A few months late, I know, but just to answer the actual question competently:
Its generally well accepted that Japanese culture is a mix between various cultures of their neighbouring countries. As an island, various cultures had come to "Wa", assimilating into the one. As an island culture, many aspects of their neighbours became accepted (be that Korean, Chinese, Siberian, etc) such as faiths such as Buddhism. Language aspects thus bear some resemblance to their Korean and Chinese counterparts.
As for their origination, despite the general consensus agreeing that Japan was a blend of cultures, this is still in heavy dispute. I'll pull out what I said in response to a similar question, in which someone was asking whether or not Japanese people, as a majority, originally came from China:
Well actually, if we're considering "ancient civilizations", in terms of their recorded history, then Korea is just as ancient as China. Both share a 5000 year history. Its generally accepted that all the areas of Asia were part of a much larger "Asia", having many states within Asia that called themselves countries. Meaning Warring States, Three Kingdoms, Koryo, "Wa" (ie. Japan), etc, scattered throughout the histories of the nations. So its just like, if China was still separated in three countries, claiming that one was originally the other. You can't possibly say. The fact there were so many states also is reasoning behind why there are so many different ethnic groups in China (whilst Korea didn't end up becoming one with China.)
If we look at genealogy, it shows that Koreans are more related to the Mongolians. This is also why Koreans, like the Mongolians, don't take in milk very well, and have a similar butt mark (as weird as that sounds for the people not in the know). But then, interestingly enough, Koreans and Japanese are extremely closely related in genealogy, more so than the Koreans or Japanese to the Chinese. Which tends to suggest that Koreans and Japanese had not be separated for a very long time, or a larger part of Japan was from Korea, or that people from Japan settled in Korea many years ago. This is always under heavy debate and flame in between the nations, and this leads to protests because of different publications of history (on top of the WWII stuff).
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Its 100% biased on how proud you are of your country. You cannot ever have the true history. And if the topic of history is ever brought up, there's always someone commenting about how the Japanese erased many records of Korean history and culture during their occupation of Korea.