What Diamond means is that human beings became much more sophisticated at this time. This is the time that he calls the "Great Leap Forward." It was at this time, Diamond says, that human beings became fully human.
We do not know what caused this change to happen, but we can see how different humans became. Before, they had very rudimentary tools and could only kill a few kinds of prey. After, they had all sorts of tools and could kill even large animals. Before, there is no evidence of what we might call culture. After, we start to see things like jewelry.
This can be seen as the beginning of human history because at that time people became more intelligent and came to have much more of social lives that are clearly identifiable as human.