The distance between the 20th Century and Severian's Urth

I imagine that there is quite a long gap between the two eras, because Severian mentions that rats in the library smear the covers of the books in their own crude writing. Either this means that rats have naturally developed this ability or somebody was tinkering with their genes. In the latter case the time might be less, but it take quite a while to develop the ability to increase even a rat's intelligence to the level where they can write. Derek