I can imagine an alien coming down to earth and studying us and thinking to itself "Wow, these humans didn't even invent nuclear weapons until a few decades ago? Didn't they look at all of that Uranium and get the least bit curious?" Lol.
I think other earth benders have tried before, but Toph was the first to crack the code
I doubt Toph was the first. But I don't doubt that metalbending is an almost dead art that had lost its importance from when it was discovered until Toph re-created it.
In the past, the only sources of metal might have been in caves or something (I don't know where metal really comes from, or if its even a natural occurring element), but point is, any and all Metalbenders probably only bended metal (like sandbenders only bending sand). And since metal wasn't a prominent source until the Fire Nation began the war, it was an almost lost art.