For example, humans are attempting to colonize an alien planet, but unfortunately, it's quite unpleasant to live on: to actually survive on the planet, you'd either have to wear space suits and build sealed cities to shield you from its atmosphere and hazards, or go native. But if you have the technology, you can do exactly the latter: Change and adapt your species to enable you to survive on the planet and become one of its native inhabitants.
There are two ways of doing this: The short-term version affects specific individual persons only, while the long-term solution affects the entire colony, becoming a part of the planet's evolutionary chain, possibly becoming a dominant species with the original humans remembered only as ancient Precursors. Over time, the transformed may also become mentally alien as well as they adapt to the planet's nature, or the mental changes are immediate and directly connected to the process of transformation.
If the transformed species ever meets its original version, expect to find An Aesop about What Measure Is a Non-Human? (if from the humans' point of view) and/or Humans Are Bastards (if from the natives' point of view).
- A Doraemon Tool allows an individual to adapt to any environmental condition, no matter how extreme (high temperature, high gravity, non-breathable atmosphere, etc). The drawback is that it must be reapplied to each individual every 24-hours. Giant and Tsuneo was nearly crushed by the oceanic pressure when they wandered away from the underwater summer camp without Doraemon's permission.
- Marvel's 1970's Guardians of the Galaxy series. Lead by Vance Astro, the secondary members of the team were former humans bio-engineered to survive the harsh conditions of the other planets of the Solar System. http://www.newsarama.com/comics/080606-FBGuardiansGalaxyv1.html
- Marvel Comics: The Atlanteans are a magic-based version of this. They are descended from worshipers of Neptune, who transformed them into water-breathers to save them from the fall of Atlantis. Their Pacific Ocean cousins, the green, scaly Lemurians, were further transformed by their rulers' use of a cursed artifact, The Serpent Crown.
- DC Comics: The Atlanteans are descended from survivors of the domed city of Poseidonis on the continent of Atlantis. Their defensive dome (originally intended to repel invaders) kept the water at bay after Atlantis sank long enough for their scientists to alter their genome to make them amphibious (though heavily water-dependent) humanoids. An offshoot that had used magical means to reclaim the neighboring city of Tritonis after a science/magic cultural schism became fully aquatic merpeople, due to their leader cursing them for accepting help from scientists.
- This is a major part of Avatar's plot, though humans don't change their bodies, instead remotely controlling eponymous avatars.
- In Pandorum the colonists were injected with mutagens designed to help them adapt to Tanis. But because the Elysium was cut off the first ones to wake up evolved into albino cannibals over the centuries.
- The Honor Harrington universe has several planets whose colonists altered their kids with genetic engineering, including multiple degrees of Heavyworlder like the Sphinxian titular character, while Grayson has so many heavy metals in the environment that even the adapted colonists need to grow their food in isolated domes.
- Clifford Simak:
- In the City series of novels, the only way to survive on Jupiter is to become a Jupiterian. When such a technology is discovered, people find out that Jupiterians are absolutely superior to humans in every single way: they are immortal, super intelligent, telepathic, etc. Thus everybody just moves to Jupiter, and humanity as a species ceases to be.
- Way Station briefly mentions a race of aliens who colonized numerous planets, regardless of the conditions, by physically and mentally changing themselves every time to perfectly fit those conditions. The main character wonders whether can they be counted as the same race.
- In one Sergey Lukyanenko short story the protagonist, who possesses the ability to automatically adapt to anything perfectly, is hired to deal with a dangerous predator on a distant planet. Gradually adapting to the Death World in question he understands that the dangerous predator in question could not possibly hunt humans for fun as described, as he actually became exactly like him, physically and mentally. When the two finally meet, they start playing as cats do... which gives the colonists a perfect opportunity to kill both with a single rocket, since both aren't technically human and they don't have to pay him this way. He survives, and briefly mentions before leaving that they are the ones who are not human.
- In the Hyperion Cantos, we eventually learn that this is how the Ousters came to be — pioneers of interstellar travel, they had to radically alter their biology to survive. When rest of the humanity finally caught up with Hawking drives and means for terraforming, Ousters became persecuted.
- The stories in James Blish's collection The Seedling Stars are all about adapting humans for new planets. For example, in "Surface Tension," humans colonize a mostly water-covered planet by creating a race of humanoids out of their own genes hand-crafted to best suit this planet, and leaving all their knowledge, up to and including how to build spaceships, in form of tablets to be read when they develop enough to manage to do so. The driving point of the story is that being made to perfectly suit the world in question includes being microscopic.
