Panspermia

Lots of people wonder about the origins of life on Earth. Were we placed here by a benevolent god? Did we arise out of a primeval soup of microbes? If the former is the case--well, why? And who? And how? And if it's the latter, how exactly did the vast chemical stew of hydrocarbons and proteins somehow get together and become self-replicating? And then how did those cute little squirmy germs grow and change and evolve enough to become, well, us?

Some stories attempt to answer these questions. But others decide to skirt this touchy ontological issue entirely, by bringing into it a different element: Outer space.

In other words, they propose that life on Earth is not of this earth.

Panspermia (get your mind out of the gutter) is a scientific idea which proposes that life on Earth came to it from outer space, as aliens. This is a real, scientific theory, mind you. (That Other Wiki, of course, has plenty of information.) This isn't the "Little Green Men"-style of alien, either. We're talking more along the lines of "cosmic pond scum hitching a ride in some water vapor in a comet." In fiction, panspermia (or exogenesis, which is similar but not identical) tends to gravitate toward the former, though--that aliens of some kind were actually the origin for life on Earth, or some other planet.

In fiction, this idea tends to pop up in one of two forms:


 * Literal panspermia and/or exogenesis. Microbes from space landed on Earth (or another planet, in more sci-fi oriented settings), and evolved into the lifeforms that now populate that planet. This can be used both to get around the idea of having to answer how life arises in the first place, or as a justification for the similarities between life forms on very different planets.
 * "Alientelligent Design." Sufficiently Advanced Aliens planted either the seeds for life or primitive multicellular lifeforms on a planet to begin with, and, depending on the type of alien, either left them to their own devices or "guided" their evolution in a large-scale Xanatos Roulette.

Frequently, this as used as part of a Reveal, and can lead to navel-gazing. Occasionally leads to Mars Needs Women, in cases of literal panspermia. See also Transplanted Humans, which is this trope in reverse (alien life coming from Earth).

It may be worth noting that while this theory may offer an explanation for how life got its start on Earth (and/or other relevant planets), it ultimately does not in and of itself answer the question how life arose wherever and whenever it did so for the first time.

'''Due to its nature as part of The Reveal, examples for this trope are frequently full of spoilers. Be warned.'''

Anime & Manga

 * In Neon Genesis Evangelion, Humanity (indeed, all life on Earth), and Angels were both created by Sufficiently Advanced Aliens who had a thing for spreading life all over the galaxy. Don't believe me? Its All There in the Manual.
 * Life on Earth was manipulated by the Creators/Advent/Uranus in Guyver as part of their goals to create a powerful bio-weapons platform. Humanity was the end result, a base form that could be adapted into specialised combatants like Zoanoids and Zoalords. The experiment was aborted near completion when they gave a base level human one of the bio-armour units that worn by the Creators themselves and it turned out not only far more powerful than expected but completely immune to their telepathic control.
 * In Super Dimension Fortress Macross both humans and giant humanoid Zentradi are stated to be descended from an extraterrestrial race of Precursors called the "Protoculture".
 * Life on Earth was manipulated by the Creators/Advent/Uranus in Guyver as part of their goals to create a powerful bio-weapons platform. Humanity was the end result, a base form that could be adapted into specialised combatants like Zoanoids and Zoalords. The experiment was aborted near completion when they gave a base level human one of the bio-armour units that worn by the Creators themselves and it turned out not only far more powerful than expected but completely immune to their telepathic control.
 * In Super Dimension Fortress Macross both humans and giant humanoid Zentradi are stated to be descended from an extraterrestrial race of Precursors called the "Protoculture".

Comic Books

 * The Polish comic series from the 80s, Expedition told the story of how Sufficiently Advanced Aliens come to Earth and help apes evolve in their image, creating the basis for several religions and mythologies in the process.
 * DC Comics Presents #1-2 has an alien race whose Living Ships' exhaust was microscopic organisms that started life on both Krypton and Earth, among other planets. (This story has been reprinted because it's also a Superman/Flash race.)

Film

 * This was The Reveal of the science fiction movie Mission to Mars.
 *  ~2001: A Space Odyssey~  - The aliens didn't necessarily seed Earth, but most definitely influenced the evolution of mankind.
 * In the Mark Hamill film Laserhawk, aliens seeded life on Earth so they could come back millions of years later and harvest us for food.
 * There is another race whose goal is to sabotage these seeding efforts. They attempt to do so on Earth but fail.

