The answer to this question can be found in the Prologue to Guns, Germs, and Steel. Basically, Diamond does not agree with the traditional theories of human development. The whole point of this book is to offer an alternative to those theories.
When we talk about “human development” in this context, we are talking about how societies develop. We are asking why it is that some societies develop more than others; why some societies become more powerful and more wealthy than others. Some traditional theories have typically focused on things such as race or culture. They have said that the people who succeed are simply better than those who did not. Other traditional theories have focused on geography in a narrow way. They have speculated, for example, that people in cold climates have harder lives and therefore have to be smarter if they are to survive.
Diamond disagrees with all of these theories. He thinks geography is the major influence on human development, but he thinks that it does not affect people in the way traditional theories say. Diamond believes that geography affected who got farming first and that fact influenced who became powerful. Thus, Diamond rejects traditional theories and offers a theory of his own.