Just for additional info,
Earth (or Terra as it seems to be called it the timeline I read) was getting overpopulated and people moved off it, up till a point where the Earth population was outnumbered 20:1. Then some time passed and Earth was hit by an asteroid. It was wiped clean, terraformed and left as a undisturbed natural habitat. Over time they lost track of it and they stopped adding it to star charts because there was no need to go there. Eventually it was lost. At least thats what I’ve read.
Further example of Earth history: The “buddislamic” religion, and "zensunni"s.
From here: http://www.dunenovels.com/news/encyclopedia.html
Also, above a quote about an intro by Frank himself stating something to the same effect.
I recall reading bits of the Dune encyclopedia, and found most of it to be, well, silly, to say the least. Like Dune taking place in some alternate history where Rome never fell.
I actually like the new novels. The Butlerian Jihad ones, especially, really put a neat spin on how you view everything (The typical “computers have taken over!!!” is complicated by the Titans). The Prelude were a little non Duneish, but since the Butlerian ones take place, IIRC, 10,000 years before Dune, things can be different then.
This implies that Guild Navigators are the product of a breeding program that has caused them to deviate from the human norm, and that this (not the Spice per se) is the reason they look so funny.
Brian H. and Kevin J. Anderson contradict this in the Dune: House series, where the Guild goes out and selects Navigator trainees from the general population.