Unearthed accidentally by a footnote from Paul Graham:
[Evolution] was harder for Darwin's contemporaries to grasp ... than we can easily imagine. The story of creation in the Bible is not just a Judeo-Christian concept; it's roughly what everyone must have believed since before people were people. The hard part of grasping evolution was to realize that species weren't, as they seem to be, unchanging, but had instead evolved from different, simpler organisms over unimaginably long periods of time.
Now we don't have to make that leap. No one in an industrialized country encounters the idea of evolution for the first time as an adult. Everyone's taught about it as a child, either as truth or heresy.
If you teach children that everything around them was some other thing in the past, and only became the things they are over the passage of time, you teach them that everything they know will change.
Many religions are essentially authoritarian societies, built on the premise of infallible knowledge. Maintaining belief in the specific allegedly infallible knowledge of the various American Christian religions opposed to evolution is impossible if you know that everything in the world around you used to be something different and will one day change again. If you merely imagine that maybe everything in the world around you used to be something different and will one day change again, maintaining belief in the specific infallible knowledge of American primitive Christianity might not be impossible, but it certainly becomes remarkably difficult.
They could in theory work around it by changing the content of their infallible doctrine, except you can't change infallible doctrine without making it fallible.
In fact, any allegation of infallible knowledge loses credibility in this context, since it also requires an infallible perpetual updating and aggregating system. Even if God exists, and even if God knows everything, for that information to percolate out to religious authority figures, you would not only need prophets who were incredibly good at listening to the word of God, you would also need God to be updating Twitter every nanosecond. (Not because the change is fast - it isn't - but because the knowledge becomes fallible if it's out of date.) Allowing the alleged infallibility of God to transfer to His representatives becomes absolutely ludicrous in this context, which means that whether God exists or not, the idea which holds up the entire power structure of American fundamentalist Christianity falls apart, and the power structure collapses. But this doesn't just mean the structure of their religious organizations; it also means the structure of their families.
This is why they aren't satisfied by getting evolution in school and creationism in Sunday school; the damage to the fundamental premise of their entire society's power structure has already been done. What they really object to is this threat to their power structure. Religions perform important social roles, and without the fundamental assumption of infallible knowledge to give their authoritarian religion the credibility to perform those roles, their sub-society faces serious and significant upheaval.
When Christian fundamentalists in America say that teaching evolution threatens the very fabric of their society, what they say is true. I don't want to get into the politics of it all, but that's what's going on. If you hear about them fighting against teaching evolution in schools, and you wonder how anyone could be so stupid, that's how.