Charles Sheffield 
Proteus in the Underworld [EPUB ebook] 

In the 22nd century biofeedback techniques to control by will the processes of one’s own body have reached their ultimate expression: the ability to transform the body into virtually any viable form whatsoever. What began as an innocent technique to reduce anxiety without recourse to drugs has raised fundamental questions about what it is to be human, since form is no longer sufficient nor even relevant.Enter the Humanity Test: in a future when other techniques can change the forms of animals, so far it has been a guaranteed one hundred percent successful means of determining whether a life form started out as human. But now strange life forms, vicious and bestial, are proliferating throughout the Solar System. They are clearly not human, and clearly their nervous systems are too underdeveloped for them to have been human. But though the beasts threaten havoc and death to all the far flung isolated stations, the simple solution of shooting the varmints is impossible: for life forms that according to the Humanity Test started out human the law is very clear: Thou Shalt Not Kill.