According to Guns, Germs, and Steel, what are the ultimate factors that address Yali's question?

The best place to find the answer to this question is in
Figure 4.1.  There, Diamond shows us what he sees as the ultimate factors that led to
the proximate factors that have allowed Europeans to have (as Yali pointed out) so much
wealth and power.

There are two or three ultimate factors
(depending on how you count them), both of which are geographical.  First, there is the
presence (or absence) of many wild species of plants and animals that could be
domesticated.  Second, there is the ease (or lack thereof) of species spreading, which
is caused largely by the length of a continent or region's east-west axis.  These are
the most basic factors that determine, according to Diamond, which societies have become
rich and powerful and which have not.