John Batchelor has written:

'The Ainu of Japan' -- subject- s -: Accessible book, Ainu, Social life and customs

'An Ainu-English-Japanese dictionary' -- subject- s -: Ainu language, Dictionaries, English language, Japanese language, Polyglot

'The Ainu and their folk-lore' -- subject- s -: Accessible book, Ainu, Folklore

'The pit-dwellers of Hokkaido and Ainu place-names considered'

James Patrie has written:

'The genetic relationship of the Ainu language' -- subject(s): Ainu language, Comparative Grammar

The Ainu, living mostly in Hokkaido, and the Kurile and Sakhalin Islands, are thought to be the indigenuous people of Japan, physically distinct from the Japanese, Chinese and Koeran genotype. The Ainu language, mostly obsoleted, bears no apparent relation to any other known language.

From this point on, most is speculation. It is thought that the Ainu were surplanted by incursions from China and Korea, but there is no historical record going that far back.

Takanaka Chiri has written:

'Ainugo eiri jiten' -- subject(s): Ainu language, Dictionaries, Japanese language, Japanese

yes, Ainu discovered japan.