There is evidence of human life on Japan before 100,000 BCE. Back then the sea level was low enough that Japan and mainland Asia were connected by a land bridge, and the Japanese population was composed of the same people as China and Korea. These were the original settlers of Japan, but during the Yayoi Period (~200 BCE to 200 CE) and afterwards there was lots of Chinese and Korean migration to Japan, especially during times of turmoil like the Warring States Period. The Koreans and Chinese spread advanced knowledge of agriculture and metallurgy, thereby advancing Ancient Japanese civilization. If you want more in depth info, check out J. Edward Kidder, Jr.