Silicon humans do exist in Software. Cobb Anderson becomes one, when his brain is processed and modelled by Mr. Frostee. Rucker's belief that neurochemistry, brain scans, etcetera, are enough to allow a computer simulation of identity--with no perceptable change in the person--is somewhat dubious (even Greg Egan admits that there will be a huge jump in the ratio of external time to "silicon time" due to limited processing power), but it's a common enough trope in cyberpunk.
Manchile is far more interesting. He's the reversal of the silicon human, a biological encoding of a bopper's thoughts in a flesh body. As he puts it:
I might as well come out and tell you. The boppers designed me from the ground up. I started out as a fertilized egg--an embryo, really--and the boppers had a meatie plant it in Della. Kind of a tinkertoy job, but it came with a whole lot of extra software. That's why I know so mucj; and that's how I can synthesize my own gibberlin and grow so fast. I'm a meatbop. My sperm cells have two tails--one for the wetware and one for the software. My kids'll be a lot like me, but they'll mix in some of their momma's wetwares.
If one posits the existance of artificial intelligences, why shouldn't they be able to tinker with the human gene pool just as efficiently as humans tinker with computers? Given smart enough AIs, maybe they really could cook up something that could replace the human being as the dominant species on Earth.