Dust DID exist before humans because the Authority condensed out of Dust - then much later Xaphania and all the other rebel angels intervened to speed up human consciousness. Humans didn't add to the Dust count until after this point.
The whole paradox of Dust pre-existing the first conscious being while also being created by thought is explained quite well by Wheat in his book on HDM
http://www.amazon.co.uk/Philip-Pullmans ... 56&sr=8-1-
Basically to do with Hegel's idea that there is potential and realised knowledge which explains the difference between Dust as a particle and Dust as thoughts / knowledge.
I still beleive that Dust wasnt created before humans,and i stand to it(must reread TAS right now to find out more)