Why can Japanese Godan verbs only have nine possible consonant sounds before the final -u?

Question

The dictionary form of Japanese verbs always ends in a -u syllable. Ichidan (one row or single-step in German) verbs will always end in -る (-ru, e.g. 食べる, taberu, to eat) while godan (five rows or five-step in German) verbs can end in any of the following:

• -う (-u) as in 会う (au, to meet)
• -く (-ku) as in 書く (kaku, to write)
• -ぐ (-gu) as in 泳ぐ (oyogu, to swim)
• -す (-su) as in 話す (hanasu, to speak)
• -つ (-tsu) as in 立つ (tatsu, to stand)
• -ぬ (-nu) as only (!) in 死ぬ (shinu, to die)
• -ぶ (-bu) as in 遊ぶ (asobu, to play)
• -む (-mu) as in 読む (yomu, to read); or
• -る (-ru) as in 有る (aru, to exist/to be/to have)

It is notable that -う (-u) is actually from -wu, which can only be seen in the (w)a-form (used e.g. in negation 会わない rather than *会あない) as all other w- syllables have merged with their respective Ø- forms. Furthermore, it is interesting that among voiced/unvoiced pairs only -く (-ku) and -ぐ (-gu) coexist as verb endings; -ず (-zu) and -づ (-dzu) do not form verbs to the best of my knowledge and neither do -ふ (-fu) or -ぷ (pu). Verbs ending in -ゆ (-yu) or an -う (-u) which is not actually a historic -wu seem not to exist at all. Finally, according to my grammar book there is only a single verb that ends in -ぬ.

What are the origins of this perceived imbalance? How come only one voiced/unvoiced pair exists in contemporary Japanese and are there any mechanisms that explain the absence of the other missing syllables e.g. by mergers? And why is it -ぶ but -つ and -す?

Answer 1

Japanese verb roots show a strong tendency to be native rather than borrowed vocabulary. In native Japanese vocabulary, original *p was lenited to [w] between vowels (based on the historical spelling of syllables with this [w] sound and the alternative development of *p to a voiceless fricative in word or morpheme-initial position, this medial [w] from *p seems to have passed through an intermediate stage of a voiceless labial fricative [ɸ]).

Because of these sound changes (*p > ɸ followed by VɸV > VwV), there is no regular source of intervocalic /p/, /h/ or /ɸ/ in native Japanese vocabulary (although these sounds do show up intervocalically in some kinds of borrowed words, reduplicated mimetic words, or compound words). I think that explains fairly well the absence of -fu and -pu verbs.

I don’t know whether there are explanations for the other gaps that you mention. I remember reading that some Japanese verbs have switched classes between time periods: I don’t know whether that is relevant.

Answer 2

I'm not qualified to comment on the rest, but I do want to note that both -zu and -nu are verb conjugations in classical Japanese. For ichidan verbs, both these forms are formed by replacing the -ru with -zu/nu:

• 見る miru "to see"
• 見ず mizu "without seeing" (modern Japanese 見なくて minakute)
• 見ぬ minu "not seeing" (modern Japanese 見ない minai)

As you can see, if -nu or -zu verbs were common, they would easily be confused with the negated forms of ichidan verbs. Godan verbs, though, are conjugated the same way as -nai, so there's less potention for confusion:

• 死ぬ shinu "to die"
• 死なず shinazu "without dying" (modern Japanese 死なないで shinanaide)
• 死なぬ shinanu "not dying" (modern Japanese 死なない shinanai)