Ainu are the people who have the Ainu culture.
Ainu culture appeared in Hokkaido in the 12th century.
Therefore, Ainu appeared in the 12th century in Hokkaido.
In Hokkaido北海道(called "Ezo"蝦夷)
Jomon culture縄文文化 >> zoku-Jomon culture続縄文文化 >>Satsumon culture擦文文化 >> Ainu cultureアイヌ文化
Ainu is the mix of "Hokkaido-Jomon people" and "Okhotsk-culture people from Sakhalin".
Japanese and Ainu have the common ancestors(the common ancestors of Jomon people and Hokkaido-Jomon people).
So, it is natural that Japanese have a part of Ainu's DNA.
Ainu culture appeared in Hokkaido in the 12th century.
Therefore, Ainu appeared in the 12th century in Hokkaido.
In Hokkaido北海道(called "Ezo"蝦夷)
Jomon culture縄文文化 >> zoku-Jomon culture続縄文文化 >>Satsumon culture擦文文化 >> Ainu cultureアイヌ文化
Ainu is the mix of "Hokkaido-Jomon people" and "Okhotsk-culture people from Sakhalin".
Japanese and Ainu have the common ancestors(the common ancestors of Jomon people and Hokkaido-Jomon people).
So, it is natural that Japanese have a part of Ainu's DNA.