Question about verbs in Old Japanese


Does anybody with in-depth knowledge of Old Japanese know why vowel-based
verbal stems could only end in -e or -i (leading to the える/いる or type 2 class
of verbs in Modern Japanese)? Why are there no cases of vowel based verbal
stems ending in e.g. -a, -o or -u?

It is a very peculiar feature when compared to other languages that have
conjugations based on the last letter/vowel of the verbal stem (such as Latin,
Greek and Sanskrit)… Unfortunately my Japanese is rather rudimentary and my
Old Japanese practically non-existent…




The capitalized _Old Japanese_ (上代日本語) certainly has two vowel-stem types of
verbs that yield today's _-eru/-iru_ verbs respectively. However, it's not
accurate to say they end in _-e/-i_ because the language in the Old Japanese
stage had 8 distinct "vowels", or to recent understanding, more like "rhymes":

> _a, i 1, i2, u, e1, e2, o1, o2_

which merged into the current 5-vowel system by the 9th century (then it
becomes called [Early Middle
Japanese](https://en.wikipedia.org/wiki/Early_Middle_Japanese)).

The two stem-final vowels you mentioned are actually _-e 2_ and _-i 2_, both
of which were formed by contraction of adjacent vowels which are thought to
have existed in earlier, pre-document ages. Since I don't have access to books
such as Frellesvig (2010) now, just let me quote
[Wikipedia](https://en.wikipedia.org/wiki/Old_Japanese) for an overview:

>   * _i_ 2 < *ui: _kami_ 2/ _kamu_ - 'god, spirit', _mi_ 2/ _mu_ - 'eye',
> _nagi_ 2/ _nagu_ - 'a calm'.
>   * _i_ 2 < *əi: _ki_ 2/ _ko_ 2- 'tree', _yomi_ 2/ _yomo_ 2- 'Hades'.
>   * _e_ 2 < *ai: _me_ 2/ _ma_ - 'eye', _ame_ 2/ _ama_ - 'heaven', _ame_ 2/
> _ama_ - 'rain', _kage_ 2/ _kaga_ - 'shade'.
>

Thus you can presume that some vowel-stem verbs were actually composed by
underlying bases that end in other vowels, and suffixing * _i_.

> _aka_ "red, bright" / _ake 2-_ "brighten"  
>  _oko 2-s-_ "raise up" / _oki 2-_ "arise"

