I watched (some) of a History of Mathmatics documentary, and what struck me was how many of the theorems ancient people came up with were just common sense. But they wrote it down: that's what made it noteable.

There were some theorems/axioms that i had trouble getting my brain to accept, but generally it came down to me not "getting" their number/tally system or simply having no context for why they needed that math in the first place (ie: it wasn't common sense to me). For example. I'm not an ancient accountant who needs to be able to calculate grain taxes and -- at the same time -- be able to assure a farmer (who can't read my number system) that i'm taking the correct amount of his food, so their method of long division using different colored stones seemed needlessly convoluted to me.