The answer to this can be found in Chapter 12 of Guns, Germs, and Steel. In that chapter, Diamond discusses the invention of writing, its importance, and how it diffused from place to place. Diamond identifies the first writing system on p. 218 of the paperback edition of the book.
The civilization that invented the first known writing system was the Sumerian civilization of Mesopotamia. This civilization invented a form of writing sometime before 3000 BC. The name of the system of writing is “cuneiform.” This is a name that modern people have given to the system. It means “wedge-shaped” and is used to describe the writing system because the markings used to write are wedge-shaped. Cuneiform was written by pressing styluses into wet clay to make marks. The clay tablets were then baked to make the writing permanent. Writing is important to this book because it was one of the factors that made civilization possible and allowed some societies to come to have more power than others.