I think it is almost certain that a vast amount of human knowledge was lost. The policy of the city of Alexandria was to copy every book that went through it, and give the copy back. As a result, they ended up with a spectacularly large collection of original materials.

The amount of ancient literature that is no longer available is vast. History, literature, you name it, so much has been lost. The odds are very good that at least a significant portion of that were in the Alexandrian library.

I'm going to ask my parents about this to see if they learned this idea in school. They didn't grow up in an English speaking country, and I don't think they're big fans of Sagan.

Maybe that facts such as the rough circumference of the earth were lost for a thousand years had something to do with it? Or is that false?