History of Japanese

[meat]

Hi, I was reading somewhere recently that the Japanese spoken today is far different than what was spoken back in feudal Japan. Why is this? Was it similar to China eventually adopting a single dialect out of many for its national language or did it just evolve into what it is today? If it is the latter, how did it evolve, as many languages today are similar or the same to what they were hundreds of years ago, why is Japanese so different?