You can find the answer to this by reading in Chapter 1. There, Diamond tells us that human beings became what we might call fully modern in a cultural sense during the Great Leap Forward. Before the Leap, people were anatomically modern. However, the artifacts they left behind imply that they were not fully modern in terms of their intelligence and their culture. For example, they had only a very few rudimentary tools. In addition, they left behind no art. This is what changed in the Great Leap Forward. People started to make and use many more kinds of tools. They also started to create a great deal of art. These achievements characterized the cultural changes of the Great Leap Forward.