Doesn't Avatar Wan's story present a plot hole with the Aang story line? While humans received the actual ability to bend from the lion turtles, it's said that humans learned bending by watching the flying bison, moon, badger moles, and dragons, etc. Sure, Wan learned the form from the dragons, but how does one explain the first earth benders Oma and Shu? Getting the power of earth from the lion turtles implies that more than one person can get it, it's not exactly a secret. Unless Omashu was built before Avatar Wan, I don't see how one can explain this. Can anyone enlighten me?