“Gill slits” by any other name…

Charles Darwin once said that he thought the evidence from the comparative anatomy of embryos was “by far the strongest single class of facts” in favor of common descent (Darwin, 1860) and while it has since been eclipsed by genetics, it remains one of most compelling subsets of evidence for evolution. And perhaps the single most striking detail in the comparative embryology of vertebrates, are the structures colloquially known as “gill slits”.  

Embryonic “gill slits” or “branchial clefts” (branchia is Greek for gill) or more properly pharyngeal clefts (grooves, folds, etc.) are part of what is called the “pharyngeal apparatus” found in front (ventral) and sides (lateral) of the head/neck region of all vertebrates in the “pharyngula stage” of development. In “fish”, and the larva of amphibians, these develop into respiratory organs used to extract oxygen from water while in amniotes (“reptiles”, birds and mammals) they are modified into other structures.

Before I go on, a brief digression about “fish”. Throughout this article I will often use “fish” in the generic sense; but it should be noted that the term as it is commonly used—to refer to any vertebrate that swims in the water, has fins and gills—is not a valid scientific classification. This is because the three main types of animals commonly called “fish” —the Chondrichthyes (sharks, rays, skates and chimaeras), the Actinopterygii (ray fined fish, which constitutes the majority of living fishes), and the Sarcopterygii (lobe fined fish, the group from which four legged land animals, i.e. tetrapods, evolved)—are not a monophyletic group. That is they are not very closely related to each other despite some of their outward similarities (like gills). For example the living Sarcopterygii, lung fish and coelacanths share a more recent common ancestor with us (and all tetrapods) than with the other “fishes”.

OK, so the “pharyngeal apparatus” consists of a series of paired pharyngeal arches and fissures which develop on the exterior with a corresponding set of pharyngeal pouches on the inside of the throat, separated from the external fissures by a thin membrane (more on the details in a moment). And in fact the possession of these structures at some point in development, along with a hollow dorsal nerve cord, a notochord and a post anal tail, are the defining characteristics of the phylum chordata to which we and all other vertebrates belong.

Copyright © 1999 McGraw-Hill Companies, Inc.

Please note that the above illustration is diagrammatic and not intended to be photographically accurate (I have to say that lest I be accused by creationists of conveying a fraud). Below are actual photographs of both a skate embryo and a human embryo for comparison. Also note: the gill structures in the embryos of Elasmobranch fishes—the subdivision of Chondrichthyes which contains sharks, rays and skates—are much less derived than in other “fishes” and therefore generally more similar to those of amniote embryos than the corresponding structures in the bony “fishes” (which are significantly modified).

(Gillis et al 2009, p.5721)

The first of the arches, the mandibular arch, forms the jaw in all jawed vertebrates (Gnathostomes). Most vertebrates develop a total of six arches but the full complement is usually only retained into adulthood by hexanchiform sharks. Hexanchiformes are very plesiomorphic which means that they are more like earlier types of sharks.  Some species of hexanchiformes even develop a seventh arch. Likewise the extant jaw-less vertebrate, the lamprey, also have seven gill openings.

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We need creationists lecturing us on evolution like we need another fenestra in the head

The Institute for Creation Research has presented the world with another taxonomic turd from the cat box of creationist wisdom. This time it comes from ICR’s “Senior Science Lecturer” Frank Sherwin. However before I get to the main event, I want to take a closer look at the litter in which Mr. Sherwin’s little jewel is nestled.

In the February (2010) issue of ICR’s monthly Acts & Facts Mr. Sherwin (whose background is in parasitology) graced us with an article titled “Darwinism’s Rubber Ruler” in which he argues that descent with modification is untestable and that “any and all scientific evidence” can be “stretched to fit” the theory.

The first thing that comes to my mind when reading this is to ask: if this is so, then how is it that I could have in my personal collection literally hundreds of books and pamphlets, many of them originating from Mr. Sherwin’s organization, that purport to contain absolutely scads of evidence that contradict evolutionary theory?

How about Dr. Duane Gish’s (the emeritus vice president of ICR) books Evolution the Fossils Say NO! (1978) and the update Evolution: the fossils STILL say NO! (1995)? How can the fossils say “no” to evolution if any scientific evidence (in this case fossils) can be “stretched to fit” the theory?

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A tiny intruder

My wife (Kathy) woke me up this morning telling me there was a “baby in the bathroom”. Naturally enough I asked “baby what?”. “A lizard” she said.

Now we generally have only two types of lizards around where I live (in Southern California), the Southern Alligator Lizard (Elgaria multicarinata) and the Western Fence Lizard (Sceloporus occidentalis). Usually when I find a lizard in the house it’s an alligator lizard. They’re nasty little buggers who bite (the larger ones can draw blood), wipe feces on you and drop their tails if you breath on them too hard.

Elgaria multicarinata

So I asked my wife which kind it was. She said, rather insistently, that she didn’t know and that I should get out of bed and look for myself. Here is what I found in all its cuteness:

baby_liz

After briefly toying with the idea of keeping it (because it was so damn adorable) we decided to let it go in an area beside the house where we frequently see its relatives. We wished it best of luck and sent our tiny intruder on its way.