consonantal system is still intact.

(The system I'm describing looks rather like the Semitic root system, but there's an important difference, namely that PIE roots contain consonant clusters which can't be broken up: for example you never see a form like li̯ek^w-.)

Answer 2

The root structure in PIE (of any root except some indeclinable particles) is as follows (s)(C)CeC(C). There were no third layer consonants, except the s-mobile and also there were specific limitations on the consonant composition.

The vowel could change from e to o, to long e, to long o and to no vowel depending on where the root was used.