TV Tropes (whose horrible power xkcd summarized rather neatly here, so beware) has a whole section devoted to Speculative Fiction.

Transplanted Humans

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 planet of humanity is somewhere else. This has become less common, however, as evidence that we’re biologically related to other Earth species has become pretty hard to talk your way around.

This trope is often combined with Ancient Astronauts.

From “A Form You Are Comfortable With” to “Zeroth Law Rebellion,” start clicking and say goodbye to several hours.

(via Dark Roasted Blend)