A couple nights ago I spent several hours reading about the first humans in Japan.
Apparently, the oldest bones they have are in the 14,000 to 15,000 year old range, and, until recently, very little research had been done on sites that might be older than that.
A pre-15,000 year old arrival date for humans would put them right at the center of the coldest part of the last ice age.
Humans could have probably walked, or floated, or island hopped into the Japanese mainland from Korea, Taiwan, Sakhalin, or Kamchatka.
In (mostly North) America there's Clovis-first-and-only; Japan's glass floor beneath which human presence can't fall is even more strict.