Take a look here. In this neat little video, Biologic Institute biologist Douglas Axe summarizes some of the findings presented in his chapter (“Darwin’s Little Engine That Couldn’t”) in Science and Human Origins (Discovery Institute Press). Axe answers the question “How hard would it be for evolution to produce a different function for a protein?”
To get one protein (A) to do the job of another (B), not a completely novel protein just a slight but functional modification, Axe working together with Ann Gauger found that it would take at the very least seven or more mutations. That doesn’t sound so bad, but what would it mean in the real world of a bacterial population? Axe gives the bottom line, a distressing one for Darwinian theorists:

It turns out once you get above the number six [changes] — and even at lower numbers actually — but once you get above the number six you can pretty decisively rule out an evolutionary transition because it would take far more time than there is on planet Earth and larger populations than there are on planet Earth.

So let’s say you’re looking at a transition between two proteins that needs eight or nine steps. You’re out of luck, buddy, because six is the most that unguided evolution can do. This by itself would seem to present a devastating rebuke to any Darwinian account of how proteins, the fundamental structures of all cellular life, came to be as they are.