- In Jack Chalker's Downtiming The Night Side, it's said that most of the humans who moved to other planets had to radically alter their bodies to survive; most don't even look human now, though they still think of themselves as such.
- In All Tomorrows, humans intentionally modified themselves to live on Mars. Then they went on to colonize other worlds, but kept their new form — large chests, bigger heads, slender hands and so on. Then the Abusive Precursors stepped in and modified the humans on each planet into something that can perfectly live on the world they colonized, but is rarely even humanoid, just to have fun.
- The Sci-fi novel I, Weapon deals with a universe that is essentially the aftermath of this with hundreds of variant human species who have adapted to the various planets of the universe, albeit in some cases with extensive genetic manipulation.
- The Vorkosigan Saga gives us the "Quaddies"; humans genetically engineered for life in microgravity, the most drastic and visible alteration being a pair of extra arms where a baseline human's legs would be. They were created as an outright Slave Race, and ended up having to flee Earth en masse when Artificial Gravity made them redundant, and by the time of the main series they've built themselves a thriving civilisation.
- Hayven Celestia: Sourang have the ability to willingly adjust their own biochemistry and incorporate genetic material from other species, which allowed them to survive the sulfur-breathing krakun's Hostile Terraforming of their homeworld. They can also inject other species with a serum that replaces the host's cells with sourang cells; they mostly use it on slaves.
- In Andromeda there are several subspecies of human genetically engineered for different planets, including heavyworlders like Dylan's mother, Castalians with gills, Inari adapted to volcanic planets, and Nietzscheans who were designed as generalists that were even more adaptable than baseline humans.
- In Transhuman Space, Mars is terraformed, but not to the extent that normal humans can cope with the conditions. Most of the population still live in pressure-domes, but a sizable minority have been "bio-modded" to fit the environment. In addition a large minority of the Rust China population are bioroids genetically engineered for Mars.
- Similarly Eclipse Phase has "Rusters" adapted to semi-terraformed Mars, as well as "Hazers" that can take the extreme cold of Titan slightly better and "Dvergar" Heavy Worlders for a few exoplanets with high gravity.
- Warhammer 40,000:
- The Tyranids start out an invasion by dropping 'nids with different mutations on a planet and multiplying the ones that survive best. They also absorb genetic material from organisms they consume.
- Fenrisian Wolves (which can grow to the size of an APC) are far more intelligent than actual canines because they're descended from genetic experiments that helped humans survive the arctic conditions on Fenris.
- In Killzone, the Hellghast are the result of colonists adapting to a Death World. They aren't happy about it.
- The Zerg of StarCraft acquire new mutations similarly to the Tyranids. Half the "evolution missions" in Starcraft II Heart Of The Swarm involve strains that evolved from exposure to hostile environments (up to and including nuclear missiles) and the other half involve killing and consuming creatures with useful genes.
- Evolva: According to the backstory, the Genohunters are used to exploration tasks because of this trope. They somewhat show it in the actual game: the DNA you acquire from the native wildlife will improve your physical condition, thus allowing you to perform better across the levels.
- Sword of the Stars has several technologies that modify your colonists to reduce the "hazard ratings" of planets, making them easier to colonize.
- In Stellaris the Glandular Acclimation tech allows you to modify the traits of species in your empire, including changing their environmental preferences on certain planets. Though it may sometimes be easier to uplift species that evolved on planets with that environment.
- After an empire has researched Gene Tailoring there's a chance of populations on undesirable planets attempting to modify themselves without waiting for the perfected tech, with random results on their traits and a significant chance of leading to racial tensions between them and the unmodified population.
- In Civilization: Beyond Earth, colonies following the Harmony affinity will adapt themselves to the planet via genetic adaptations taken from the native wildlife, to the point that the normally toxic and corrosive miasma will heal their units like it does the alien life.
- The 'Amphibians' of Gene Catlow colonize other planets using this method - but on Earth, they screwed up somehow. Seeing as Earth is mainly ocean, they figured that an amphibian form would work best there - while failing to notice intelligent life evolving on dry land at the same time.
- Florence, an uplifted wolf, is a 'proof of concept' for a process designed to uplift and educate a species from a fertile-but-biologically-hostile world called Pfouts - rather than altering humans to be alien, it's a case of altering aliens to be human.
- Winston is a human whose parents gave him "spacer genes" for long-term life in space. No hair, resistant to radiation, no skeletal degeneration in microgravity, and the ability to hibernate through months-long voyages. Ironically, he's space-phobic.