I read this in the late 80's early 90's as a long paperback novel. The earth hits a cosmic string, humans DNA is damaged. The main character is a "White Card" his DNA is pure. He is a slave/worker for someone who recognizes that he is a white card and is trying to make him into a "Protoman". He kills the guy and tangles his body in a fishing net to give the appearance that it was an accident. He escapes. The rest I don't remember much.
It finally dawned on me that I hardly ever bought books randomly. They were either from one of my favorite SF authors or won a major award. So I looked through all the SF award winners for that time and found it.
Radix - 1982 Nubula award winner