What might be causing this blocky shading on my model? I have a model that I sculpted and retopologised in another application, and then imported the retopologised mesh into Blender with a normal map.

The mesh is around 9200 verts, and I have smooth shading turned on.

I am getting an odd shading artifact in the form of jagged edges: 


While the surface is not perfect, it seems more than smooth enough that this artifact should not be happening.

The object has a normal map applied (baked from the high quality sculpt), but I can turn that off an get the same thing:



Here is a shot of the wireframe:



Is this abnormal like I think it is? What might be causing it? I have checked the mesh for everything I can think of.

EDIT: I have made the file available here (20M, sorry):
The mesh is fine. The primary cause of the geometric shadow is the texture co-ordinates you have given the HDR environment. The simple HDR setup here...



...results in a smooth shadow.



As reference: Trouble Viewing Whole HDR Image as cycles world

The PBR node you have added obvious effects the edges...