Finding Your Inner Fish
This chapter touches on how common descent and the fossil record are intertwined. It spends the majority of the time discussing the actual discovery of Tiktaalik.
Shubin asks this question early in the chapter
How can we visualize events that happened millions and, in many cases, billions of years ago?
That is a very common question when talking to those who are not educated in modern science. Then he reminds us that 99 percent of all species that ever existed are extinct. And that the percentage of organisms that have been fossilized is miniscule and that only certain species will ever be fossilized because of where they lived. It’s a daunting task. Fortunately, scientists like Shubin didn’t give up or claim it to be impossible. They went out and did the work.
He then uses an example of a zoo to describe the nested hierarchy of living things. It should be obvious to even a casual student of science that chimpanzee limbs are very similar to ours. Indeed, every single terrestrial amphibian, reptile, mammal and bird has exactly the same limb pattern. One major bone, two lower bones, a wrist, and five fingers. Isn’t that curious?
Why would every single vertebrate land animal have exactly the same bone structure in the limbs? It would make sense if every single vertebrate land animal was descended from a common ancestor that had that particular structure.
Here’s the hypothesis:
If all vertebrate land animals descended from a common ancestor that had that limb structure, then we should be able to find that common ancestor in rock of an appropriate age.
Work of other paleontologists have found rocks of about 165 million years old that have fully formed amphibians. There are no amphibian fossils in any rock so far discovered that are 185 million years old. There are fully formed bony fish in rocks of that age.
So, next hypothesis:
If there are amphibians at 165 million years ago and no amphibians at 185 million years ago, then we should be able to find the transitional fossil between those two ages.
Fish live in water, mostly salt water. Amphibians live in water and moist land, usually fresh water. So the transitional fossil should be in fresh, shallow water.
There are very few exposed rock surfaces on the Earth that are of this age (Devonian for those keeping track) and are of freshwater river deltas and freshwater marshes. One of those places is Ellesmere Island in the Arctic.
The story of the expeditions and discovery of Tiktaalik is interesting and Shubin really gives you the feeling of being there.
Needless to say that after six years, they found it. Tiktallik is nearly perfectly intermediate between fish and amphibians. It has a scales and fins with fin webbing, but the fins contain the standard limb structure of one upper arm bone, two lower arm bones, and a wrist. It also has a flattened head and neck of a land living creature.
To quote Shubin
It took us six years to find it, but this fossil confirmed a prediction of paleontology: not only was the new fish an intermediate between two different kinds of animal, but we had found it also in the right time period in earth’s history and in the right ancient environment. The answer came from 375-million year-old rocks, formed in ancient streams.
There it is. That’s how science works. Shubin and his crew weren’t on Ellesmere Island for the fun of it. They were there because the evidence led them there. Fortunately, they were able to find that animal.
I will relate on story from Chapter 1. After all the press releases, Shubin’s son’s preschool teacher asked him to bring Tiktaalik to the class. So he brought a cast of it and asked them what it was. The first boy said it was an alligator because of the head and teeth. ANother child said it was a fish because of the fin and scales. Another child shouted “Maybe it’s both.”
Even preschoolers can see it.
Tiktallik can tell us a lot about what it is to be human. It’s the earliest living thing with a neck… so it can turn it’s head and look around independently of its body. It has a limb structure that is just like humans and horses and everything else.
That’s the lesson of common descent. The basics of our anatomy were present in fish 375 million years ago.