It's been touched on already in this thread, but I'll reiterate what I posted on this very topic about a month ago:
For years I gave the matter major serious thought, (ironically, my devout JW Mom had nevertheless instilled in me a respect for science, reading, and education), and put together a breakdown of the reasons why evolution is the slipperyest slope imaginable for fundamentalists and Biblical literalists/innerentists of all stripes:
1. If evolution is true, then the Eden account in Genesis cannot be literal history, and must therefore be allegorical. However, once you start recategorizing the essentially supernatural aspects of Genesis as allegorical, where do you stop? The Deluge? The Ten Plagues? The crossing of the Red Sea? the Virgin Birth? the Miracle of the Loaves and Fishes? The Resurrection? The Revelation to John of Patmos? Eventually the temptation to explain away or dismiss ALL the supernatural events in the Bible can become almost overwhelming.
2. If evolution is true AND the means by which God (assuming He exists), created and propogated life on Earth, then its very nature reveals a "God" who is fluid, changing, and enigmatic; the very opposite of an authoritarian deity as He's portrayed in fundamentalists' interpretation of the Bible.
3. In the late 19th/early 20th Century, orthodox Christians finally got fed up with having to reinterpret the Bible and Christianity (as they percieved it) with what seemed to be every other discovery science made, and with the publication of Darwin's Origin of the Species, drew a line in the sand and said "NO MORE!" (plus, they found the suggestion that we could be descended from creatures like chimps and orangutangs kinda gross).
4. (A big one) If evolution is true, Adam couldn't have existed as an actual historical figure, therefore he could not have committed Original Sin, therefore humanity (as a whole) did not need redeeming, therefore Jesus' (who some theologians have called the "Second Adam") "Redeeming Sacrifice" isn't the literal canceling out of Adam's Fall, as defined by the Apostle Paul and Augustine of Hippo, therefore fundamentalist Christians' understanding of Christianity's entire purpose may be potentially flawed.
5. If evolution is true, then all the conditions (or restrictions) that fundamentalist Christians claim as having their origins in the Garden of Eden are suspect, not the least of which is the assumption that Christianity is the restoration of authentic worship as was ostensibly laid out in Eden, thusly threatening Orthodox or fundamentalist Christianity's claim to hold the "One True Path". (Yes, it sounds weird, but there are a few obscure Christion sects that subscribe to this)
6. Biblical literalists/innerentists are not at all comfortable with a whole lot of symbolism, because it's "fuzzy"; open to REinterpretation. They like it black and white, and evolution has the potential to turn virtually everything important to them into shades of gray; it is therefore exceedingly threatening to their worldview.
7. Biblical literalists/innerentists also believe that humanity has entered the End Times or Last Days as foretold in the Book of Revelation, and that if people are to take the last book of the Bible seriously, then they better take the first book in the Bible seriously (forgetting, of course, that one CAN take something seriously without necessarily taking it literally).
8. Many conservative Christians interpret the Revelation passages referring to the post-Armageddon era as a millenium-long righteous utopia where justice and peace prevail under the benevelent rule of the returned Messiah, of which Eden is clamed to have been the prototype. If evolution is true, and Eden is allegorical, it potentially casts doubt on that hope.
9. The vast majority of religious conservatives take it as an axiom that humanity is inherently bad (and needs to be kept on a tight leash for its own damn good, hence their often hawkish and clenched-fist political and social inclinations). A “perfect God" wouldn’t create “imperfect” worshippers, so something must have happened to screw us and the rest of the world up. The Genesis creation narrative of the Fall from Grace in Eden provides a neatly packaged theological (or, more accurately, ideological) explanation, but it really only works if it’s taken literally.
As mentioned before, it’s not so much about Genesis per se; it's (mostly) about Eden. This is just my opinion, but it seems to me that evolution is FAR more threatening to Biblical literalists and fundamentalists than gay marriage, stem cell research, or Roe vs. Wade.
Interestingly, while researching “The Case for God” Karen Armstrong concluded that Biblical literalism was sort of an unexpected byproduct of the Enlightenment; the rational/scientific method for explaining the world around us had had unparalleled success, but an unexpected side effect was the discrediting of mythology, to the point where the word “myth” even became, for all intents and purposes, associated with “lie”.
I light of that, how could people who revered the Bible associate their cherished scriptures with lies? They couldn’t, of course, and so Young-Earth literalism, well, evolved into its the modern-day (and often quite militant) form.
I doubt the vast majority of Biblical literalists have actually sat down and worked out these details in their heads; I had to think about them for a while myself to clearly quantify them. I suspect, rather, that they grasp it more on an unconcious/intuitive level; this might explain the more visceral "reptile brain" response one often gets when one suggests evolution and Abrahamic monotheism might be compatible.
