A question about Leto II's other memory

I've read basically every book in the original and extended sagas. I don't believe any comment has ever been made about the origin of the worms. We know that they are not native to Arrakis; in Children, we learn they must have been introduced. Does anyone know if I've missed anything, and if either of the Herberts have stated where the worms come from?

Related to this, I think if the worms were moved by humans, Leto II should actually know their origins; he has literally every human memory, back to neolithic humans. Does he know? If he doesn't know, then the origin of worms must be alien, or that the worms have their own mode of travel..