Yemenites were for a long time a very isoldated community. They might have a mesorah on some things such as identifying grasshoppers or their dikduk, but the Rambam had to teach them a lot of basic things. When it comes to learning in the Ashkenaz world you can trace american yeshivas back to who their roshei yeshiva learned under in Europe. When you go back enough you can trace it back to Rashi and Rabbeinu Gershom Meir Hagolah. In the sefardi world it some came from Spain and some of them (Mizrachi Jews) were there from the beginning of exile.
