The Seedling Stars — James Blish

You didn't make an Adapted Man with just a wave of the wand. It involved an elaborate constellation of techniques, known collectively as pantropy, that changed the human pattern in a man's shape and chemistry before he was born. But the pantropists didn't stop with biology. Education, thoughts, ancestors and the world itself were changed, because the Adapted Men were produced to live and thrive in the alien environments found only in space. They were crucial to a daring plan to colonize the universe. And millennia later, it is only fitting that they should return to a long forgotten planetary system to colonise a hostile world called . . . Earth.