I addressed the topic briefly in my post on the spice trade, but our ancestors were actually well aware that the world is round- they knew about it since the time of the ancient Greeks. The ancient Greek philosopher Eratosthenes actually calculated the circumference of the Earth to within 15%. The only reason we think that they believed in a Flat Earth is due to a series of feuds in history. The Protestants and Catholics began making claims that the other side believed the Earth was flat, and then some crappy 1800s English historians picked up those claims and ran with them to try and discredit religion.