With regard to Guns, Germs, and Steel, how can geography be used to answer Yali's question?
Basically, the answer to this question is the entire book. Diamond spends all of Guns, Germs, and Steel telling us how geography answers Yali’s question.
The basic idea here is that geography determined where agriculture would arise first and, thereby, it determined where technology, organized government, and infectious diseases would arise. Because Europeans got all of those things, they were able to have more “cargo” than the people of New Guinea and other non-Europeans.
Diamond argues that geographic luck caused Eurasia to get agriculture earlier than any other area of the world. Eurasia had more kinds of plants and animals that could be domesticated. It also had topography that allowed agriculture to spread. Because Eurasia developed agriculture first, it also developed settled, high-density civilizations first. This allowed it to have more technology than anyone else by the time that Europeans went exploring. It also allowed infectious diseases to arise in Eurasia but not in places like the Americas or Australia.
In short, geography gave Europe agriculture. Agriculture gave Europe guns, germs, and steel. Guns, germs, and steel allowed Europeans and their descendants to become the richest and most powerful people in the world by the time that Yali asked his question.
Jared Diamond states that his work, Guns, Germs, and Steel, is an attempt to answer Yali's question: "Why is it that you white people developed so much cargo and brought it to New Guinea, but we black people have little cargo of our own?" Many critics have described Diamond's approach to answering this question as a form of "geological determinism."
Diamond argues that the differences in technology and wealth we now observe in different areas of the world have nothing to do with differences in people's innate abilities, cultural traditions, or how hard they work, but are instead grounded in geography, including such factors as climate, transportation links, and natural resources.
He argues that certain geographical regions have plants and animals that can be easily domesticated, something that facilitated the "neolithic revolution" in agriculture and created the food surpluses that allowed specialization of labor and development of technology. Easily accessible metal and coal facilitated development of more advanced technology as did convenient east-west trade routes which accelerated the development of agriculture.