In the book Guns, Germs, and Steel, what significance do the continents' differing dates of settlement have on subsequent history?
Jared Diamond poses and answers this question in the last part of Chapter 1 of Guns, Germs, and Steel. The answer is, in essence, that the difference in dates of settlement of the various continents had no impact on the subsequent history of the world.
On p. 50, Diamond poses the question that you posed. He posits the idea that we would probably predict that the continent that gets settled first would be the one to dominate the world. If you think about it, this makes some amount of sense. When people got to a continent, they could start to settle down and build a civilization. The longer the time that they had to build the civilization, the more advanced they would become. But Diamond goes on to demolish that theory. He points out that Africa was the first continent inhabited by people and it did not end up dominating the world. In fact, there is little correlation between when a continent was settled and how rich or powerful its people later became. Thus, we can say that the difference in the dates of the settlement of various continents did not end up having an impact on world history. As Diamond says at the end of Chapter 1, we need to look elsewhere for the factors that caused Eurasians to get guns, germs, and steel and dominate the world.