When did humans start thinking the earth was flat?
In ancient sculptures and drawings, the Earth was depicted as being round and Atlas was shown as holding the Earth in the form of a globe.
At what point did “flat earth” become the dominant theory?
If you’re referring to the educated (leaders and elites) the answer is simple, They DIDN’T!
Very few humans ever thought the world was flat in recorded, due to the simple fact that when one looks at the horizon one can see the curvature of the earth, and the fact that if one watches a ship sail out to sea, it sinks below the horizon before it actually gets too small to see (unless you have poor eyesight, of course). It is a modern myth that people thought the earth was flat. As for the middle ages, they only maintained the “truths” described in ancient greek texts (which were given the same weight as the bible). Thus, Copernicus and the likes were suppressed due to Greek incompetence, really, and Medieval Europe’s willingness to believe in the Greeks. There was much intellectual vitality, and advancement in many fields — technological (many key inventions, e.g., in agriculture and navigation, including those that made the “Age o Discovery” possible), scholarly (those ‘ignorant’ religious folks FOUNDED the university system), economic (beginnings of modern economic systems, stock companies, banking), social and governmental (foundations of modern legal and political structures, end of Roman system built almost entirely on slavery).
As for the question at hand, the notion that most educated people of Columbus’s day thought the world flat is total nonsense — a lie spread by later determined opponents of religion (from Voltaire to Huxley to A.D. White, to Sagan and Dawkins).
In fact, “All educated persons of Columbus’ day, very much including the Roman Catholic prelates, knew the earth was round. The Venerable Bede (c. 673-735) taught that the world was round, as did Bishop Virgilius of Salzburg (c. 720-784), Hildegard of Bingen (1098-1179), and Thomas Aquinas (c. 1224-74). . . . . Sphere was the title of the most popular medieval textbook on astronomy, written by the English scholastic John of Sacrobosco (c. 1200-1256)” http://www.taemag.com/issues/articleid.17713/article_detail.asp
Note that Columbus’s proposed voyage was NOT opposed because some prelates thought he’d “fall of the edge of the earth”! Rather, they believed the world was much larger than HE his calculations (and incidentally, they were right!!) so that one could not safely make such a long voyage.
Sadly, the anti-religious bias that perpetuates the popular notion of the “Dark Ages”, keeps turning up. Even Daniel Borstein gets it wrong. In his book *The Discoverers* he suggests that the Church “forgot” the world was round but rediscovered in perhaps a century before Columbus –but his evidence is pitifully thin (and the examples above, with many others, contradict him).