Copyleft uses copyright law to enforce a different scheme, a social contract. The license, which can only be assigned by the holder of copyright on a work, insists that licensees (you, if you have access to my work which I've put under an appropriate license) must also allow copying by anyone, even when you modify my work.
I don't think this is an evolution, (the parent to your comment doesn't actually claim so); I think it was the entire original intent by Stallman. What has evolved is the terms of the GPL license, attempting to keep up with changes in technology.
--- some old TV show