How Did Shmini Atzeret (8th day Holiday) Become Simhat Torah? with Rabbi Jonathan Feldman on Jewish Matters Podcast

Simhat Torah is one of the most joyful days of the Jewish calendar, yet is is a relatively recent holiday.  Some say that Shmini Atzeret, the 8th day of the Sukkot festivities, is the most special day of the year. How can that be if there is not specific mitzvah associated with it, and why did it get merged with Simhat Torah?