One of the delusions of the creationists is about their own origins. They like to think that they're the guardians of an old truth, that has been under attack since only one or two centuries. But creationism is itself a recent invention, and that should not be forgotten.
Nietzsche said that myths were beginning to die when people started believing in them. (He was more specifically speaking about Greek myths, if I remember correctly, but that's besides the point.) The story of Adam and Eve was, during thousands of years, a vivid myth that was innerving the mystery of the origins of mankind, and which was used as the center for the theological or esoteric meditations of the learned classes. It is important to see that, for Christians and Jews, the Genesis was naturally open to multiple interpretations, which weren't mutually exclusive: as the Bible was supposed to be given by a being whose intelligence was infinite, it was only logical to seek in it other meanings than the pure literal one.
That's what the Jews made, for example, with the compilation of the Talmud, after the desctruction of the Second Temple, and later, with the Kabbalah. For the Rabbis, the story of Adam describes the drama of the incarnation of the soul, of divine nature, in flesh ("unto Adam also and to his wife did the Lord God make coats of skins, and clothed them" -- Genesis 3:21 : meaning that before the Fall, before the birth in this world, souls are immaterial).
In the Christian world, Saint Augustine wrote a short book on the literal interpretation of the Genesis, where he explains that the purpose of the Bible is not to be a book about natural history, but about salvation, that the alleged six days of the creation must be seen as a metaphor, and that what reason and intelligence allow the men to discover must not be shadowed by too much respect to the letter of Scriptures. This view, shared by many other Fathers, is still today the official view of the Catholic Church.
So, why and how did creationism appear ? I blame the Puritans. When this fringe of Protestantism decided that they didn't need professional priests, or specialists in theology and in exegesis, and that they didn't want any mediation between them and the Scripture, they closed their minds to three thousand years of wisdom. That was only the translation in the spiritual field of the austerity they imposed to themselves in their lives: a people of merchants, of bankers, obsessed by usefulness and thrift, scared by anything that could be related to pleasure, scared then by the pleasure of learning for learning's sake, of the joy given by the speculation of a bondless mind, with no sight of practical applications. Creationism is the product of a strong hatred for freedom of thought, itself the product of a strong hatred for idleness, for culture, and for anything that has no practical uses. And that's why it's so dangerous, and why it must be fought at all costs, not only by atheists, but also by all partisans of a religion from where spirituality is not absent.