Humans have all but left Terra (Earth) and spread out over outer space.
It starts out with a man and women dis-formed and mutated. They go to a disreputable embryo dealer. And they are told it is the last pure blood Terran.
The child is raised on a planet of dactyl humanoids. When a slave circus comes to town, he sneaks in by climbing a wall. He is caught and made a slave.
There is a giant rock creature and other weird beings. Ultimately he makes his way to Terra, and almost dies there due to a lack of Terran antibodies.