I was going to write exactly what you said, that we underestimate the our ancient ancestors. Ancient certainly does not mean unintelligent. I believe many people already realize this, but there are more likely than not some who think that the ancients were simple (rofl). Look at the aquaducts for example. Simple in function but the design was so well executed that they still stand and I think are still used today! Or even the forms of government or societal systems they all had in place. Culture was thick even thousands of years ago all across the world. One of the signs of greatness if you ask me. Many things may have been lost due to war, disaster, etc. but that means we ought to cherish and protect what is still left even more. Thank goodness we live in the modern age where if something were to happen (I certainly hope not) it is most likely well documented and in duplicate all over the place, written and electronic. 

Lastly, the mirror thing. I did not realize they used and made mirrors that early on. If that is the case, I am surprised the telescope was not invented sooner since they figured out how to amplify/magnify a small light source. They were on to something!