Watching a show on the Smithsonian Channel right now about the Stonehenge civilization where they found that the people were really quite advanced, even performing successful surgeries on the skull. I've heard of a lot of instances like this, where we only now discover that they knew of something in the past that our current people didn't know until much later on. How could we have just forgotten so many of these things? How did they develop it so much sooner than us?
You should read How The World Became Modern by (I think) Steven Greenblatt. He goes into detail how knowledge, at least in the western world, was lost and then found again.
A lot Stevens assertions deal with the loss of knowledge of the past coming from books being expensive, difficult to reproduce, fragile, and certain philosophies not necessarily meshing with current theology.
Sorry if this breaks this subs rules.