Michio Kaku stated somewhere that the first Japanese people were sent from China under the orders of Emperor Qin to find the fountain of youth in 200BC. Is this true? Actually to be fair I don't think he stated this conclusively but rather I think he said something like "The first Japanese MAY have been..."
I heard the same story and actually watched a documentary that mentioned it. A descendent of the official appointed to find the fountain of youth was interviewed. As far as whether the Japanese originated from China, it is important to know that there are 3 different ethnicities in Japan; the Ainu in Hokkaido from Siberia, The Ryukuyans in Okinawa from Austronesia, and the Japanese Japanese who are of Sino-Korean origin which means they came from China/Korea. Also, my mom told me that most Japanese who have dry earwax share this recessive trait with the Chinese and Koreans while the Ainu tend to have the wet variety, so this means that the Japanese, Chinese and Koreans must come from common ancestors. read the link in the source to find out more.