The debate starts many centuries back. There was this new disciplin called Science, and the religious types were greatly interested in it. With science, you could prove the existance of God. What you did was to accumulate evidence until such a point as the existance of God became undenyable. So all sorts of religiously trained folks went out and looked at the world.
Meanwhile, science produced many other advances, like the high temperature furnace for producing early steel and early steam technology. Both these things needed one thing: power. Where did you get power: trees or coal. Most of the trees were gone, so that left coal. Science turned it's focus towards the process of finding and retrieving coal. The other thing that steel needs is iron, and so science worked on finding iron.
Those who were experts on finding coal were geologists. These earliest geologists had one, simple sorting mechanism: something helped them to find coal, or it didn't. Over time, they revised their system and made it work better. What they discovered and they theorized based on these discoveries was much different than the Creationsist stand that backed science in that day. What really got them in trouble, though, was their estimates for the age of the earth. Their early estimates hit 20,000 years!!! That's twice as old as the "official" age of the earth according to the Bible. Something was amiss.
Meanwhile, other scientists were out looking at life. All those new creatures in the new world were darned interesting. There were many creatures closely related, but a little different. But why? They knew from animal husbandry and plant experiments showed that traits were inheritable. Som traits were domainant, while others were recessive. The naturalists also wrestled with this question. It was Darwin who named the mechanism: natural selection (rather than, say, human selection). The environment acted as a breeder, removing less favorable individuals while allowing more favorable individuals a better chance to reproduce.
From there on, science and conservative religion have been at odds. Science, which was to prove the existance of God, instead went on to demonstrate that the Bible doesn't agree with anything at all.
So when hearing about Creationsim, just recall that science, natural selection, and geology were invented to help demonstrate Creationsism. The evidence was so against creationism, in the end, that creationism was entirely abandonded by science.