Chapter three of "Your Inner Fish" talks a lot about ZPA tissues. The story of our relationship to fossil animals is not just based on anatomy. It can also be seen in our genes. Genes are a major part of the body and makes each of us who we are. In this chapter, Neil Shubin talks about a lab which is divided into two parts. The first part involves the study of fossils, while the other one focuses on the study of DNA and embryonic development. According to Shubin, the majority of our cells have the same copy of DNA.
So, our limbs exist in three dimensions: a top and a bottom, a pinky side and a thumb side, a base and a tip. Each bone is different from the other, and our body knows to develop in the way that Shubin describes because of chemicals produced by the cells in our our developing body. Researchers found that a certain patch of cells in the body were responsible for all of the limb development going on. If you remove that patch, no limbs develop. If you cut that patch in half, a person would end up with two limbs. All of this was tested on chickens.
Certain geneticists began to work with flies and found a gene they called hedgehog. This gene determined which end of the fly was which, and this concept is quite similar to that of the patch of cells. Therefore, researchers began looking for something similar to this in humans. The chicken version of the hedgehog is called the Sonic hedgehog. Apparently, every creature on the planet has the Sonic hedgehog gene. This gene can be controlled by a vitamin A injection, resulting in changes in how the limbs develop.
Researchers injected some protein that the Sonic hedgehog makes into a shark embryo. The protein was that of a mouse. Because the mouse gene was similar to the shark gene the limb development was affected the same way as if it were injected with vitamin A injection. This means that the evolution from fish fins to limbs likely involved the use of ancient genes.
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