WMG for The Long Earth. Spoilers may be unmarked.
In 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.
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?
- A possible problem with this theory are Joker worlds — are they just worlds that went through weird random event recently? Or worlds that went through it long ago and then they just never split again? (The insect world is mentioned to diverge because pterosaurs never evolved there, which would be at least 100 million years ago.) And what about the Gaps which were, so far, only one world thick? I still guess there is splitting, but it looks like it might not be governed by a simple random algorithm.
- It's possible that Jokers are simply worlds where split hasn't occured for a long time for random reasons. If they DID split further, they would stop being so obvious and they would become just another group of similar worlds. In other words, worlds where a rare event happened but which then split further become groups, while worlds that stop splitting but have no especially rare event look just like their neighbours and don't attract attention — only when both happens, you get a Joker.
- But it would introduce more problems. For example, if Datum Earth split after humans evolved, where are the human artifacts? Were they also eliminated? That comes with the problem that there is no sharp boundary between what is or is not an artifact. Is a road artifact? A grave? A midden? If this rule was in effect, there would probably have to be a sapient entity overseeing the splitting process.
- Sapiency is almost something that emerges pre-artifact generation level culture. Another possibility is something involving sapiency halts the splitting.
- One possibility is that there have been no Datum splits more recently than 75,000 years ago, right before the Toba eruption. This event is theorized to have crippled the planet's ecology to the point where a certain species of hominid was reduced to a few thousand breeding pairs, skirting dangerously close to extinction. Perhaps homo sapiens was wiped out by this event in all the nearby worlds (early human remains are notoriously difficult to find, so it's possible that the archaeologists simply missed them), surviving only in the datum.
- Possibly confirmed in The Long Mars: It is possible to Step on Mars, but the Long Mars does not run parallel to the Long Earth.
- If the crumpled string/intestine view of Long Mars is right then there may not be a "North" and "South" vs "East" and "West", but up and down the string. Soft Spots would be the North and South. You just can step those "directions" because there rarely is a place to step to.
- Confirmed in The Long Utopia. The book's ultimate antagonists are aliens invading from M31 globular cluster some 30 000 light-years from the Milky Way, who found a North-South leap between Long Earth and one of their planets in the globular cluster.
- Confirmed even more so in The Long Cosmos, where an extra-terrestial intelligence contacts Long Earth and supplies them with plans to a machine powerful enough facilitate huge North-South jumps.
- If the crumpled string/intestine view of Long Mars is right then there may not be a "North" and "South" vs "East" and "West", but up and down the string. Soft Spots would be the North and South. You just can step those "directions" because there rarely is a place to step to.
- Or the Earth-Moon universe is the same one as the beanstalk universe — it's possible that Long Earth and Long Mars intersect in more than one place or even that they cover the same worlds, but with completely different numberings. This could be related to the "soft places"
- Except the Mars expedition moved East, not West, so even if the Long Mars and Long Earth weren't each their own separate chains of worlds, they'd be traveling away from the Earth-Moon when they found the Beanstalk.
- The Long Cosmos reveals that the Beanstalk Builders survived to some extent - their monoliths and language were discovered on a world in the Milky Way's core by the Thinker expedition.
- Voyager and (the first) Lobsang both use biogel-based computing.
- Lobsang can appear in several places at once, like the holo-doctor, and both need special equipment to extend their range — the doctor uses a "holoemitter"; Lobsang uses various Android bodies and transmitters.
- 'Stepping' is warp travel — going quickly to other worlds that are otherwise unreachable within the lifetime of the traveller. The soft spots then become an equivalent to Borg Transwarp Conduits.