Second, that Columbus was not afraid of sailing off the edge of the world does not make the claim that at one point in history people assumed the world was flat false. It only means that you haven't gone back far enough in history. Even a few thousand years may not be enough, depending on one's standards. But if you keep going back, there is a point at which this was true. Why? Because if one goes back far enough eventually technologies are so primitive that there is no evidence to suggest otherwise. Let's remember that even if boats are sailing towards an island and tribes on the island see that at a further distance they saw only the peak of the ship, while as it got closer they saw it's entirety, such an observation still requires a ship. Then we can go back to before ships/vessels were made, which is sometime before Christ (that's already two thousand years), and so on. The point is: eventually there is a point in human history where it was assumed by default that the world is flat because (1) it intuitively seemed that way (2) there were no technologies and scientific methods of measurement known to man to suggest otherwise.