This is probably one of the most readable and succinct pieces I've seen online about Japanese origins. I write that as plural because there is more than one major group of DNA in Japan.
During the last Ice Age, which ended approximately 15,000 years ago, Japan was connected to the continent through several land bridges, notably one linking the Ryukyu Islands to Taiwan and Kyushu, one linking Kyushu to the Korean peninsula, and another one connecting Hokkaido to Sakhalin and the Siberian mainland. In fact, the Philippines and Indonesia were also connected to the Asian mainland. This allowed migrations from China and Austronesia towards Japan, about 35,000 years ago. These were the ancestors of the modern Ryukyuans (Okinawans), and the first inhabitants of all Japan.
I love how the Ainu dude with the mouth harp is playing along with the Hawaiians (at the 4:15 mark).
Some interesting documentary footage from Japan about the Ainu.
This video is more about the warfare between the Ainu and later settlers of Japan. Lots of cultural information, photos, music.
I can't think of another ethnic group that actually hunts (or used to hunt) bears. Gnarly!