What aspects of Diamond's evidence in Guns,Germs,and Steel does Diamond prove or explain for lay readers?

In this book, Diamond does what I think is a very good job
of explaining all of his evidence.  That is, however, not the same thing as saying that
he proves it.

Some of Diamond's evidence is of the sort
that can easily be proved.  For example, when Diamond says that various animals like
zebras have never been domesticated, that is a claim that is quite easily proved.  Other
aspects of Diamond's evidence are well explained but are not proved because they simply
cannot be proved.  For example, it is not possible to prove that Aboriginal Australians
would ever have domesticated plants if they had had the chance.  Diamond argues that
they would have and he presents good points to support that argument.  But that is not
the same as proof.  The very nature of this sort of argument (that a group of human
beings would have acted differently in different circumstances) makes them impossible to
ever prove.

So, Diamond explains all of his evidence, but
not all of his explanations constitute proof.