Yes, the known history is neccessarily a subset of the actual one - but I think we often don't imagine how large the gap is and also often make the mistake of assuming that this gap is smaller than it is. Consequentely, our "models" of ancient societies or the biology of the dinosaurs are far too limited.
I think one reason for us not understanding this gap well is the way history is taught. We learn about the earliest civilizations. We assume that we know a comparable amount from later periods in other regions about other civilizations, but we often don't. In Europe we have a wealth of information about ancient Greece and the Roman Republic, but we know relatively little in comparison about Scandinavia even hundreds of years later.
