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

The
thesis of Diamond's book is that, in the words of the author:

History followed different courses for different peoples because of differences among
people's environments, not because of biological differences between peoples themselves
(25).

He claims that this view is largely eschewed by
most academics. Historians in particular think it diminishes the importance of human action, and
argue, according to Diamond, that it is deterministic. In other words, some say that Diamond's
view seems to suggest that a people's environment determines everything else about their
development, from their culture to the technology they develop. Diamond aspires to write what he
calls a "unified synthesis" of a range of disciplines, including human genetics,
history, archaeology, evolutionary biology, and epidemiology (26). To put it another way, he
says that human history must be established as a "historical science, on a par with
recognized historical sciences such as evolutionary biology, geology,...