How does Diamond determine the "subject" of Guns, Germs, and Steel (p.16)?

Diamond makes it clear in the Prologue to Guns, Germs, and Steel that he set about writing the book to answer "Yali's Question," a query posed by a man in New Guinea by that name. Yali asked why white people had so much "cargo," or manufactured goods, when his people had so few. This leads to Diamond's question, articulated on page 16:

[W]hy did human development proceed at such different rates on different continents? Those disparate rates constitute history's broadest pattern and my book's subject.

He goes on to say that this question is of "overwhelming practical and political importance," because the interactions that proceeded from and in many cases created these inequalities have continued effects today. Not only that, but Diamond's answer to this question downplays many of the cultural and ideological (not to mention racial) explanations for inequalities between societies. A host of the world's most important problems, Diamond argues, "result from the historical trajectories implicit in Yali's question." So, in short, Diamond regards the origins of human inequalities as among the most important subjects confronting scholars today, and he attempts to provide an answer to why these inequalities exist in Guns, Germs, and Steel.