In the mid-twentieth century, biologists were doing experiments with chicken eggs, cutting up the embryos and grafting tissues to other places to figure out what would happen if embryos’ development was interfered with. Chicken eggs were perfect for these experiments; they were large enough for biologists to carefully choose which parts to cut up and graft, protected by the eggshell, and available in large quantities everywhere. These experiments would become important to scientists studying evolution because the experiments eventually revealed that the basic genes in all animals are the same; they’re simply put to different uses in different animals.
Scientists performed a variety of experiments that eventually led to the discovery of a tissue that controlled how fingers and toes formed and made each finger different. One set of experiments, performed by a group of biologists that included Edgar Zwilling and John Saunders, revealed a tissue that controlled limb development. They removed it at different periods of development, and the earlier it was removed, the less of the limb had developed. Meanwhile, Mary Gasseling had discovered that if the tissue Zwilling and Saunders had found was moved to the opposite side, a mirror image of the limb would form. The tissue was named ZPA, and a variety of experiments were done with it to discover how it controlled the development of limbs, fingers, and toes. A variety of molecules were suggested, but none of them were capable enough.
Then, in the early 1990s, new technology became available for scientists to discover what was really going on with ZPA and how it controlled the development of fingers and toes. At that point, Tabin, McMahon, and Ingham came up with the idea of comparing the chickens to flies. One gene, called hedgehog, made one end of a fly’s body segment look different from the other, so the three laboratories began looking for a similar gene in chickens. They found one: Sonic hedgehog, which was active in the ZPA tissue. Knowing the structure of Sonic hedgehog, other researchers could look for it in animals with fingers, and they found the gene active in the ZPA tissue in every one of those animals.
At that point, Randy Dahn, who was part of Shubin’s lab, decided to connect skates, which have fins, with the fingered animals, which includes humans, to find an inner fish. He soon found a Sonic hedgehog gene in skates that did the same thing to the skates’ fins as the gene in fingered animals did to those animals’ fingers. Dahn knew that the skeletal rods in a skate fin looked alike, so would an injection of a mouse’s Sonic hedgehog gene make the rods develop differently? He tried it, putting a bead that leaked the protein in a skate embryo. After developing, the rods looked very different from one another, and the rods closer to the bead developed differently than the rods further away, just like they would for a person, meaning that the skate’s Sonic hedgehog genes were identical, or nearly identical, to a person’s Sonic hedgehog genes. Dahn had discovered that, when fish evolved into people, the genes stayed the same; they just did things in new ways.
The experiments done in the mid-twentieth century were important to the story of Your Inner Fish because they ultimately led to Dahl’s discovery that human genes and skate genes are similar and reinforce the idea that humans evolved from fish. The experiments also show that there are many connections between living animals, often in unexpected ways.
I really enjoyed reading Handy Genes and found it quite interesting as well as informative. It taught me a lot about genes and DNA, especially the genetic connection between humans and other animals, such as sharks, skates, chickens, and flies. At first, it was a bit hard to understand, since it’s filled with a variety of information, but after a while of rereading it became clear. I would recommend this chapter, and the entire book, to anyone interested in genetics, Tiktaalik, biology, evolution, or DNA and rate it 9.6/10.
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