At some point in history, ない replaced *あらない as the negative of ある, at least in the Kantō dialect (Kansai seems to have あらへん; あらん is also apparently attested in some dialects). When did this happen? ...
Yes, this sounds like a really confusing question. But I suddenly realized that する seemed to be a perfectly regular 上二段活用 (kami nidan; upper bigrade) verb with a stem of s-: 未然形: し (as in しない) 連用形: し ...
I'm looking at this picture of the 日米和親条約 (Kanagawa convention) from the late Edo period and it seems rather strange: It looks like the non-kanji parts are rather haphazardly written in katakana ...