A remarkable feature of Proto-Indo-European (PIE) is the restrictiveness of the constraints on its root structure. It is generally agreed that all PIE roots were monosyllabic, containing a single underlying vowel. In fact, the vast majority of the roots are thought to have had a single underlying vowel, namely *e. (Some scholars reconstruct a small number of roots with underlying *a rather than *e; others do not, and reconstruct underlying *e in every PIE root.) It is also commonly supposed that every root had at least one consonant on either side of its vowel; in other words, that there were no roots which began or ended with the vowel (Fortson 2004: 71).
I have no dispute with the first of these constraints; though it is very unusual, it is not too difficult to understand in connection with the PIE ablaut system, and the Semitic languages are similar with their triconsonantal, vowel-less roots. However, I think the other constraint, the one against vowel-initial and vowel-final roots, is questionable. In order to talk about it with ease and clarity, it helps to have a name for it: I’m going to call it the trisegmental constraint, because it amounts to the constraint that every PIE root contains at least three segments: the vowel, a consonant before the vowel, and a consonant after the vowel.
The first thing that might make one suspicious of the trisegmental constraint is that it isn’t actually attested in any IE language, as far as I know. English has vowel-initial roots (e.g. ask) and vowel-final roots (e.g. fly); so do Latin, Greek and Sanskrit (cf. S. aj- ‘drive’, G. ἀγ- ‘lead’, L. ag- ‘do’), and L. dō-, G. δω-, S. dā-, all meaning ‘give’). And for much of the early history of IE studies, nobody suspected the constraint’s existence: the PIE roots meaning ‘drive’ and ‘give’ were reconstructed as *aǵ- and *dō-, respectively, with an initial vowel in the case of the former and a final vowel in the case of the latter.
It was only with the development of the laryngeal theory that the reconstruction of the trisegmental constraint became possible. The initial motivation for the laryngeal theory was to simplify the system of ablaut reconstructed for PIE. I won’t go into the motivation in detail here; it’s one of the most famous developments in IE studies so a lot of my readers are probably familiar with it already, and it’s not hard to find descriptions of it. The important thing to know, if you want to understand what I’m talking about here, is that the laryngeal theory posits the existence of three consonants in PIE which are called laryngeals and written *h1, *h2 and *h3, and that these laryngeals can be distinguished by their effects on adjacent vowels: *h2 turns adjacent underlying *e into *a and *h3 turns adjacent underlying *e into *o. In all of the IE languages other than the Anatolian languages (which are all extinct, and which records of were only discovered in the 20th century), the laryngeals are elided in pretty much everywhere, and their presence is only discernable from their effects on adjacent segments. Note that as well as changing the quality (“colouring”) underlying *e, they also lengthen preceding vowels. And between consonants, they are reflected as vowels, but as different vowels in different languages: in Greek *h1, *h2, *h3 become ε, α, ο respectively, in Sanskrit all three become i, in the other languages all three generally became a.
So, the laryngeal theory allowed the old reconstructions *aǵ- and *dō- to be replaced by *h2éǵ- and *deh3– respectively, which conform to the trisegmental constraint. In fact every root reconstructed with an initial or final vowel by the 19th century IEists could be reconstructed with an initial or final laryngeal instead. Concrete support for some of these new reconstructions with laryngeals came from the discovery of the Anatolian languages, which preserved some of the laryngeals in some positions as consonants. For example, the PIE word for ‘sheep’ was reconstructed as *ówis on the basis of the correspondence between L. ovis, G. ὄϊς, S. áviḥ, but the discovery of the Cuneiform Luwian cognate ḫāwīs confirmed without a doubt that the root must have originally begun with a laryngeal (although it is still unclear whether that laryngeal was *h2, preceding *o, or *h3, preceding *e).
There are also indirect ways in which the presence of a laryngeal can be evidenced. Most obviously, if a root exhibits the irregular ablaut alternations in the early IE languages which the laryngeal theory was designed to explain, then it should be reconstructed with a laryngeal in order to regularize the ablaut alternation in PIE. In the case of *h2eǵ-, for example, there is an o-grade derivative of the root, *h2oǵmos ‘drive’ (n.), which can be reconstructed on the evidence of Greek ὄγμος ‘furrow’ (Ringe 2006: 14). This shows that the underlying vowel of the root must have been *e, because (given the laryngeal theory) the PIE ablaut system did not involve alternations of *a with *o, only alternations of *e, *ō or ∅ (that is, the absence of the segment) with *o. But this underlying *e is reflected as if it was *a in all the e-grade derivatives of *h2eǵ- attested in the early IE languages (e.g. in the 3sg. present active indicative forms S. ájati, G. ἀγει, L. agit). In order to account for this “colouring” we must reconstruct *h2 next to the *e. Similar considerations allow us to be reasonably sure that *deh3– also contained a laryngeal, because the e-grade root is reflected as if it had *ō (S. dádāti, G. δίδωσι) and the zero-grade root in *dh3tós ‘given’ exhibits the characteristic reflex of interconsonantal *h3 (S. -ditáḥ, G. dotós, L. datus).
