WMG for TheLongEarth. Spoilers may be unmarked.
The bugs in the dead world are an invasion species - and a forshadowingIn the dead world Joshua find some dull bugs under a patch of dirt. He regards them as yet another depressing part of the dust world and moves on, but the existence of these bugs make no sense at all. Insects are highly complex organisms, and beetles didn't evolve until a lush and thriving biosphere was already in place. If the world was truly as dead as the characters think, then the bugs are as out of place as they are themselves - and there for the same reason. Only sapient beings can Step, but they can carry animals with them without problems. This means that they can also carry microorganisms, insects, seeds and so on with them without noticing it. Humans are now moving plenty of invasion species with them from Datum and outwards, and Trolls and Elves have carried things with them for a long time. The bugs in the dead world came over in the fur of a troll, dropped off and since then they have made a living with what meager resources they can find. This forshadows another being's ability to travel by hitchhiking on unaware steppers - First Person Singular. Joshua was swimming in direct physical contact with Her, and by the end of the first book they have carried her spores to every world between the Gap and Datum. This may become something of a problem.
Absence of mankind on other worldsGiven that there seem to be an infinity of earths, the absence of mankind on other worlds is a conundrum, if one rules out theistic guided evolution, then one assumption to make is that there are other worlds in which mankind exists but that these worlds are inaccesible from datum earth. Given that massed humanity is described as having a the effect of psychic repulsion, this repulsive force increasing with the number of humans, what if two worlds where to have similar numbers of people? The effect would be like bring two similar poles of a magnet together and they would repulse each other. Since worlds next to each other are the most similar we can surmise that the two adjacent worlds with mankind in it would have a similar number of humanity inhabiting it and be strongly repulsive. One can work on the analogy of a deck of cards used in the book. Placing a joker in the middle of a deck, one can imagine a deck laid out on an infinite floor, with cards laid to the east and west of it dissappearing into infinity, these are the worlds that datum eath has access to. Now imagine a wall of cards, in addition to the cards to the east and west there are cards above and below going off to infinity, these are the worlds to the North and the South, the ones where mankind has evolved. As one moves away from datum eath the repulsive force from above and below will weaken and it may be possibe for a stepper to 'leap' up or down a stack, and by mving back come to a world inhabited by mankind other than datum earth :And if a wall, why not a cube of cards with datum earth at the centre, and if a cube why not a hypercube of worlds?
The worlds are capable of splittingThe Long Earth was created by splitting of one world into two adjacent worlds that happens at certain rare moments. The Datum Earth's history split from the other worlds millions of years ago, but even if every world split only once in ten million years (on average), 4.5 billion years of Earth's existence would lead to 2^450 worlds. Splitting once every 100 million years would still be 2^45, roughly 35 billion worlds (and that doesn't even count universes split before Earth or solar system were formed). In this case, the true structure of Long Earth would be circular with West and East eventually meeting, but not before extremely large values are reached. This would also explain why neighbouring worlds tend to be similar (they split comparatively recently) and why there are "bands" of similar worlds that differ from their neighbours (they are all descendants of single world altered by a significant event). There will be a split event in the series; apart from messing with the numbering system a bit, it could result in duplicating towns and people, creating further legal problems.
The worlds split, but sapient beings are not duplicated.It would explain the lack of humans on the other Earths.
Stepping North or South will become viable in later books.Stepping to other Earths is usually indicated by directions East and West. In "The Long War", Joshua's friend Bill recounts a story of one of his friends who claimed to have "stumbled" and Stepped North, ending up on a world with no day, with a Galaxy in the sky overhead. Joshua doesn't believe him. More than likely, Stepping North and South will be a way to deal with First Person Singular.