Fredric Brown's "Keep Out" (short story, racism, free)

First batch of 20 year olds, humans adapted for Martian conditions by giving their volunteer parents a drug called "daptine" before conception & then by raising them in gradually changing environmental conditions that are now nearly Martian, have reached their graduation day: tomorrow, they will be released unprotected into Martian wilderness - to make the world their own.

What their older handlers haven't accounted for, however, is what the story says is a key human attribute: hating others that don't look like us.

See also.

Changing human anatomy physically to colonize other worlds or to inhabit greatly changed living conditions is an old trope. Some other famous stories of the kind:

1. Poul Anderson's "Call Me Joe" (download comic book adaptation as part of a larger package), Clifford D Simak's "Desertion": Human colonists ... kind of ... mind dump themselves into animals especially created to live on the solid surface of Jupiter.
2. James Blish's "Surface Tension" (download MP3): Stranded humans on an alien world are about to die, & must create humans in new microscopic bodies where they can not only survive, but eventually discover their human roots.
   
   Among the most famous stories of this kind.
3. Murray Leinster's "The Mad Planet" (download): Major changes in oxygen/carbon dioxide ratio of earth's atmosphere means only the tiny fraction of those humans with bodies substantially changed to withstand this atmosphere can live.

Fact sheet.

First published: Amazing Stories, March 1954.
Rating: B.
Download full text from Project Gutenberg or Manybooks. [via QuasarDragon & Marooned]
Related: Stories of Fredric Brown.