But in many cases there does not seem to be any particular evidence for the reconstruction of the initial or final laryngeal other than the assumption that the trisegmental constraint existed. For example, *h1éḱwos ‘horse’ could just as well be reconstructed as *éḱwos, and indeed this is what Ringe (2006) does. Likewise, there is no positive evidence that the root *muH- of *muHs ‘mouse’ (cf. S. mūṣ, G. μῦς, L. mūs) contained a laryngeal: it could just as well be *mū-. Both of the roots *(h1)éḱ- and *muH/ū- are found, as far as I know, in these stems only, so there is no evidence for the existence of the laryngeal from ablaut. It is true that PIE has no roots that can be reconstructed as ending in a short vowel, and this could be seen as evidence for at least a constraint against vowel-final roots, because if all the apparent vowel-final roots actually had a vowel + laryngeal sequence, that would explain why the vowel appears to be long. But this is not the only possible explanation: there could just be a constraint against roots containing a light syllable. This seems like a very natural constraint. Although the circumstances aren’t exactly the same—because English roots appear without inflectional endings in most circumstances, while PIE roots mostly didn’t—the constraint is attested in English: short unreduced vowels like that of cat never appear in root-final (or word-final) position; only long vowels, diphthongs and schwa can appear in word-final position, and schwa does not appear in stressed syllables.
It could be argued that the trisegmental constraint simplifies the phonology of PIE, and therefore it should be assumed to exist pending the discovery of positive evidence that some root does begin or end with a vowel. It simplifies the phonology in the sense that it reduces the space of phonological forms which can conceivably be reconstructed. But I don’t think this is the sense of “simple” which we should be using to decide which hypotheses about PIE are better. I think a reconstructed language is simpler to the extent that it is synchronically not unusual, and that the existence of whatever features it has that are synchronically unusual can be justified by explanations of features in the daughter languages by natural linguistic changes (in other words, both synchronic unusualness and diachronic unusualness must be taken into account). The trisegmental constraint seems to me synchronically unusual, because I don’t know of any other languages that have something similar, although I have not made any systematic investigation. And as far as I know there are no features of the IE languages which the trisegmental constraint helps to explain.
(Perhaps a constraint against vowel-initial roots, at least, would be more natural if PIE had a phonemic glottal stop, because people, or at least English and German speakers, tend to insert subphonemic glottal stops before vowels immediately preceded by a pause. Again, I don’t know if there are any cross-linguistic studies which support this. The laryngeal *h1 is often conjectured to be a glottal stop, but it is also often conjectured to be a glottal fricative; I don’t know if there is any reason to favour either conjecture over the other.)
I think something like this disagreement over what notion of simplicity is most important in linguistic reconstruction underlies some of the other controversies in IE phonology. For example, the question of whether PIE had phonemic *a and *ā: the “Leiden school” says it didn’t, accepting the conclusions of Lubotsky (1989), most other IEists say it did. The Leiden school reconstruction certainly reduces the space of phonological forms which can be reconstructed in PIE and therefore might be better from a falsifiability perspective. Kortlandt (2003) makes this point with respect to a different (but related) issue, the sound changes affecting initial laryngeals in Anatolian:
My reconstructions … are much more constrained [than the ones proposed by Melchert and Kimball] because I do not find evidence for more than four distinct sequences (three laryngeals before *-e- and neutralization before *-o-) whereas they start from 24 possibilites (zero and three laryngeals before three vowels *e, *a, *o which may be short or long, cf. Melchert 1994: 46f., Kimball 1999: 119f.). …
Any proponent of a scientific theory should indicate the type of evidence required for its refutation. While it is difficult to see how a theory which posits *H2– for Hittite h- and a dozen other possible reconstructions for Hittite a- can be refuted, it should be easy to produce counter-evidence for a theory which allows no more than four possibilities … The fact that no such counter-evidence has been forthcoming suggests that my theory is correct.
Of course the problem with the Leiden school reconstruction is that for a language to lack phonemic low vowels is very unusual. Arapaho apparently lacks phonemic low vowels, but it’s the only attested example I’ve heard of. But … I don’t have any direct answer to Kortlandt’s concerns about non-falsifiability. My own and other linguists’ concerns about the unnaturalness of a lack of phonemic low vowels also seem valid, but I don’t know how to resolve these opposing concerns. So until I can figure out a solution to this methodological problem, I’m not going to be very sure about whether PIE had phonemic low vowels and, similarly, whether the trisegmental constraint existed.
Fortson, B., 2004. Indo-European language and culture: An introduction. Oxford University Press.
Kortlandt, F., 2003. Initial laryngeals in Anatolian. Orpheus 13-14 [Gs. Rikov] (2003-04), 9-12.
Lubotsky, A., 1989. Against a Proto-Indo-European phoneme *a. The New Sound of Indo–European. Essays in Phonological Reconstruction. Berlin–New York: Mouton de Gruyter, pp. 53–66.
Ringe, D., 2006. A Linguistic History of English: Volume I, From Proto-Indo-European to Proto-Germanic. Oxford University Press.