What is Jared Diamond's thesis in Guns, Germs, and Steel and what question is he trying to answer?

Jared Diamond introduces Guns, Germs, and
Steel with a question asked him by Yali, an aboriginal New Guinean
leader:

"Why
is it that you white people developed so much cargo and brought it to New Guinea, but we
black people had little cargo of our
own?"

The term "cargo" means
manufactured goods and other rare and expensive objects. As Diamond reflects on this
question, he also becomes curious about the reasons for unequal development in various
areas of the world. He mentions that it obviously has nothing to do with hard work or
natural ability, as there are both smart and less intelligent and hard-working and lazy
people in all cultures and regions.

Diamond's main thesis
is that geography and physical environment have, to a large degree, placed constraints
upon how societies can develop. He investigates various topics such as the availability
of domesticable plants and animals, the barriers to travel to encounter and trade with
cultures at the same latitudes, and other characteristics of natural resources and
environment to argue his main thesis, that it was differences in external environmental
factors, rather than intrinsic superiority, that led certain civilizations to become
wealthy and powerful.