yeah I'm confused cause Oma and Shu, the people whom the city of Omashu is named after, learned earthbending by watching them badger moles. So if they could just pick it up so quickly, however did it become a genetic thing passed from one generation to another? Maybe they were always genetically gifted that way, but Oma and Shu were just the first to find the gift within themselves?
Maybe there are more than four elements to be controlled but the fifth hasn't been discovered by those who posses it! Possible?