The Copyright Act of 1976 is the big recent overhaul of US copyright, which switched copyright to automatic registration and established regimes for unpublished and orphaned works, which was previously largely handled by state laws, usually under common law.

The requirement that the author must hold copyright seems to be implicit in earlier versions of copyright law all the way back to the 1790 Copyright Act; I don't see any provision that would let one copyright a work one was not the author of.

Registration creates prima facie evidence that the registrant is the legitimate owner of the copyright. Anyone who disputes the claim in the face of a registered copyright has the burden of proof to demonstrate that the registration was erroneous.