In response to Neil Shubin’s recent paper on the subject, and Carl Zimmer’s summary, the creationist Michael Denton criticizes evolutionary explanations for the vertebrate limb. It’s a bizarre argument.
First, here’s the even shorter summary of the Shubin work. Ray-finned fish have, obviously enough, rays in their fins — rigid bony struts that provide structure. These rays are formed dermally. That is, osteoblasts deposit bone on the surface of a connective tissue matrix to build the rods of bone that prop up the fin. It’s called dermal bone because the classic example is the assembly of bony elements within the dermal layer of the skin.
Another way of making bone is endochondral. You start with a framework of cartilage, and cells within the cartilage (which is what “endochondral” means) gradually replace the cartilaginous tissue with bone. Your limb bones, for instance, are endochondral, starting out as fetal rods of cartilage that were replaced by the action of osteoblasts.
What the researchers in the Shubin lab found is that fish fins, which have dermal bone, and tetrapod limbs, which have endochondral bone, use the same cell signaling pathways and cell movements to build the cellular structure of the limb/fin, but differ in the final steps. Fish switch on the dermal bone pathway, while tetrapods use the endochondral pathway. We have a fundamental similarity than simply uses a different end-product, which ties the assembly of fins/limbs more closely together than we had thought.
Not to Denton, though! He wants to make some strangely saltational argument that these two modes of bone formation are somehow incompatible. I’ll show you his final conclusion, so you can decide to bother with the rest.
In short, my assessment is this: There never were any transitional forms making both dermal bone and endochondral bone. Organisms made one or the other.There never were any transitional forms with fin rays and digits. And I predict that no matter how extensively the fossil record is searched, the phenotypic gap between fins and limbs will remain even as the genetic gap continues to diminish.
That’s an incredibly stupid statement. You make both kinds of bone; your skull, for instance, is largely formed of dermal bone, while your limbs are endochondral. Fish have both endochondral bones (for example, in their vertebrae) and dermal bones (skull and fin rays). The ancestral fish-like form had both kinds of bones. We have fossils of transitional forms with both a central endochondral core and fin rays.
We’re done. Denton’s claim has already been addressed and refuted. How can he not know this?
You can just stop now. His whole argument is dead in the water. But you could take a look at the thought processes that led to that stupid conclusion, and they’re kind of weird, too.