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. Contrast Ultraterrestrials, where the Transplanted Earthlings aren't human.

Comic Books

 * The Post-Crisis DC Universe used this trope to justify the Human Alien Planets of Hats most of the Legion of Super-Heroes came from: after the Invasion Crisis Crossover, Mon-El Valor seeded several worlds with humans who had gained powers from alien experimentation.

Literature

 * The Norby books by Janet and Isaac Asimov have "The Others".
 * Ursula K. Le Guin's Hainish novels use the reversed version; humans and their (e.g.) fuzzy and androgynous neighbors are so many subspecies of the parent race.
 * The Novels of the Jaran by Kate Elliot had a planet full of humans that, millennia ago, an alien removed from Earth and genetically modified before placing them on the planet. His reasons for doing so were never explored, but that may be because the series is currently unfinished.
 * In Jack Vance's Planet of Adventure, the earthman protagonist encounters transplanted humans living side-by-side with (and frequently dominated by) several species of sapient bona fide aliens on the titular planet.
 * In David Weber's Empire From the Ashes trilogy, humans on Earth are transplants descended from the stranded crew of an interstellar warship which has disguised itself as the Moon. Humans were originally native to another planet, Mycos (as is all the other life on Earth - explaining why it is biologically related to humanity).
 * In Larry Niven's Known Space universe all the primates on Earth, including humans, are descended from a race called the Pak that were stranded on Earth a few million years ago. The reason humans are related to other life on Earth is that the Pak terraformed earth and wiped out most of the native ecosystem doing so. Furthermore, nearly all carbon-based life in the galaxy is descended from food yeast seeded on developing worlds 2 billion years ago by the first few alien races.
 * The Narnia series has a magical version of this to explain why there are humans in Narnia. They're all descended from a London cab driver and his wife, and/or Mediterranean brigands and their islander "wives", who came later.
 * Gor.
 * In Sergey Volnov's Army of the Sun trilogy, this is the main theory (although there is little actual evidence) behind the similarity between humans and three human-like races. Each one looks like a "sub-race" of Earth: Caucasians, Africans, and Asians. It is also mentioned that an ancient wall carving on the "Asian" race's homeworld is the name of a legendary figure in their culture, whose name sounds a lot like "Genghis Khan." This is all mentioned as a side-note, however, and is unimportant to the main storyline.
 * In William Shatner's Quest For Tomorrow series, this is revealed to be the fate of the Neanderthals. A ship belonging to a Lizard Folk race found Earth a long time ago and found two sentient species on the planet. The homo sapiens were dismissed as unimportant, while the Neanderthals turned out to be telepathic. Desiring to learn the secret of telepathy, this particular faction transplants a good number of Neanderthals to a remote planet and then wipes out the rest with a virus, engineered specifically to kill them. After a series of failed experiments, the lizards decide to kill off all their subjects as well. They release the virus and leave. This time, however, some of the Neanderthals survive and build a civilization of their own, albeit much slower due to the lack of writing (telepaths don't need to write). Decades before the series initial timeline, the Neanderthals have only reached Industrial Revolution, while humans are already a star-faring race. They then Ascend to a Higher Plane of Existence by turning their star into a black hole.
 * Last Exile : Though kind of a subversion, since humans transplanted themselves and two of the three nations became Fantasy Counterpart Culture Lost Colonies at the mercy of the more advanced, Crystal Spires and Togas Deadly Decadent Court faction.
 * The Algebraist, by Iain M Banks, has 'aHumans' (the humans who were taken to join galactic civilisation in about 4000 BC) and 'rHumans' (tho ones who made it out themselves seven thousand years later). There is some animosity between these groups. Slightly averted, though; these two groups make up only a small fraction of sentient beings even among the 'Quick' species (of which humanity is a member).
 * This troper once read a novel (can't remember the name) about a small mountain town that gets transplanted along with the buildings and a chunk of the land to another world by an unknown astronomical phenomenon. After a brief period of infighting, they are chased off the Earth land by flying creatures. After many hardships and adventures, they meet an intelligent race of hunter-gatherer centaurs, with whom they ally against a common enemy. The novel ends with a stable human society on this world, and the protagonist telling this story to his grandchildren. Of course, this small town would need a pretty large population if the human race is to survive long-term on the new planet.
 * The Saga of the Skolian Imperialate, by Catherine Asaro, posits a small group of Mayans who were kidnapped by an unknown species and transplanted to the planet Raylicon. The kidnappers left their starships behind. The Raylicans experiemented with the discarded technology, created an interstellar empire, fell, reachieved starflight, and then encountered humans from earth just as they invented stardrive theory from scratch.
 * S.M. Stirling's novels The Sky People and In The Courts of the Crimson Kings update the Burroughs depiction of Mars and Venus by being set in an alternate timeline in which aliens terraformed Mars and Venus millions of years ago and have since periodically transplanted species from Earth to the other planets.

