One stable of Star Trek is that various alien life forms can breed together. Spock, for example, is a hybrid of Vulcan and Human. In real life, this isn’t really likely even when superpowers and suchlike aren’t added into the mix. (Could Lois Lane carry a half-Kryptonian child to term?) Cross-race sex might be possible, but not hybrids.
What if it WAS possible?
Humans have a wide range of physical appearances and some of us are dumb enough to believe that people are superior or inferior depending on something as minor as skin colour. If humanoid life evolved upon different worlds, it may well look different to humanity with still have the same basic form.
Let’s assume that some long-gone (perhaps) race sowed the seeds of life in the galaxy billions of years ago. Perhaps something happened to wipe out most other forms of life and the sole survivors (the Planters, for want of a better term) started a long-term project to reseed the galaxy with life. Engineered life was dumped on suitable worlds, programmed to turn them into habitable planets which would eventually evolve intelligent life. Nothing was mapped out completely; the Planters were wise enough to leave some latitude for evolution that would alter the seeds for new worlds. Not all of the seeded worlds blossomed to life. Some worlds – maybe like Mars or Venus – had problems and couldn’t cope with the transformation into a habitable world. The remains of the seeds died out, leaving little trace of what had happened millions of years ago.
Having completed their work, the Planters went into stasis. Or transcended. Or died out. No one knows for sure.
Fast forward a few million years later. One of those seeds has grown into humanity. Others have grown into humanoid races, all descended from the same basic stock. The ones that have developed space technology have discovered some Planter technology, portals that take people from star to star. Some races have fought wars over the portals, or made friends with nearby races. And they have discovered that they can interbreed. Some hybrids are mentally subnormal, others are more intelligent and capable than the ‘pure’ races. Different combinations of races produce different results.
But some of the Planter technology is very dangerous.
And the Planters are coming back…