Humans of Four Worlds

Any good biologist will tell you that just because species look alike, doesn't mean they are.  Indeed, in some instances, those creatures that have the same phenotype (appearance) have a radically different genotype.  There are a number of reasons for this, including evolution's choices are sometimes limited in certain niches; probability is just going to go a certain way sometimes.  Other times it is deliberate in an attempt to capture the shape of a more deadly, more dominant species in any attempt to use their power as cover against potential predators.

The Four World are not even close to all of the potential worlds that they are, but they are 'compass points' by which the terrifying inhabitants of Outer Fae behave, and thus Inner Fae and thus Terra and Gaia took their axioms regarding all human worlds in the immediate vecinity.  Each world and its situation has given the humans of that world tremendous abilities, despite their similar appearances.

Terra: Growing up so close to Inner Fae, it was totally logical for these humans to interbreed with them.  As such, the humans of Terra have tremendous magical capability and the ability to generate off spring with most anything.  There are half fae, half vampires, half ghosts and many other sundry such things running around Terra.  This also means that as creatures of magic, they tend to be great story tellers but not the best makers of tales, so the same stories get used and reused many different ways.

Gaia: Their proximity to the great old ones makes their sanity fragile, since their ability to self decieve and find hope where there is none is legendary.  Humans of Gaia are able to also stop 'mandelbrot probabilities' before they happen; postponing inevitable events.  They are also tremendously vulnerable to mutation, corruption and mind control. Any form of cloaking works well on the inhabitants of Gaia.

Mana: The inhabitants of Mana live in a world where their belief actually shapes their reality.  Stories can be very dangerous for them, because while they are not story crippled like Terrans, they are not as great a story tellers as the inhabitants of Earth.  However, once they believe a story, their belief increases the quantum possibilities of it, making even the most fantastic or unbelievable thing more real.  This at one time made their world a fascinating, deadly and wonderous place.  Now it is among the most boring in all creation.

Earth: Humans from Earth have the highest quantum wave form probability; more than all three other worlds combined.  They are more 'real' and if they are given reason to disbelieve something can collapse the quantum wave form of a thing just by observing it.  They are also master story tellers, and if a story from earth spreads to Mana, it can become real in many many worlds; especially Terra or Gaia.  They are only able to master invisible or unseen magics however, since the slightest variation of their observed beliefs often causes them not to be.