Creationists come up with the weirdest criticisms. Serge at Imago Dei disagrees with my claim that humans build their face using "the same embryonic foundation that fish use to build gills", calling them "pharyngeal phantasies". He bases this on the peculiar notion that there ought to be a simple one-to-one mapping of cranial nerves to pharyngeal arches, and that by his understanding of the arrangement, the cranial nerve that innervate the derivatives of the first pharyngeal arch in us (the trigeminal nerve) ought to innervate the first gill arch in teleosts.

These are both false assumptions. I'll explain why.

While you can try to memorize cranial nerve targets by associating them in order with pharyngeal derivatives, the rule only works for a few arches and breaks down just about everywhere else. I can sympathize a bit with the idea that it sure would be tidy if CNS structure were tightly segmented and in clean register with peripheral segmentation, though. I did my graduate work in Chuck Kimmel's lab at the University of Oregon, at the time when we were just figuring out the segmental organization of the zebrafish hindbrain, and my graduate work was on spinal innervation of the somitic musculature, which does have a simple organization. But I'm afraid it just doesn't work out that nicely cranially—heads are peculiar and convoluted. Anyway, here's a piece from a paper my grad advisor wrote on fish segmentation:

The segments of the CNS and the mesoderm can be related to one another by considering the patterns of motor innervation. For the spinal myotomal segments of the trunk and tail this relationship is simple and direct: the single set of primary motoneurones present in each segment in the spinal cord projects through a single ventral root and their synapses are restricted to the immediately overlying myotome. [shoot…there's half my thesis summarized in a single sentence—pzm]

In the head pharyngeal region, there does not appear to be a simple 1:1 relationship between the series of hindbrain neuromeres and the pharyngeal arches, although the pattern of innervation is not completely known. Motor nuclei present in some of the hindbrain segments, including the first segment and two located more caudally, do not supply the pharyngeal arches at all, but project to the extrinsic eye muscles. If relationships that have been well shown for other teleosts hold for the zebrafish, then two motor nuclei (of nerve V) supply the first, or mandibular, pharyngeal arch and two motor nuclei (of nerve VII) supply the second or hyoid arch.

There's more detail, but Chuck concludes,

…the 'segmental' pattern of innervation of the gills is not one in which a single arch is innervated by motoneurones derived from a single neural segment of the embryo.

So one assumption by Serge is faulty; the first quote above also demolishes his other assumption. He claims that consistency would demand that nerves that innervate the first pharyngeal derivative in us should innervate the first gill in fish. That's nonsense. Teleosts are jawed vertebrates, like us, and have jaws and hyoids that ought to be innervated in the same pattern as the jaws and hyoids of humans. And they are. We do not expect cranial nerves V and VII to innervate gills, but instead the homologous structures in fish derived from the first pharyngeal arches. Just as they do in us.

These facts have already been pointed out to Serge, but he doesn't seem to comprehend them. He's now requesting evidence that each gill is innervated by a different cranial nerve (which just doesn't make sense, isn't predicted by homology, and isn't expected), and doesn't think much of the homologies between first and second pharyngeal arch derivatives between fish and people.

Evidence of both of these may exist, and I'd be interested in it. We often here of how the pharyngeal arches "look like gills", but I find the distribution of the cranial nerves are far more interesting.

I suspect that the only reason he finds that more interesting is that, as he admits, he doesn't know much about fish cranial nerves (and didn't even like head and neck embryology in his training!), so he can more easily imagine that they contradict evolutionary expectations. But OK, if he's sincerely interested in cranial nerve distribution, here ya go. This is a table I consolidated from two comparative anatomy dissection guides, one on the shark and the other on the cat.

Gosh, cranial nerve distributions are nearly identical in fish and mammals! There are some interesting differences—cats lack lateral lines and gills, of course, and they are separated by several hundred million years of evoluton—but overall, the similarities are overwhelming.

Alas, poor Serge. He has made three conceptual errors that are trivially refuted, with 20-30 year old sources, no less.

• His naive expectation that there'd be a one-to-one correspondence between cranial nerves and pharyngeal arches is mistaken. We don't see such a thing, nor do we expect such a thing.
• He has the homologies of the pharyngeal arch derivatives out of register. Cranial nerve V does not innervate the first gill, and evolutionary homologies would predict that it shouldn't innervate the gill.
• If he thinks the anatomical and functional correlations of the cranial nerves between fish and tetrapods support a creationist model, well, then he should be persuaded otherwise. They line up very, very well, and reflect common descent.

I strongly recommend that he pay more attention to that boring head and neck embryology stuff if he wants to argue about it in the future.

Kimmel CB, Sepich DS, Trevarrow B (1988) Development of segmentation in zebrafish. Development Suppl. 104:197-208.

Gilbert SG (1975) Pictorial anatomy of the cat. University of Washington Press.

Gilbert SG (1975) Pictorial anatomy of the dogfish. University of Washington Press.