Embryology and Evolution

I wasn't going to do a third post about this in a single day, but I shouldn't have too much more to add on this topic *fingers crossed.* Besides, I just looked at the sample exam questions and damn near had a panic attack. So here goes...

So basically the main theme of this post is "ontogeny recapitulates phylogeny." Ontogeny is the development of the individual, and phylogeny is the development of the species. The idea is that as the embryo develops, it moves from a primitive one-cell organism and then gets more and more complex until it becomes a human or other animal, and this mirrors the development of organisms from simple one-celled organisms to more and more complex things. The idea isn't perfect, but it's a useful concept in understanding how many structures develop. In this post, we're going to see how this relates to the pharyngeal arches of the embryo.

In primitive jawless fish, the pharyngeal arches are the gill arches, with the first arch making up the mouth. In less-primitive fish with jaws, the first arch makes up the upper and lower jaws. The lower jaw is made up of several parts: the dentary (holds the teeth) and the articular and quadrate (form the jaw joint).

In mammals, like us, our first pharyngeal arch also develops into the mouth. Instead of the dentary, we have Meckel's cartilage, which also serves to hold the teeth. It develops into the mandible, and eventually articulates with the temporal bone but only after the articular and quadrate cartilages have buggered off. Where do they bugger off to? Well, the articular cartilage forms the malleus ("hammer") bone of the middle ear while the quadrate cartilage forms the incus of the middle ear. The other arches contribute to other structures of the neck, such as the larynx, but I won't go into those now.

And, um, that's all I need to cover on this topic. Main point is that the development of our jaw starts off very much like the development of the jaw in fish and reptiles. But then other stuff happens because we're more complex. Or something.