I am sad to say that I’ve been putting this off. Make no mistake, this is a fascinating chapter, but trying to condense it is difficult. Shubin really is a good writer and doesn’t write a lot of fluff (unlike a particular creationist author we’re all aware of, who takes more than 500 pages to say absolutely nothing). Anyway, here we go….
This chapter is all about the head. Specifically it shows us more of our common ancestry. Creationists will claim that common design is a valid opposing hypothesis, but that really doesn’t hold water. Here’s why.
Design, human design (because in spite of claims, we still don’t know any other kind), is about the finished product. No one cares what the intermediate stages are. However, in human design (unlike organisms), efficient intermediate designs allow for easier manufacturing. So, you get a car and a truck, even though they are built in the same factory, they will have different intermediate stages, because they are different vehicle. While it makes sense to attach parts to the engine before it goes in a (tight) front-wheel drive, that doesn’t hold true for a big truck with plenty of engine bay room.
But the curious thing about human development is that we start from the same beginning as sharks. The pieces of the embryo that form the head of humans and the head and gills of fish are in the same place, they are same shape, and there are four of them. In fact, every vertebrate animal starts with these four arches.
Here they are:
There’s an embryo. The four folds underneath the eye spot labeled I and IV (II and III are between I and IV) are the arches.
Now, I said that every vertebrate animal has these 4 arches. Here’s a comparison of several embryos.
Here, the arches are labeled “Branchial arches”.
In humans, the first arch becomes the upper and lower jaw, the malleus and incus bones in the inner ear, and the blood vessels and muscle that attach to them. The second arch becomes the third ear bone (stapes), a bone in the throat, and the muscles that control facial expressions. The third arch forms the muscles, bones, and nerves of the throat (swallowing). The fourth arch forms the larynx and other parts deep in the throat.*
Isn’t in interesting that our ear is made from parts of two arches? Isn’t it interesting that parts of the throat are made from three different arches.
That doesn’t seem to make any sense… unless you understand the evolutionary path of these structures.
We are all modified sharks.
The first arch in sharks becomes the jaw, same as us. The second arch in sharks becomes a bar of cartilage and muscle. In humans that bar breaks up to become the stapes and some small structures of the throat. In the shark, the bar becomes the structure that allows sharks to extend their jaw slightly out of their mouth while feeding. The bone that allows this in sharks, if you carefully trace it’s evolutionary history becomes our stapes.
The third and fourth gill arches in sharks become the support structure for the gills.
If you carefully follow the control nerves through embryonic development AND through evolutionary history, you will find that the muscles and nerves that we use to swallow and talk are used by sharks to move the gills.
Our heads are amazingly complicated structures. However, they have a simple pattern that is the exact same in EVERY VERTEBRATE ANIMAL.
You can play tricks with developing embryos. If you cut off an arch, the embryo won’t develop the structures that the arch makes. You can go even further and turn off the genes that build those arches (Otx, for example) and get a similar result.
Your Inner Fish goes more into detail than I can or want to here. Including explaining why we humans sometimes get ruptured disks in our spines. Those disks are a hold over from Amphioxus, a worm. Yes, a non-vertebrate that does have a notocord and gill slits, the very precursors of our own spine and spinal cord and the arches that end up developing most of our face, jaws, and throat (including the ability to speak).
So, by this point, we’ve seen 4 pieces of evidence that support common descent. Yes, any one of the four, by itself, could be a ‘just so’ story or a coincidence. But when you start combining various types of evidence from a broad range of areas (head anatomy, embryonic development, wrist anatomy, genes, teeth and bones, plus all the evidence that isn’t anatomical or genetic) then you begin to realize that the idea of common descent isn’t just a fairy tale that scientists need to support evolution, it’s an extremely well support point.
In fact, it’s so well supported that the information developed from the concept of common descent can be used to make predictions and find new information (Tiktaalik).
* For those that are really into anatomy, the indentions between the arches are also very important. They form much of the soft tissue of the throat, eustachian tubes, glands, etc.