Scattered humans

Many authors and gamemasters want cultures which are strange, and evolved independently from Earth, but not really alien. Often they want these beings to be biologically compatible with Earth-humans, to the point that they can breed with humans.

However, the chance that there are alien species which look, think and act like us is very small, and the chance that they could breed with humans is as good as zero. Such aliens regularly occur in bad SF, but if there is no convincing explanation for their similarity to us, the work does not deserve the name of science fiction.

Coming up with a reasonable explanation for parallel evolution is a scientific tour de force. A better and more convincing explanation for the humanlikeness of the `aliens' is simply that they are really no aliens at all. They are humans, just like you and me. The only difference is that they live on alien planets, have developed strange cultures, and that their origin lies hidden in the obscurity of the past. Often, only one thing is certain: they came from Earth some day. Their genes prove that.

Of course, there must be a reason why they are out there. There are basically two possibilities:
There has been a technical civilization on Earth thousands of years ago, which colonized many planets and later vanished, leaving behind humans scattered about dozens, hundreds or even thousands of planets, without any way to contact each other.

This is especially appropriate for campaigns in the far future.
An alien species colonized the space around our solar system long ago. For some reasons, they took humans from Earth to their planets (numerous reasons can be imagined for this: using humans as slaves or pets, scientific curiosity, etc.) Then the aliens vanished. The humans survived, but failed to take over the aliens' technology. Either it was destroyed during the catastrophe which wiped out the aliens, or it was simply incomprehensible to the humans.

A campaign using this approach can just as easily be set in the near future, or -- if Earth itself doesn't play a part -- even in the past. The things described above could have happened as far back in the past as 50000 B.C.; on some planets, people might already have FTL starships right now.

Both paths lead to the same result: as many planets as you like, inhabited by low-tech (perhaps even stone age) humans. These human societies, isolated by the impassable vastness of space, then evolve as separately as real aliens, leading to a vast diversity of cultures, among which any kind of imaginable human culture might exist.

However, scattered humans also have one drawback. You can't set up something biologically alien. The scattered humans are still humans; they don't lay eggs, they have no antennae, they can't hear radio waves or see infrared, they don't change their gender, they won't form hive-mind societies, etc. At least unless you go for posthumanism (see below). Of course, a campaign might feature both scattered humans and real aliens.