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.