Long ago, visitors came down from the stars, and took humans (and possibly other Earth species) away to other worlds. Now that we have the ability to travel between those stars, we keep running into our long-lost cousins.
This is an increasingly common (and admittedly pretty good) way to justify Human Aliens, by making alien humans.
One variation is that that's how we got to Earth, and the origin [...] [tvtropes.org]