Science fiction is full of stories in which extraterrestrials engineer life on Earth — it's one of the main ways we explain humanoid aliens, after all. But why would aliens want to create the human race? It seems to be a lot of trouble to go to. Here are the most outlandish reasons for aliens to put human beings here.
The Hitchhiker's Guide to the Galaxy by Douglas Adams
Purpose: To find the answer to the question of Life, the Universe and Everything.
Turns out the whole Earth is a kind of biological computer, designed to find the answer to the ultimate question — and the mice are actually the aliens who designed it, keeping an eye on their experiment. (While pretending that they're letting us experiment on them.) Too bad the Vogons destroy the whole thing just a few minutes before the result appears.
Doctor Who, "Image of the Fendahl"
Purpose: To resurrect an ancient space monster who is Death
There are about 100 species that claim to have guided human evolution, or somehow helped to steer the human race, on Doctor Who. Including Scaroth in "City of Death." But perhaps the most complex scheme belongs to the Fendahl, which sends its essence in a skull (with a pentagram on it) to Earth, millions of years in the past. This skull then reprograms the DNA of all life on Earth until it leads to humans — so that eventually, Professor Fendelman will do his experiments with time visualization and find the skull, leading to his assistant creating a weird Satanic science cult. The skull planned all of this in detail.
The Sirens of Titan, by Kurt Vonnegut
Purpose: Manufacturing, Communication Technology
Okay, the Tralfamadorians didn't precisely create humans, but they did manipulate our entire development just so we could grow up and produce a small metal part needed by one of their stranded brethren. Stonehenge? The Great Wall of China? Just ways of giving the stranded Tralfamadorian an update on our progress.
The book was—unsurprisingly—a major influence on Douglas Adams:
"I've read The Sirens of Titan six times now, and it gets better every time. He is an influence, I must own up. Sirens of Titan is just one of those books – you read it through the first time and you think it's very loosely, casually written. You think the fact that everything suddenly makes such good sense at the end is almost accidental. And then you read it a few more times, simultaneously finding out more about writing yourself, and you realise what an absolute tour de force it was, making something as beautifully honed as that appear so casual."
Bio-Booster Armor Guyver, by Yoshiki Takaya
In this popular manga series, humans were created by a group of aliens who hoped to use us as bio-weapons (the plan never came to fruition).
The series later served as the source material for a number of film and television adaptations, including 1991's camp-tastic The Guyver, starring Mark Hamill.
Assassin's Creed Franchise
Purpose: Slave labor
The Roman gods are actually slave-masters in this video game franchise. Known as the First Civilization (a prior Earth-based people), they created humans that looked like them but were also a sturdy and obedient workforce.
In another 1990's sci-fi spectacular featuring Mark Hamill, aliens came to Earth 250 million years ago to plant a delicious crop of humans. Now they're back, and they're hungry.
South Park, "Cancelled"
Originally intended to be the series' 100th episode (although it actually ran as number 97), "Cancelled" reveals that life on Earth is actually one enormous alien-run reality show. To produce Earth, the aliens import various "species" from their actual home planets—Asians, bears, Jews, and ducks are among the imports. The show is threatened with cancellation when the stars discover they're being filmed, but a little blackmail and some memory-alteration sorts the whole thing.
"Attention Universe! Be sure to tune in next week for another exciting episode of Earth! The Asians are really stewed at the Russians, the zebras try to get along with the buffalo, and Americans and Iraqis have an all-out brawl!"
Purpose: Spare Parts
When an enormous bio-weapon intended as an interplanetary invasion system has to crash-land on a nearby planet, it finds itself in need of repair. The solution? Create a bunch of humans you can use for spare parts (it only takes 10,000 years).
Bonus That Doesn't Quite Fit: "Jokester" by Isaac Asimov
Purpose: An Extraterrestrial Experiment
In this Asimov short story the human sense of humor was actually implanted into our heads as part of an alien experiment on human psychology—an experiment described as being similar to making rats solve mazes. Once the characters in the story become aware of the experiment it loses its usefulness, humor is taken away, and nothing will ever be funny again.