The "origins" of Allomancy seems like a question that has two parts. When & How.
The How part does seem to be addressed at the end of WOA, with the quote posted earlier by Vin.
However, it seems like Allomancy existed before that. Does anyone remember any references about Alendi burning metals?
The connection that made me think about the "When" was Vin and Alendi. They both felt the thumping. There does seem to be some "interesting" and "abnormal" effects when it comes to vin and sensing Allomancy, however, the fact remains that she only hears the thumping (I think) when burning bronze.
If that's the case, it seems like Alendi would of been burning bronze as well, meaning that Allomancy existed before that point.
I'd have to reread WoA and see if there is evidence for this. I seem to recall at least one point where she turned on bronze and felt the thumping, but it also wouldn't suprise me if Vin had bronze on pretty much all the time. If that was the case, than maybe she didn't need to be burning it to hear the thumping, it's just that she happened to be doing so.