In the Preface of Guns, Germs and Steel what is suggested about the roots of European dominance in the modern world?

If you are asking about the Preface (rather than the
Prologue), Diamond is arguing there that modern European dominance comes from events and
conditions that were found far back in history.  He says that explanations for this
dominance tend to focus on "proximate" causes like capitalism or religion.  He argues
that this is insufficient.  Instead, he says, any explanation of European dominance
should look deeper into the "onion" that he uses as a metaphor for world history.  He
says that the real roots of European domination are not to be found in recent things
like capitalism but rather in much older things.  He does not say what these ultimate
causes are in the Preface, but we know that they are things like the geographic
distribution of plant and animal species.