This is an old conundrum. English has no proper neutral pronoun to either a male or female person. It is usually used to refer to inanimate or not-human things.
You could say:
The power to become habituated to one's surroundings is a marked characteristic of mankind.
But the usage of one as a pronoun has come to sound a bit pretentious. I believe there is a shift occurring in english where their is being used in the singular and becoming acceptable.
The power to become habituated to their surroundings is a marked characteristic of mankind.
Some would take it a step further.
The power to become habituated to their surroundings is a marked characteristic of humans.
But that's somehow less poetic. Humans seems more clinical, and mankind more familial and grandiose somehow.
one'ssurroundings – shinynewbike Jan 18 '11 at 7:57