In regards to humans, I have a few ideas for my faction. This one is probably the most feasible, unless multiverse theory is fair game.

Since the origin of life on Earth is a bit blurry, if we were to presume that life was seeded on Earth by an asteroid or extraterrestrial, what if, by some chance, that same "seed" broke up and crashed into another, earthlike planet, resulting in the evolution of a species very similar to homo sapiens?