|Image courtesy of this Let's Play|
Possibly more than you might think.
The problem with writing ancient civilizations is striking the right balance between "our ancestors weren't stupid" and "they probably didn't know all the mysteries of the universe now lost to us, either". This post is on the former issue.
The image for this post came from a video game called Wild Arms. The characters are invited to a festival that will celebrate ancient knowledge in a way that only a steam-punk fantasy setting built on the ruins of a lost technologically advanced world can: by throwing a bunch of rusted metal into a nearby public park and posting signs about how mysterious it all is. One of the "exhibits" on display is a skeleton, which is "found in several places" but its "use is unknown". Unknown. To an agrarian community who make their living slaughtering sheep in between being menaced by random encounters of goblins outside the city walls. Huh.
The Wild Arms example is, I think, probably satirical, but that doesn't mean that Stupid Ancestors don't show up in a wide variety of writings. Stupid Ancestors have been invoked to claim that a divine authority would be needed in order to make some of the biologically sensible claims in the Bible, like the fact that if you lose too much blood, you die. You'd think that a people who work with sharp farming implements in between slaughtering livestock and very occasionally going to war with their surrounding neighbors could have worked that out on their own, but no, Stupid Ancestors it is.
And then there's the book I reviewed last year that I really wanted to enjoy, but the author was apparently so terrified of including any detail that might be wrong, that she went overboard and gave almost no details whatsoever. (I couldn't even tell if the characters wore clothes.) What was worse, though, was that her ancient peoples were all possessed of not a single ounce of skepticism: the main villain shows up and takes over their society simply by virtue of the fact that he claims God exists and sends him direct divine communication to boss people around. When the one person who opposes the villain mysteriously winds up dead when spending time alone with him, these innocent flower children don't for a moment consider the tragedy to be anything other than the sheerest of coincidence. Stupid Ancestors strike again!
The hardest part about overcoming Stupid Ancestors in writing is trying to imagine what our life would be like if it were completely different. For instance, I've never seen a skeleton before, outside of models in school and in shows on television. If you took my schooling and my television watching away from the equation, I wouldn't know what a skeleton looks like! And thus... Wild Arms.
The problem with this, of course, is that characters aren't created by taking yourself and subtracting bits away. Everything that is subtracted has to have something added to take its place, and these new sources of knowledge will change what your character does and doesn't know. Cut out modern medicine and you're going to have a race of people without a germ theory, perhaps, but they're also going to have a touch of experience treating all the open wounds that the EMTs and emergency personnel are no longer handling for them. Cut out grocery markets and they might not know much about high fructose corn syrup, but they may know a thing or two more about how to grow vegetables or kill a rabbit.
This is scary for an author. It's hard enough to build a plausible new world without trying to think up how every little aspect affects every character's life and inner knowledge. The good news is that in some ways, you don't have to. The key isn't "sit down and map out everything that every character knows", but rather "look back over what I've said they do and don't know and sanity check it". You can have characters interested in a skeleton without making them Stupid: make it a medical display ("we don't know how it works, but we know it's a part of us"). You can have easily duped characters without making them Stupid: give them reasons for why they can't cope with the harsh reality of the situation.
In other words, the key to avoiding Stupid Ancestors isn't rewriting your story to have different events, but rather rewriting specific areas to justify the events that come before and after. Your characters can behave essentially the same as before, but for different reasons than outright stupidity. And, really, all you need in order to do that is to write them from a perspective that has the same raw intelligence that the rest of us -- and most of our house pets -- were born with.