Welcome to the forum, obsinate. Glad to see a new posters, great stuff!
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I'm not sure I agree.
Sorcery itself stems from the ability to see/grasp the Onta - which is a hereditary trait, at least for the Nonmen and the Inchoroi.
Sorcery has also slowly evolved over time, incrimental improvements which are mentioned vaguely throughout, mostly the Mandate iteratively improving the Gnosis and the Anagogic schoolmen doing the same. If its being incrementally improved, then you should probably be able to incrementally go back back back, all the way to the start.
As for not evolved, I suppose, but the exercise of sorcery is a mental one, not so much physical, so I liken its 'discovery' to something more like language or mathmatics. Calculus, for example, was 'invented' by Isaac Newton, but that doesn't mean that it was given to him by Aliens. He took basic principles and used them in a novel way. Sorcery, as well, must have taken an initial cognitive leap, but that doesn't make it unusual.
Kellhus did, in fact, derive his own sorcery after only a few days being given the basic principles of the Gnosis, weaving in a second innutteral.
There are few Origin Stories in Earwa. We have some rumors/tales of Shawmen who wielded sorcery before the Breaking-of-the-Gates. Anagogic sorceries were derived without the Nonmen.