Live-Action TV

 * Stargate SG-1 is probably the best-known modern example; the vast majority of the Adventure Planets the SG team goes to were colonized with human slaves by the Goa'uld.
 * Likewise, Stargate Atlantis populates its galaxy with human societies planted by the more benevolent Ancients. People in the Pegasus galaxy are typically aware of how to use the stargate, so some colonies may have settled on their own.
 * C. J. Cherryh's Morgaine Cycle novels used a similar concept (complete with Cool Gates) decades earlier.
 * "The Paradise Syndrome," the Forgot the Call episode of Star Trek: The Original Series, featured a group of Native Americans transplanted to another star system by Precursors as a sort of cultural preserve.
 * The Star Trek: Enterprise episode reveals that the race with whom Archer and his crew are making first contact are
 * And yet another has them finding an Old West-style planet which was populated by abducted humans previously used as slave labor by the aliens who now work as their slaves.
 * The Star Trek: Voyager episode "The 37's" has the crew find a planet populated by descendants of humans abducted by aliens in the 1930s as slaves but rebelled against the abductors. Some of the abductees (known to the locals as the 37's) are still in stasis (including Amelia Earhart). Why someone would go to another quadrant to get slaves, given that the galaxy is teeming with intelligent (and humanoid) life, is another question.
 * The Mystery Science Theater 3000 episode Future War featured a race of alien cyborgs who had visited Earth at some point in the past and kidnapped both humans and dinosaurs—the humans they used as slave labor and the really cheesy forced-perspective puppet dinosaurs they used as "trackers". They failed everything forever.
 * Farscape:
 * Battlestar Galactica had this in both versions: "There are those who say that life here began out there."
 * Kind of. In both versions, it was humans leaving Kobol of their own accord to settle the Thirteen Colonies of Kobol. This has been offered by fans as an alternative explanation to in the re-imagined continuity, however.

Music

 * The Something Wicked Saga by Iced Earth is told from the point of view of the Setians, a race of Reptilian Humanoids who were indigenous to Earth and nearly got wiped out by invading humans.

Radio

 * In The Hitchhiker's Guide to the Galaxy, the humans currently on earth were descended from telephone sanitary workers and hairdressers, set to crash into the Earth by the other people on their planet, who were later wiped out by a disease that occurred due to contaminated telephones.

Tabletop Games

 * In the Traveller RPG and its offshoots the Ancients brought humans and other species to thousands of planets and also created the Vargr by uplifting wolves. The end result being that Terrans went out to the final frontier and discovered they were already there, in fact one of those transplanted races, the Vilani, had established an interstellar empire thousands of years earlier.
 * Done in a more restricted fashion in the Forgotten Realms. Humans are native to Abeir-Toril (that is why they count as a Creator Race, and elves don't)... but they are also native to other worlds, and certain cultures are descended from such non-Toril humans (the Mulan, for example, are the descendants of slaves taken from another world that by Word of God was Earth).

Video Games

 * The Backstory for StarCraft mentions that the first Terrans (humans) in the sector were exiled criminals transported off Earth as their punishment, a la Australia. In the Expansion Pack, Brood War, those who stayed on Earth catch up with them. And while the Earth humans are absent in StarCraft II, a good chunk of the tech that Rory Swann develops is reverse engineered from their tech.
 * Since the UED Expedition force was wiped out by the Zerg with no survivors, those left on Earth have no idea what's going on in the Koprulu sector. Sending another fleet would probably not be their priority.
 * The Daribi, a race of alien invaders from the Global Guardians PBEM Universe, are transplanted Neanderthals. They were rescued from extinction on Earth by Sufficiently Advanced Aliens and taken to a world that wasn't actively killing them off.
 * While there aren't any actual humans in Spore (except maybe Steve) you can find planets inhabited by tribes or civilizations of your own species and abduct specimens of other species and drop them off on planets.
 * The reason why humans, orcs, goblins, elves, dwarves etc can be found on so many worlds in the old Might and Magic verse is that the Ancients (who may or may not be humans/transhumans from Earth) put them there (you can even visit part of the ship that brought your ancestors to Enroth in Might & Magic VI).