Evolution of the vertebrate jaw

This earlier article came up today in “A Taste of Pharyngula.” PZ Myers wrote a detailed article, with illustrations, of how Hox and Dlx genes help to guide the development of both the lamprey’s suction-cup mouth and the jaw of a fish. It’s called “Evolution of the jaw .” He cites

…a new paper on the molecular evidence for the origin of the jaw, which describes gene expression in the lamprey pharynx. And as a plus, it contains several very clear summary diagrams to show how all the bits and pieces and molecules relate to one another.The short summary is that there is a suite of genes (the Hox and Dlx genes, which define a cartesian coordinate system for the branchial arch elements, Fgf8/Dlx1 genes that establish proximal jaw elements, and Bmp4/Msx1 genes that demarcate more distal elements) that are found in both lampreys and vertebrates in similar patterns and roles, and that vertebrate upper and lower jaws are homologous to the upper and lower “lips” of the lamprey oral supporting apparatus.

Start here. This is a simplified diagram of the pharyngeal structures of a typical embryonic chordate— there is a series of repeated, similar bars of mesenchyme and developing cartilaginous frameworks and other tissues, from a mandibular arch (MA) in front, to a hyoid arch (HA), to a chain of branchial arches (Ba). These are tissues that contribute to a cage of supporting elements that will form various features of the face and neck as development proceeds, with each getting modified in characteristic ways. MA, for instance, will go on to form the core of the upper and lower jaws, while HA contributes to the hyoid bone in our neck, and the Ba structures make gill supports in fishes and diversify into other features in us gill-less tetrapods.

As the diagram illustrates, the information that specifies the identity of each pharyngeal arch is defined by a Cartesian grid made up of the overlapping expression patterns of a set of well known genes. The anterior-posterior identity is set by the array of Hox genes; no Hox genes are turned on in the mandibular arch, making that a kind of default state, while more and more Hox genes are activated farther and farther back.The dorsoventral identity is encoded in the pattern of Dlx gene expression.

And that’s just for starters. Read it, take it slowly, and see if it makes sense to you. It’s a lot more detailed than Stuart Pivar’s book Lifecode about the hypothetical development of balloon animals, about which Myers has written:

I had to point out that Pivar hadn’t actually addressed any biology, and that his modeling was little more than an extended flight of fancy, unanchored by any connection to any embryology…..Much of the book is filled with sketches in which he starts with something like his toroidal tubes, and then imaginatively transforms the tube into some animal. These transformations are completely unfettered by data or even the slightest familiarity with the embryology or evolution of the organism in question. Here, for instance, is a tube transformed into a polyp like creature and then into a spider.

Nothing in the development of a spider comes even close to looking like that. No evolutionary intermediates looked like that. Chelicerates did not evolve directly from some kind of collapsed coelenterate, and the intermediates don’t even make functional sense. The radial tentacles of coelenterates are not homologous to the legs of spiders. This is artistic self-indulgence, nothing more.

Quite a contrast in styles, wouldn’t you say? Data vs. fancy. The spider even has the wrong number of legs. Lifecode isn’t science.

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