I can completely identify with Grey's troubles with arguments about Guns, Germs and Steel. While I've not had discussions/arguments about that book specifically, I often find myself in very heated discussions about politics and society.

When I was studying Human Geography at university we spent a lot of time thinking about the dichotomy of structure vs. agency. It seems that in many case people come to arguments from the agency end of the spectrum where people have free will. This is often the position that economists take when looking at the world, especially when talking about free market ideals. In my view that approach is overly reductive and ignores the unimaginable levels of complexity in the world.

At the other end of the spectrum, structure recognises that there are inherent environmental, physical, economic, social, political and any list of other constraints that we all face in life. From what I know of Guns, Germs and Steel, that's exactly the argument that it puts forward.

I think that reality sits somewhere on that spectrum and precisely where is the point of the whole debate. Yes people overcome constraints but they also make decisions which seem odd if you assume that everyone has full agency and only makes decisions that benefit them directly.

Of course, this being the internet, people adopt and become entrenched in extreme positions to the exclusion of other viewpoints and just shout at one another without considering that maybe they don't actually disagree as much as they think.