The story of our relationship to fossil animals is not just based on anatomy. It can also be seen in our genes. Those sequences of DNA that make us who we are. Indeed makes every living thing what it is.
Shubin’s lab is divided into two parts. One to study fossils and one to study DNA and embryonic development. For these two things go hand in hand (if you’ll forgive the pun).
This chapter is pretty thick and a summary will be difficult, so bear with me if this gets long.
We all know that DNA is what makes us what we are. What we’re not always sure about it how. So, while Shubin was in the Arctic, another researcher in his lab was busy injected a form of Vitamin A into shark and skate embryos. Why?
To quote Shubin
Our limbs exist in three dimensions: They have a top and a bottom, a pinky side and a thumb side, a base and a tip. The bones at the tips, in our fingers, are different from the bones at the shoulder. Likewise, our hands are different from our thumbs.
How does our body know to develop this way?
In short, it has to do with concentrations of chemicals produced by the cells in our our developing body. All of the genetic switches that control this do their thing between the third and eighth week after conception. Yep, before most women even know they are pregnant, the basic structure of the embryo is already set up.
What researchers in the 50s and 60s found was that a certain patch of cells was responsible for all of the limb development. Remove that patch and no limb develops. Turn the patch of cells over and the limb grows backwards. Cut the patch in half and you end up with two limbs. Note that this was all done on chickens.
Later, geneticists working with flies discovered a gene they called hedgehog. This gene controlled which end of the fly was which. This was very similar in concept to the patch of cells that controlled the orientation of the limbs, so the researchers went looking for this in other creatures.
They named the chicken version of the gene (which is surprisingly similar to the fly version) Sonic hedgehog. Yes, after the video game. But what did it actually do?
With a neat trick that allows the region of the body that Sonic hedgehog is active to glow, the researchers found that gene was only active in those cells that I mentioned in limb development.
Interestingly, the form of Vitamin A that was mentioned previously causes Sonic hedgehog to activate. So by injecting Vitamin A, one can control the development of limbs in a chicken embryo. It’s easy to make double wings for example or mirror wings (top and bottom).
It gets even more interesting, because every limbed creature on the planet has the Sonic hedgehog gene. It is effectively the same in everything and it can be controlled by vitamin A injections.
In fact, the same gene is present in sharks and skates, which are very primitive versions of sharks. In fact, the vitamin A injections work exactly the same in sharks and skates, resulting in changes in how the limbs develop.
The researchers went a step farther and injected a small amount of the protein that Sonic hedgehog makes into the developing limb of a shark embryo. Except that it was the protein from a mouse. If the mice gene is too different from the shark gene, nothing will happen. If the mice gene is very similar, then it will affect the limb development in exactly the same way as vitamin A injections. Anyone want to guess what happened?
Yep, the mouse protein injection had the same effect on the shark. This means that the protein and therefore the gene are effectively the same in a shark and a mouse. How’s that for evidence of common descent?
What does all this mean?
I’ll let Shubin speak again.
What does this mean for the problem we looked into in the first two chapters – the transition of fish fins into limbs? It means that this great evolutionary transformation did not involve the origin of new DNA: much of the shift likely involved using ancient genes, such as those involved in shark fin development, in new ways to make limbs with fingers and toes.
Now, I’m sure that creationists will pipe up with all the things we don’t know. Of course, if we knew everything, then scientists would be out of a job. However, research like this is just stunning, especially compared with the research that creationists are doing…
And this shows a stunning example of common descent. If the exact same gene controls limb development in everything from sharks to humans, then it is a much more parsimonious explanation that we all had a common ancestor.