The answer to this can be found on the very first page of Chapter 1 of Guns, Germs, and Steel. This is p. 35 in the paperback edition of the book. The reason why Diamond draws a starting line is because we need to compare all the continents of the world at a given point in history. We need to find out whether there was some continent or region that was already way ahead at that point in history.
In Guns, Germs, and Steel, Diamond is attempting to determine why it is that the people of Eurasia came to dominate the world. This means that he needs to find a time at which the Eurasians were not yet dominant. He needs to find this so that he can ask what happened between that time (the starting line) and the time when the Eurasians came to be dominant. Diamond sets his starting line at about 11,000 B.C. He says that it is clear that there were people on every continent at that point. He also says that people would start to domesticate plants and animals “within a few thousand years of that date (p. 35).” He wants to ask whether, at that point, the people of some continents “already (had) a head start or a clear advantage” over others.
In Chapter 1, Diamond takes a brief look at all of the populated continents of the world at the “starting line.” He is trying to determine, as we mentioned, if one continent or region had a clear head start. In this chapter, Diamond argues that no continent had a head start. He says that all continents had advantages and disadvantages at that point. For example, he mentions on pages 51 and 52 that Australia and New Guinea had very few areas that were good for supporting human life but that, on the other hand, they were the people who were first to have watercraft that could cross open oceans. This meant that they might have had a technological advantage. By looking at all the continents, Diamond concludes that there was no advantage. He says that a person looking at the world in 11,000 B.C. “could not have predicted on which continent human societies would develop most quickly.”
It is important for Diamond to draw such a starting line because it lets him know that he needs to look at times after that starting line to find out why Eurasia came to dominate the world.