Surely you are not contending that organisms which share an evolutionary lineage should therefore have widely disparate embryologies? Two lineages share characters as adults because they developed along similar lines – i.e., they share embryonic similarities.
When species diverge, it is not the case that suddenly their entire embryonic development changes from that of the parent stock. The degree of similarity between the developmental paths of two species will typically be a consequence of their relative closeness in evolutionary terms. Closely related species will very nearly mirror one another’s development, while more disparate species will share more basic similarities. All vertebrates, for example, develop along certain lines up to a point, after which they diverge according to their particular lineage.
In other words, I would expect that if organisms are presumed to share an evolutionary history, then we ought to see some degree of similar developmental pathways in those organisms. And we do.
It sound like you’re saying that it’s just a renaming of the structure formerly known as gill slits because they won’t eventually lead to gills. The same structure appears in fish, but does eventually lead to gills, so gill slits they remain. It doesn’t change the fact that the structures are the same in both embryos. Or have I missed a point?
I don’t see how Blake’s point has anything to do with my question, which is:
Is there actually a structural difference between what are called “gill slits” in embryonic fish and “Pharyngeal arches, clefts, and pouches” in human embryos, or is it just that they use different terminology to describe them because of what they eventually become?
I’m sorry - the first draft of my post went into this in a bit more detail, but I deleted it because I felt I wasn’t really being clear. I’ll try to address this later today when I have more time.
Okay, if we can subsume embryonic similarities under the general catagory of anatomical similarities shared by species with shared evolutionary lineage, then I can understand what you’re talking about. I mistakenly thought there was something different about the embryonic similarities. I was misled by Haeckel-inspired talk.
I think it may be the case that they should not really be called ‘gill slits’ in embryonic fish, at least not until they are actually slits, and have gills developing in them. Maybe I’m wrong.
Some of each. To be fair, *pharyngeal *arches/clefts/pouches are also often referred to as *branchial *arches/clefts/pouches. Branchial comes from the Greek for gills. Strictly speaking, I suppose one would reserve the term branchial for those structures that would give rise to gills, but there is overlap in the terminology, so one finds references in the human medical literature to branchial cleft cysts, fistulas, etc. So the terminology is at least theoretically different based on what the tissues will eventually become.
But there are structural differences as well. In both fish and humans, as the embryo develops, a characteristic set of alternating ridges and grooves forms on the outside of the embryo, while simultaneously inside the embryo’s gut tube a series of pouches form - wide spots in the gut tube. The ridges are the pharyngeal/branchial arches, the grooves are the clefts, and the pouches are the, uh, pouches. In between the embryo’s outer covering of ectoderm and the endoderm which lines the gut is mesoderm. In humans, the grooves deepen and the pouches enlarge and the endoderm thins out to the point that the ectoderm is in contact with the endoderm in places. The mesoderm soon moves back in and separates the two layers, however. In the case of the human ear, the first cleft deepens and contacts the first pouch. The tissue of the cleft forms the outer ear canal and eardrum. The mesoderm of the first and second arches that moves back into place between the two layers forms the bones of the middle ear, and the first pouch forms the lining of the middle ear and the Eustachian tube.
In the case of fish, the grooves deepen and the pouches expand, and the endoderm and ectoderm come in contact and then break down, so there is open communication between the pouch and cleft. This is a gill slit. The branchial tissues then go on to form the jaw and gills of a fish.
That’s a long way to go just to arrive at the statement that humans don’t have gill slits. Humans don’t have gills at any point in embryonic development. They do, however, share a basic vertebrate body plan with fish that says that ‘things between the head and heart will form from this wrinkly patch here, that from the outside bears a superficial resemblance to the gills of a mature fish’. Humans and fish also form eyes and brains from the same basic precursor structures, but we don’t form fish eyes or fish brains, either.
So are you going to tell Britannica ?After all we are fighting ignorance here, and if they query my info source all I will be able to say is “Brossa!”, and then point expectantly at you.
Sure, what the heck- I’ll float 'em an email. I’ll even ask them to send you an updated Vol. 7: Enthalpy-Ezra.
Seriously, though, I would let the ‘gill slits’ thing go as being a very entry-level, accessible explanation, but they go on and compound the problem later:
These structures develop quite fully, just not into gills. I happen to be quite fond of my mandible, thyroid, thymus, parathyroids, laryngeal cartilages, anterior neck muscles, and middle and external ears, thankyouverymuch.
All this talk about gill slits makes me continually misread **Mangetout **as Mangetrout.
Thank you for the very clear explanation, but I get the whole development process. I’m not confused at all about how the different things develop, or that they develop differently depending on the animal the embryo will eventually become. I was just wondering about the particular terminology and whether the two structures were actually, at that point, identical and just bore different names because of what would happen in the next step.
All chordates, which includes vertebrates like humans and fish as well as some invertebrates, have the following characteristics at some point in their development:
-a dorsal, hollow nerve cord
-a notochord
-pharyngeal slits (aka gill slits)
-a muscular post-anal tail
There are better sources than Wikipedia for information on biology in general and embryology and evolution in particular. For example, the story of Haeckel’s embryo drawings can be found here by the authors of one of the most popular introductory college biology texts.
A further explanation of similarities and differences among vertebrate embryos can be found at Pharyngula, which is the blog of a developmental biologist.
Whenever you read a claim that the evidence for evolution is missing or fraudulent, you should look it up at the TalkOrigins Archive. Here is one of their pages on the misrepresentation of Haeckel by creationists.