What happened in human history by 11,000 BC that Diamond finds significant in Guns, Germs, and Steel?
You can answer this question by reading at the very beginning of Chapter 1. There, Diamond tells us why he is using 11,000 BC as the time at which he wants to evaluate the relative levels of development in each of the inhabited continents of the world.
Diamond chooses 11,000 BC for three main reasons. First, there was clearly a move towards civilization going on in some parts of the world. Diamond says that village life was beginning. Second, the Americas clearly had people in them by this point, meaning that all of the continents except Antarctica were populated. Finally, this was the end of the last Ice Age.
These events make 11,000 BC a good time, in Diamond's mind, to compare levels of development in the various parts of the world.