The step by step emergence of "new body parts" is a concept some are simply unable, or unwilling, to wrap their heads around. I imagine some could consider an eye a "new body part", and yet through intermediate stages, for the most part visible in living organisms today, we can witness the progress from a simple light sensitive cell, to a full blown eye, with lens, iris and asscociated paraphanalia. Likewise, if we consider the growth of the eye within an embryo each stage is directed by one or more coding sequences in the embryo's DNA and each of those coding sequences was originally a mutation.
The counter-argument to this is a string of empty denials that may convince their author, but leave any informed individual bemused by the perversity of human thinking.