If evolution occurred throughout billions of years of Earth's history, then why has it ground to a halt? Why are there not any transitional life forms alive today, let alone no fossils? If three billion year old monkey species are now humans, then why aren't the 2 billioin year-old monkey species sub-humans?
And that doensn't even scratch the surface of trying to explain the duckbillled platypus!! Besides, the age of the earth is debateable, since there are few reliable records of history much older than 3000 years. Carbon dating and methods like it are fundamentally flawed in that they assume to know the starting quantities of what they are measuring.
Besides, no one is discrediting natural selection itself. It is an observable phenomenon. Creationists do discredit the idea that it can change fish into birds, or apes into people, though.