Jack Williamson 
Brother to Demons, Brother to Gods [EPUB ebook] 

Brother to Demons, Brother to Gods begins with a genetic engineering program conceived by the Smithwicks, a family of "Creators" who first improve on inadequate Homo sap ("preman") with two laboratory species and then come up with the "stargods." These powerful Frankenstein fabrications, however, send the Creators chasing back to their labs in a last desperate attempt to undo the consequences of their folly. A thousand years later, two "preman" waifs, Davey and Bugler, find unsuspected powers genetically programmed into their minds and bodies, and realize that they are to be the parents of the liberating "ultiman."