As most of us know (and all of us who have read WEIT), all vertebrate embryos develop “branchial clefts” (also called “pharyngeal arches” or “branchial arches”) at an early stage, and these are almost certainly the vestigial remnants of the clefts of our fishy ancestors, which develop into gills. Those branchial clefts are sometimes called “gill arches,” even in species, like reptiles, bird, and mammals, that never have gills. In humans, for example, the clefts disappear and transform into other parts of the body, including the jaw, the middle ear, and the larynx. Here’s a photo from Wikipedia showing them in humans:
And here they are in a cartilaginous “fish,” a skate (yes, some will take issue with that name, but it’s irrelevant for our purposes):
The second photo is taken from a wonderful new post on the weirdly-named website Playing Chess with Pigeons, written by Troy Britain. The post is called “Gill slits by any other name,” and it’s not only an explanation of where these things come from and what they become in vertebrates, but also an eloquent attack on creationist (and intelligent-design) claims that these “gill slits” in non-fishy vertebrates have nothing at all to do with evolution (here’s an example of one such attack on my book).
The creationist/ID attack on “gill slits” in amniotes (basically, mammals, birds, and reptiles) goes as follows (this is from Britain’s post).
- Pharyngeal structures of amniote embryos never function as gills and therefore should not be referred to as “gill slits”.
- Whatever resemblance to the gills of aquatic vertebrates the pharyngeal structures of amniotes has, it is superficial.
- Seeing the pharyngeal structures of amniote embryos as being gill-like and calling them gill-slits despite their not functioning as gills is “reading evolution into development”.
Britain takes these claims completely apart. Go read his post: it’s long but really educational and accessible to everyone. If you want to spend half an hour learning some great evolutionary biology, by all means invest it in reading Britain’s discussion.