This story is pretty obviously inspired by The Road Not Taken, though I think it’s interestingly different in significant ways.
Interstellar travel (of sorts) is, it turns out, quite easy. It’s so easy in fact that entire species of animals have evolved to be able to do it naturally, including humans.
The ability is called “worldwalking”, and it allows you to move yourself and a reasonable amount of additional mass (say, on the order of about 100 people with associated gear as an upper limit) to other planets. The connection between these other planets doesn’t seem to map to positions in real-space terribly well, if it maps at all, and the planets you can move between are typically quite similar (so any planet you move to will probably have a broadly similar range of temperature, gravity, atmosphere, etc).
This talent is distributed unevenly, but typically in species that have it, more than half of the population can do it too some degree (carrying themselves between worlds) and some can do it to a much larger degree.
The difficulty of moving between planets varies. There’s effectively some sort of potential function which moving up it is hard and moving down it is easy. This potential changes gradually over time. There’s also some sort of distance factor. Imagining the placement of worlds as being on some sort of 2D mountainous land doesn’t put you too badly wrong.
At some point in the last 50k years or so, earth’s potential reached a peak where it was basically a worldwalking everest – quite easy to leave (there’s no equivalent of going “splat” when you jump off the peak), almost impossible to climb.
This created a very strong selection pressure against the worldwalking talent. If you ever worldwalked away (which was very easy to do even for extremely low levels of ability) you would basically never come back. So anything with non-trivial worldwalking talent very rapidly removed themselves from the gene pool as soon as their ability manifested. As a result the talent is basically entirely absent on earth.
Since peaking a while back, the potential level of earth has been gradually going down. It’s recently reached the point where travel to it has become reasonably feasible – as recently as 100 years ago, probably fewer than 1% of world-walkers could make it here and they probably couldn’t bring more than one or two people with them. The rate of decline seems to have increased recently and it’s got to the point where most world-walkers could reach earth if they had to and a reasonably large percentage can bring about a dozen people with them.
All of a sudden it’s become impossible to dismiss world walkers as random crazies and earth-bound humanity has basically been forced back into contact with humanity at large.
The rest of humanity is quite like us, by and large – most of them can probably interbreed with us just fine. They’re technologically much less advanced than we are, though they’re not primitives. They’ve likely got quite a decent grasp of basic science, maths, philosophy etc. but they’ve never been particularly driven to create a high functioning technological society because world walking was just easier. Sure we could build roads and trains and stuff, but why bother when we can worldwalk? We could build large cities, but there are entire virgin planets only a few hundred days travel away, so why?
They’re also incredibly racially diverse. There are millions of inhabited planets, each with their own subtly different selection pressures (different animal and plant species, more UV, more solar radiation, higher gravity, less water, etc) and there’s been a lot of time for those to take effect, but there’s also been enough migration that you can see ancestries from a wide variety of different locations wherever you go.
There’s also a lot of them. They’ve never experienced any population pressure or mass die offs, and they’ve been reproducing merrily for as long as we have. A million planets with maybe a million people apiece? Sounds plausible, though the true number is basically unknowable, but even given that we’re already outnumbered 100:1.
So we’ve just met our cousins. They’re more primitive than us, they look different and scary, and they have a large pool of natural resources they’ve never learned to exploit and a vast amount of living space for us to move in to.
This probably isn’t going to go well, is it?
(though a potential saving grace is that there is so much space and resources available that we’re likely not going to step on anyone’s toes too soon).