I superbly enjoyed this book and the concepts therein.
Evolution of sentience (which relates to stepping) seems to be a "Joker" probability, in the range of Earth being destroyed or the Moon never forming. It appears to have happened twice within the ~two million worlds we see (our own ancestors on the Datum and the Dino civilization). We don't know how far the avatar of FPS has travelled, so its hard to say how that factors into the odds.
Stepping and civilization seem to be two more-or-less exclusive upshots of sentience. If you can step, you don't need to settle down. In fact, as Sally notes near the end of the book, regression is likely. Fast forward a dozen generations and the stepping population is likely to be incresingly nomadic. This isn't the first time the events of the book have happened. The "phobics" are the future of civilization.
The Dino civilization, I suppose, went the same way. If any of them could step, they stepped. Those who stayed to build the temple and worship uranium were phobics, thus no evidence of them on surrounding worlds. (Except where Joshua meets Sally, we do have upright dinosaurs who appear to be sentient... Stepping cousins?)
Stuttering and the "soft places" interest me also. We see in these instances that stepping is not merely a matter of the "subsequent term" operator. You can skip. You can perform addition. I'm from a math background and this question nagged me the entire time I read the novel: Why is the long Earth discrete?
That is, why must I go to West 1, or West 50. Why not West .5? Or West pi? Or West square root of two?
With the exceptions of Jokers, neighboring worlds seem to share similar properties. This suggests, to me, that what we are seeing are snapshots of a piecewise continuous function. So why are our steps so big? If I can jump a thousand steps at once, why can't I baby-step?
Maybe humanity formed as a singularity, yes, but maybe it formed on an extremely small sliver of worlds? And if we could get to West 0.0000001 or so, we might find other people? Themselves potentially stepping, but relegated to West 1.0000001, West 2.0000001 and so forth? The step from 1 to the integers is microscopic compared to the step from the integers to the reals (see cardinality). The Long Earth as we've been introduced to it might be a zero-dense subset of what is really out there.
Is TPS the Silence? Or the migraines? Or both?
Who/What is manipulating the population at Happy Landings. Why? Some kind of cosmic bet?
What is the significance of 1,890 humans in one "area"?