Consider DNA polymerase. We’re told that this complex molecule copies DNA at a rate of 200 bases per second. Note that in order to perform this task correctly, the molecule has to identify which of the four bases (CATG) it is attached to, then it has to snatch the complementary (not the identical) base out of the cytoplasm, which means it only has a 25% chance of grabbing the right one, yet it does so 99.9999% of the time.
But wait, there’s more. If the DNA polymerase happens to insert the wrong base into the growing chain, it is able to detect its mistake, back up, extract the errant nucleotide, and then proceed where it left off.
To reiterate: this is a molecule we’re talking about. Never mind where this molecule came from: explain to me how it works right now, inside the cells of your body, my body, and pretty much every eukaryotic organism on the planet.
This molecule, however complex, is an inanimate collection of atoms. It has no nervous system. Even if it performs its assigned task inside a neuron in your brain, still: it’s inside the neuron, inside the nucleus of that neuron, copying a small stretch of DNA on a single chromosome. Common sense tells us that no inanimate molecule can do what DNA polymerase is doing: not without an intelligent, guiding Hand.
So how do we explain the behavior of this thing? Is God personally moving every single molecule in every single cell inside your body? Moving every molecule in the universe? Is God personally pushing protons together in the heart of every star in the universe to form helium? Going down that path leaves us with an animist religion, where God is the universe, and vice versa.
Another problem with this approach is that DNA polymerase does make mistakes that go uncorrected. This happens about once every 100,000,000 bases, which is an A+ anywhere in the galaxy. Yet even with such a low error rate, an average of 30 mistakes are made every time a human genome is copied. In some situations, those mistakes lead to birth defects, degenerative diseases, and cancer. Is God personally picking victims, seemingly at random?
Here’s an even worse problem. If God is moving molecules around inside our bodies, is there any point in trying to cure diseases? Surely we wouldn’t be able to do such a thing, and even if we did manage to succeed, surely we would pay the price for having thwarted God’s will. So is all medicine an affront to God?
None of those answers seems especially appealing, which leaves us with the alternative: that DNA polymerase is indeed performing its task without divine intervention, being driven instead by Brownian motion, 3d stereochemistry, electromagnetic attraction, and the like. For fundamentalists, this isn’t a good answer either, because it opens the gate. If we can accept that this obviously intelligent behavior is being perpetrated by an inanimate molecule, why point to something like the eye and say “that’s obviously too complex to simply have evolved”?
If you can accept that the present-day behavior of DNA polymerase is driven by natural laws, then accepting that evolution is driven by natural laws is trivial by comparison.