In Guns, Germs, and Steel, Diamond claims that there is a relationship between farming and societal development. Where does he get the evidence to support his claim?
Did humans living at the time create documents (like diaries and newspapers) that survive to this day?
There are no diaries or newspapers that survive from the times that Diamond discusses in most of his book. However, there are written documents of sorts. For example (as Diamond discusses in Chapter 12) there are ancient Sumerian clay tablets that survive to this day. We can see from these that some of the earliest writing has to do with things like inventories of agricultural products. This provides some evidence of the link between farming and societal development. The other major source of evidence that Diamond uses is the evidence of societies that survived into modern times. Diamond can (as he does in Chapter 14) look at, for example, Native American societies to show that farming typically went along with the development of more complicated societies.