Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body by Neil Shubin Read Free Book Online Page A

long-dead animals. Experiments are great because we can actually manipulate something to see the results. For this reason, my laboratory is split directly in two: half is devoted to fossils, the other half to embryos and DNA. Life in my lab can be schizophrenic. The locked cabinet that holds Tiktaalik specimens is adjacent to the freezer containing our precious DNA samples.
    Experiments with DNA have enormous potential to reveal inner fish. What if you could do an experiment in which you treated the embryo of a fish with various chemicals and actually changed its body, making part of its fin look like a hand? What if you could show that the genes that build a fish’s fin are virtually the same as those that build our hands?
    We begin with an apparent puzzle. Our body is made up of hundreds of different kinds of cells. This cellular diversity gives our tissues and organs their distinct shapes and functions. The cells that make our bones, nerves, guts, and so on look and behave entirely differently. Despite these differences, there is a deep similarity among every cell inside our bodies: all of them contain exactly the same DNA. If DNA contains the information to build our bodies, tissues, and organs, how is it that cells as different as those found in muscle, nerve, and bone contain the same DNA?
    The answer lies in understanding what pieces of DNA (the genes) are actually turned on in every cell. A skin cell is different from a neuron because different genes are active in each cell. When a gene is turned on, it makes a protein that can affect what the cell looks like and how it behaves. Therefore, to understand what makes a cell in the eye different from a cell in the bones of the hand, we need to know about the genetic switches that control the activity of genes in each cell and tissue.
    Here’s the important fact: these genetic switches help to assemble us. At conception, we start as a single cell that contains all the DNA needed to build our body. The plan for that entire body unfolds via the instructions contained in this single microscopic cell. To go from this generalized egg cell to a complete human, with trillions of specialized cells organized in just the right way, whole batteries of genes need to be turned on and off at just the right stages of development. Like a concerto composed of individual notes played by many instruments, our bodies are a composition of individual genes turning on and off inside each cell during our development.
     

Genes are stretches of DNA contained in every cell of our bodies.
     
    This information is a boon to those who work to understand bodies, because we can now compare the activity of different genes to assess what kinds of changes are involved in the origin of new organs. Take limbs, for example. When we compare the ensemble of genes active in the development of a fish fin to those active in the development of a human hand, we can catalogue the genetic differences between fins and limbs. This kind of comparison gives us some likely culprits—the genetic switches that may have changed during the origin of limbs. We can then study what these genes are doing in the embryo and how they might have changed. We can even do experiments in which we manipulate the genes to see how bodies actually change in response to different conditions or stimuli.
    To see the genes that build our hands and feet, we need to take a page from a script for the TV show CSI: Crime Scene Investigation —start at the body and work our way in. We will begin by looking at the structure of our limbs, and zoom all the way down to the tissues, cells, and genes that make it.

    MAKING HANDS

     
    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 one side to the other. Our pinkies are shaped differently from our thumbs. The Holy