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

opposite directions. This feature is critical: think of trying to walk with your kneecap facing backward. A very different situation exists in fish like Eusthenopteron, where the equivalents of the knee and elbow face largely in the same direction. We start development with little limbs oriented much like those in Eusthenopteron, with elbows and knees facing in the same direction. As we grow in the womb, our knees and elbows rotate to give us the state of affairs we see in humans today.
    Our bipedal pattern of walking uses the movements of our hips, knees, ankles, and foot bones to propel us forward in an upright stance unlike the sprawled posture of creatures like Tiktaalik. One big difference is the position of our hips. Our legs do not project sideways like those of a crocodile, amphibian, or fish; rather, they project underneath our bodies. This change in posture came about by changes to the hip joint, pelvis, and upper leg: our pelvis became bowl shaped, our hip socket became deep, our femur gained its distinctive neck, the feature that enables it to project under the body rather than to the side.
    Do the facts of our ancient history mean that humans are not special or unique among living creatures? Of course not. In fact, knowing something about the deep origins of humanity only adds to the remarkable fact of our existence: all of our extraordinary capabilities arose from basic components that evolved in ancient fish and other creatures. From common parts came a very unique construction. We are not separate from the rest of the living world; we are part of it down to our bones and, as we will see shortly, even our genes.
    In retrospect, the moment when I first saw the wrist of a fish was as meaningful as the first time I unwrapped the fingers of the cadaver back in the human anatomy lab. Both times I was uncovering a deep connection between my humanity and another being.

 

    CHAPTER THREE

    HANDY GENES

    W hile my colleagues and I were digging up the first Tiktaalik in the Arctic in July 2004, Randy Dahn, a researcher in my laboratory, was sweating it out on the South Side of Chicago doing genetic experiments on the embryos of sharks and skates, cousins of stingrays. You’ve probably seen small black egg cases, known as mermaid’s purses, on the beach. Inside the purse once lay an egg with yolk, which developed into an embryonic skate or ray. Over the years, Randy has spent hundreds of hours experimenting with the embryos inside these egg cases, often working well past midnight. During the fateful summer of 2004, Randy was taking these cases and injecting a molecular version of vitamin A into the eggs. After that he would let the eggs develop for several months until they hatched.
    His experiments may seem to be a bizarre way to spend the better part of a year, let alone for a young scientist to launch a promising scientific career. Why sharks? Why a form of vitamin A?
    To make sense of these experiments, we need to step back and look at what we hope they might explain. What we are really getting at in this chapter is the recipe, written in our DNA, that builds our bodies from a single egg. When sperm fertilizes an egg, that fertilized egg does not contain a tiny hand, for instance. The hand is built from the information contained in that single cell. This takes us to a very profound problem. It is one thing to compare the bones of our hands with the bones in fish fins. What happens if you compare the genetic recipe that builds our hands with the recipe that builds a fish’s fin? To find answers to this question, just like Randy, we will follow a trail of discovery that takes us from our hands to the fins of sharks and even to the wings of flies.
    As we’ve seen, when we discover creatures that reveal different and often simpler versions of our bodies inside their own, a wonderfully direct window opens into the distant past. But there is a big limitation to working with fossils. We cannot do experiments on