Allomancy was, indeed, born with the mists. Or, at least, Allomancy began at the same time as the mists’ first appearances. When Rashek took the power at the Well of Ascension, he became aware of certain things. Some were whispered to him by Ruin; others were granted to him as an instinctive part of the power. One of these was an understanding of the Three Metallic Arts. He knew, for instance, that the nuggets of metal in the Chamber of Ascension would make those who ingested them into Mistborn. These were, after all, fractions of the very power in the Well itself.