Literature

 * The backstory of the Lensman novels states that all life in Earth's galaxy (and I believe the Second Galaxy as well) came from Arisian spores. Mentor tells at least one Lensman that this is why he's offering the Lens to the Galactic Patrol -- they're "family".
 * This appears to be the case in Dan Brown's Deception Point.
 * The Hainish Cycle by Ursula K. Le Guin used this trope for human and semi-human life, spread by the Precursors in the title.
 * The more scientific sort is a suggested origin of all life in the CoDominium universe (at least until The Mote in God's Eye introduces truly alien aliens.)
 * The Ellimist (an extremely powerful Last of His Kind god-like being) in Animorphs was responsible for creating the Pemalites. His evil counterpart, Crayak, also created his own species--the Howlers.
 * What's more, he created the Pemalites specifically so that they would go out and "spread life around the universe".
 * Also, several Earth vegetables were apparently imported by crab people during the Cretaceous period.
 * This is alluded to -- and it's all the more memorable because it comes out of freakin' nowhere (it's very against the grain of the overall tone of the novels)- in the Quintaglio Ascension trilogy. Turns out that there was a ridiculously powerful species that lived in the universe that existed before this one (stay with me), all but one died when the universe we know was formed, and that one last being seeded different planets with life forms -- from Earth, which is the only planet where life formed naturally. Heady stuff.
 * Larry Niven has done this at least twice. In his Known Space future history, nearly all alien species evolved from food yeast grown to feed the Thrint and their subjects. Then, about 3 million years ago, a species called the Pak colonized the Earth and became Homo habilis.
 * One Authority story had the team face off against an alien life-form the size of the moon -- "the closest thing to God" that had "planted" life millions of years ago. Subverted in that life on the planet didn't develop as it should have, leading to, among others, the rise of humanity.
 * The Hitchhiker's Guide to the Galaxy reveals what earth really is. It is implied though that no organism on Earth is older than a few million years, and was likely brought in from other places. And, furthermore, humanity is.
 * Discworld example: In Eric Rincewind drops a sandwich in a tide-pool and the narrator wonders what life would have been like with mustard rather than mayonnaise.
 * The Darkness from Michael Grant's Gone (novel) is an alien virus that was riding a meteorite when it crashed into Earth -- specifically a nuclear plant, which caused it to rapidly mutate and somehow become a mind-devouring god-like being.
 * At the end of Last and First Men the solar system is about to be destroyed by increasing radiation from the sun. The Last Men devote their remaining time to sending out "the germs of life" on the solar wind.
 * This was part of The Reveal in H.P. Lovecraft's At the Mountains of Madness; it turns out that
 * In Sergey Lukyanenko's A Lord from Planet Earth, it is common knowledge among Human Aliens (no Starfish Aliens) that life on all of their planers began with intentional panspermia by the mysterious Seeders, who also left behind numerous Forgotten Technology and temples on each planet . What they don't know is that the Seeders are, in fact,.
 * One of the novels in Mikhail Akhmanov's The History of the Galaxy series reveals that biological life in the galaxy (possibly, the Universe) is an unintentional side-effect of an Energy Being's attempts to survive. The being was "born" in the magnetic fields of a gas giant but foresaw that its homeworld would eventually die. In order to escape, it created artificial proto-plasmic semi-sentient creatures called Forerunners. They would travel through space and spread the Energy Being to other compatible gas giants. The Forerunners were themselves mostly energy, but parts of them were organic and composed of rudimentary DNA (how else do you program an organism to do something?). Over billions of years, the Forerunners spread through the galaxy and, possibly, beyond. Some of them died, and their remains ended up on planets, unintentionally starting the process of life. This, of course, raises more questions than it answers, such as how the Energy Being came to be, and how it was able to create a semi-biological organism.
 * Not Earth per se, but with all the life in that galaxy far, far away, the Star Wars Expanded Universe must use this trope. The Precursors have definitely created the Corellian system, and a cluster of black holes known as the Maw.
 * It's actually that most sentient life colonised most of the habitable planets in the Galaxy for so long, people aren't completely sure of its origins, but Coruscant is suspected to be the homeworld of most Humans, along with most other Humanoid deviations. Most Planets do seem to have their own native species on them, so it's not COMPLETELY Panspermia.
 * The Celestials definitely made the Corellia System artificially, though the exact details of their influence, origins, and status as resident Sci-Fi Creator Gods are in question.

