Around 200 BC, Eratosthenes [1] (a regular young Greek poet) calculated the circumference of the Earth just by measuring shadow lengths at high noon, and his guess was pretty much spot on (plus he figured out how far the Moon and the Sun are from the Earth, also very accurately), whereas Robert Hooke complained that too many people believed in a flat Earth in the 1600's [2]. Eratosthenes also invented Geography. Literally.
Pythagoras (~500BC) was a crazy smart guy, too. And then there was Hannibal, who used some insanely clever tactics to defeat a massive Roman army in what's considered the most decisive military victory in history (Hannibal lost 6,000 men, the Romans lost 60,000) (~216BC). [3]
My other favorite instance of mind-blowing genius is the Roman poet Lucretius writing about the time he stared at some dust floating around in a sun beam and deduced the notion of Brownian Motion, where the movement of atoms bouncing off each other on an invisibly small level "gradually emerges to the level of our senses... as bodies in motion." [4]
We have a whole lot of very smart people doing awesome things today, but when you control for population sizes, available knowledge, and living conditions, etc, the intellect of historical populations seems a lot more impressive than ours today.
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1: http://en.wikipedia.org/wiki/Eratosthenes
2: http://en.wikipedia.org/wiki/Flat_Earth#High_and_Late_Middle...