Personally, I began to wonder about the literal reading of Genesis back in high school. I had been raised in an environment that did not even consider any other viewpoint, so this was based purely on the text itself. First, I was seriously into ancient history (went on to get a BA degree in ancient history) and became familiar with how literature was written back then, what people believed about the past and how they wrote about the past. I do not recall the specific things that clinched it for me (since it was a wide variety of little clues, both historical and literary), but before I was a senior, I was almost certain that Genesis could not have been meant to be read literally.
This had nothing at all to do with science or evolution in particular, since at that time, I still held YEC beliefs. It was only after reaching this conclusion, that I began to wonder whether the YEC beliefs may be wrong as well. I didn't think so, since I had been told that YEC beliefs were not based solely on a literal reading of Genesis, but were supported by the evidence as well. I found out this was just wrong, and that YEC'ism was based almost exclusively on a literal reading of the text.
There were three primary reasons why I concluded that Scripture was most likely not meant to be read literally:
1. The literary clues: language is very poetic and symbolic, using a framework that rises above a straightforward reading. The idea that God was giving us a symbolic and typological presentation of the truth, rather than a historical narrative, seems very clear to me.
2. A knowledge of the cultural setting, in both mindset and literature, taught me that a non-literal presentation would not have been viewed as any less "true" or correct than a literal historical record. So, reading it non-literally did not cause me any concerns over "not believing the Bible" or "not trusting God".
3. There were just too many consistency problems with the literal reading. Sure, the literalists since the middle ages have developed "work-arounds" for these problems, but that is just what they are: "work-arounds". The two creation stories which disagree regarding the order of creation, the day on which the sun and moon are created, the problem of Cain, the fact that Adam and Eve did not physically die on the day they ate the fruit, the fact that there was a tree of life, when supposedly they were already immortal, etc, etc. These made it clear to me that some, at least, of the text could not be read in its most straightforward manner. If it were not for the first two reasons, I may have stuck with a literal reading and just accepted these work-arounds. But all added up, the choice was pretty clear for me.
The scientific verification of the age of the earth came next, which was a slam dunk. The facts of evolutionary development were equally obvious once I reviewed the evidence (now without any bias in favor of a literal reading of Genesis, but still not with any predeliction to accept evolution, since I still had OEC sources telling me the earth could be old and the development of species over time could be explained without evolution). The theory of evolution as the explanation for those facts of evolutionary development came last, which I accept as the best theory going to explain those facts, and which fits the known evidence as well as any theory in science.
Here is a very interesting article describing why Genesis 1 should be read non-literally.
What makes it interesting is that it is written by someone that obviously is opposed to Darwinism.
http://www.asa3.org/ASA/topics/Bibl...f%20Genesis%201
Here are two more interesting discussions of the literal/nonliteral issue:
http://www.wcg.org/lit/bible/OT/sixday.htm
http://www.wcg.org/lit/booklets/science/genesis_1.htm
I will add that, as a person with a historical background, and one who follows the archealogical and historical fields pretty closely, I believe that starting with Abraham, we are pretty solidly in historical narrative. This does not mean that some of the stories may not be overlaid with legendary additions (as God chose to present His message to us), but the persons and overall events I think are historical.
http://www.wcg.org/lit/booklets/science/genesis_1.htm