Live Action TV

 * Stargate Verse - All humans and human-looking aliens are Ancient-seeded one way or another.
 * Apparently, there's a machine that can be programmed to do this.
 * The Asgard appear to have evolved independently, though, despite the fact that their original form (seen only once) is very similar to that of the humans. It was the millennia of cloning that resulted in Clone Degeneration (apparently, keeping the original DNA on file is beyond a race of Sufficiently Advanced Aliens).
 * Star Trek uses the Sufficiently Advanced Aliens version to explain all its Rubber Forehead Aliens--they actually are related. (This still doesn't explain all the Rubber Foreheads that Voyager runs into in the Delta Quadrant...)
 * On the other hand they'd still be about as related as birds and mollusks.
 * In the 6th season Star Trek: The Next Generation episode "The Chase," it's revealed that the first starfaring species, which emerged 4 billion years ago, was so lonely that they seeded the entire Milky Way galaxy with a "genetic program" which would cause emerging life to, eventually, evolve into a form that physically resembled them. Since such a genetic program would likely herd all the different planets' developing genomes along the same narrow path, it actually makes sense that all humanoid species would be genetically similar enough to interbreed.
 * The Star Trek: Voyager episode "Year of Hell" establishes that life on all planets in the Krenim sector is the result of the same comet passing through their orbits in distant past.
 * In Doctor Who, the Time Lords were one of the earliest races to evolve, and they either seeded the universe with their genetic material or affected the fundamental properties of the universe itself, so it's not that Time Lords are humanoid, but humans are "Time Lord-oid."
 * Red Dwarf inverts this trope. Every single lifeform encountered, no matter how alien, is ultimately of Earthly origin. The first novelisation even claims that it has been proven that no other life exists in the Universe, although exactly how you can prove this isn't clear.
 * In the final episode of Space: Above and Beyond it was revealed that the aliens evolved from Earth bacteria that was deposited on their moon through panspermia.
 * In perhaps its biggest predictive stretch, an episode of Life After People proposed that this could happen to Europa, one of Jupiter's moons, which is believed to have water oceans. With no humans left to direct it, a deep-space probe crashes there and introduces Earth bacteria to these oceans, which gradually give rise to an entire complex aquatic biosphere.

Tabletop Games

 * Traveller, to some extent, as a means of explaining why Humans and Earth-Like life is everywhere.
 * In Magic: The Gathering, the plane of Mirrodin was created by Karn. He didn't put life on the plane, though. Memnarch, a creation of Dominaria's Mirrari, did. All that life is from other planes. However, Karn is responsible for some inhabitants of Mirrodin.

Video Games

 * In Fossil Fighters, it is eventually revealed that
 * And in the end,
 * Spore uses the proper definition of the theory. That is, alien bacteria hitching a ride on a rock to a different planet. This is the opening animation for the cell stage (the first stage). This was added to avoid answering the always difficult (and for now unanswerable) question of how life actually arose. And also to try to explain why all life in the universe is made of the same handful of parts.
 * Xenogears - life on the planet was created by
 * Though it does have some native life forms, as evidenced by Balthasar's paleontological studies. While their planet's fossil record goes back millions of years, no evidence of humans or related beings can be found any further back than ten thousand.
 * The Big Bad reveals this during his Hannibal Lecture at the end of X-COM.
 * The H'riak in Alien Legacy are a violent race whose goal is to seed the galaxy with violet life that attacks anything not related to the H'riak. The Centaurians and the Empiants are the two known examples of their work.
 * Not life per se, but human evolution is this way in Chrono Trigger: "Grown like farm animals, waiting for the slaughter. All our history, all our art and science, all to serve the needs of that...beast."
 * In both the Pact and subsequent OG-verses of the Super Robot Wars metaseries, all life in the universe was actually created by an ancient lost civilization originating on Earth, handily justifying the numerous invading Human Aliens that appear as antagonists. Strangely, other continuities, such as Super Robot Wars Judgement feature Earth life being created by aliens, which they're going to have a hell of a time working into the OG-verse.

Western Animation

 * According to the South Park episode "Cancelled", all life on Earth is one big intergalactic Reality Show, in which different species from other planets had been brought together for the amusement of the viewing public.
 * In the "Bolero" segment of Allegro Non Troppo, life on a planet (maybe Earth, maybe not) evolves from the gunk at the bottom of an astronaut's discarded Coca-Cola bottle.

Real Life

 * It's possible life on Earth started that way, but as of right now there is no solid evidence of life anywhere else in the universe for that life to have come from.
 * It has been pointed out that if life on earth did arrive this way, it still doesn't provide us any answers as to the origin of life - it just displaces where it originated from in the first place.
 * While this may or may not happen with other life forms, it can happen with viruses. Well, for SARS, at least.