Is there a known rule of correspondence between Latin and Greek *p and *kʷ - in other languages?

Question

It seems to me that some words that have -p- in stem in Latin have clearly reconstructible -k^u̯- based on other Indo-European languages. Some examples include

• *u̯lpes - *u̯lk^u̯os ("wolf")
• *u̯esper - *u̯esk^u̯eros ("evening")
• *apa - ak^u̯a ("water")

The "k^u̯" reconstruction is supported by the majority of languages, still many authors consider the roots with "p" as genuine and/or separate. For example, Katz tryed to proove the connection between *u̯esper, Hettite *u̯esp "cloth of the dead" and Greek ospros "pulse".

Mallory & Adams reconstructed the PIE water deity as *H₂epom Nepots instead of evident *Ak^u̯am Nepot(s) (note that the laryngeal here is not supported by the Nostratic evidence).

Starostin lists both *ap- and *ak^u̯a; *welp- and *welk- (and also *lup-) as separate roots.

I wonder why the correspondence between *k^u̯ and *p is not evident to some authors?

Answer 1

(a slightly updated answer)

Latin "lupus" is usually believed to be a borrowing from Sabellic - PIE *k^w corresponds to Sabellic (Osco-Umbrian) *p. Other examples of this phenomenon is Latin popina vs. Latin coquina or Latin Pontius vs. Quintius. Another stop in Sabellic borrowings: Latin "bos". And then metathesis is usually proposed for Greek "lykos".

As for Latin "vesper", you might not be seeing a bigger picture. Compare Greek "hesperos", Armenian "gisher", Hittite "ispant-", and Sanskrit "ksap-". It has been suggested that the PIE root was *ue(k)spero, a derivative of PIE *kwsep- "night", the zero grade *kwsp-, the consonant cluster was simplified in different daughter languages with various outcomes.

Of course, you can dispute the link between Latin "vesper" (Greek "hesperos" and the rest), on the one hand, and Lithuanian "vakaras" and OCS večerъ, on the other hand. OCS večerъ has no clear etymology, and various hypotheses have been proposed. The hypothesis I mentioned above clearly has one weak point: it offers no explanation of what might have happened to "sp" in Balto-Slavic; although it was suggested that some sort of taboo was involved there (see Havers 1946, Specht 1944) or possibly another word (Muehlenbach 1923-1932). There is another proposal that OCS večerъ is a not a simplex, but a root *vek- with a suffix erъ (like OCS severъ). Or maybe it is related to Lithuanian ukanas "cloudy" and ukti "overcast" etc. So, the last two proposals do not posit any connection between OCS večerъ and Latin "vesper" (and other comparanda mentioned above).

It's not clear how the hypothesized PIE form *wesk^wer- would explain the Balto-Slavic data (what happened to *-s-?). Nor it is entirely clear why two roots with the same meaning would exist in PIE.

As for Latin "quinque" and PIE *penk^we, it is usually suggested that in Italic as well as in Celtic (!) languages *p was assimilated to the following *kw (distance or non-adjacent assimilation). You can find it even in intro textbooks, e.g. Campbell 2004: 29. Another example of this assimilation: PIE *pek^w- vs. Latin coquina.

You need to understand that in historical linguistics, esp. in etymological research, there is no ultimate theory that explains everything, a theory that cannot be refuted. Quite often several hypotheses coexist, with different degrees of probability. And quite often these hypotheses are ad-hoc, that is they are created to explain one or, at best, a couple of words. This is undesirable but, I'm afraid, unavoidable, at least at the moment.

Answer 2

Alex B has given a wonderfully detailed answer, but I'd like to sum up some extra information for future readers.

The general consensus, to my understanding, is that the forms with *kʷ are the original ones. They're found in a wider variety of languages, and /kʷ/ → /p/ is a common sound change, which pretty much never happens the other way around.

The change /kʷ/ → /p/, as mentioned, shows up a lot in unrelated places. The two main sub-branches of Celtic are known as P-Celtic and Q-Celtic because the former went through this change and the latter didn't; in more modern times, Romanian lost the Latin labiovelars in the same way (Spanish cuatro ~ Romanian patru). Most relevantly here, it happened in Attic all dialects of Greek (Attic hippos ~ Latin equus), and in the whole East Italic branch (Oscan, Umbrian, a few others—Latin quattuor ~ Oscan petora).

So to my understanding, all the alternate forms with /p/ are later developments. For example, I haven't seen any solid evidence for a PIE root **h₂-p "water"; forms like Romanian apa come straightforwardly from Latin aqua, with the regular loss of labiovelars. Where there's an alternation within the same language, it can usually be explained by borrowing: Latin seems to have borrowed lupus "wolf" from Oscan or another East Italic language, where the change from *kʷ to /p/ was also regular.

Answer 3

I'm not sure I understand the question correctly.

*g^w > b is a known change in Celtic. E.g. for Irish bé "woman" from the same root as Greek gyne, Gothic quno, English queen.

*k^w > p occurred so late in Old Irish, that it is actually attested. One might speculate on the nature of this change, whether it's complete coincidence.

I had noted something similar in a comment somewhen, for Latin or Greek if I remember correctly, but no general rule.