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

use other materials, such as calcium carbonate or chitin. Also, unlike us, these animals have an exoskeleton covering the body. Our hardness lies within.
    Our particular brand of hardness, with teeth inside our mouths and bones inside our bodies, is an essential part of who we are. We can eat, move about, breathe, even metabolize certain minerals because of our hydroxyapatite-containing tissues. For these capabilities, we can thank the common ancestor we share with all fish. Every fish, amphibian, reptile, bird, and mammal on the planet is like us. All of them have hydroxyapatite-containing structures. But where did this all come from?
    There is an important intellectual issue at stake here. By knowing where, when, and how hard bones and teeth came about, we will be in a position to understand why. Why did our kind of hard tissues arise? Did they come about to protect animals from their environment? Did they come about to help them move? Answers to these questions lie in the fossil record, in rocks approximately 500 million years old.
    Some of the most common fossils in ancient oceans, 500 million to 250 million years old, are conodonts. Conodonts were discovered in the 1830s by the Russian biologist Christian Pander, who will reappear in a few chapters. They are small shelly organisms with a series of spikes projecting out of them. Since Pander’s time, conodonts have been discovered on every continent; there are places where you cannot crack a rock without finding vast numbers of them. Hundreds of kinds of conodonts are known.
    For a long time, conodonts were enigmas: scientists disagreed over whether they were animal, vegetable, or mineral. Everybody seemed to have a pet theory. Conodonts were claimed to be pieces of clams, sponges, vertebrates, even worms. The speculation ended when whole animals started to show up in the fossil record.
    The first specimen that made sense of everything was found by a professor of paleontology rummaging through the basement at the University of Edinburgh: there was a slab of rock with what looked like a lamprey in it. You might recall lampreys from biology class—these are very primitive fish that have no jaws. They make their living by attaching to other fish and feeding on their bodily fluids. Embedded in the front of the lamprey impression were small fossils that looked strangely familiar. Conodonts. Other lamprey-like fossils started to come out of rocks in South Africa and later the western United States. These creatures all had an exceptional trait: they had whole assemblages of conodonts in their mouths. The conclusion became abundantly clear: conodonts were teeth. And not just any teeth. Conodonts were the teeth of an ancient jawless fish.
    We had the earliest teeth in the fossil record for over 150 years before we realized what they were. The reason comes down to how fossils are preserved. The hard bits, for example teeth, tend to get preserved easily. Soft parts, such as muscle, skin, and guts, usually decay without fossilizing. We have museum cabinets full of fossil skeletons, shells, and teeth, but precious few guts and brains. On the rare occasions when we find evidence of soft tissues, they are typically preserved only as impressions or casts. Our fossil record is loaded with conodont teeth, but it took us 150 years to find the bodies. There is something else remarkable about the bodies to which conodonts belonged. They have no hard bones. These were soft-bodied animals with hard teeth.
    For years, paleontologists have argued about why hard skeletons, those containing hydroxyapatite, arose in the first place. For those who believed that skeletons began with jaws, backbones, or body armor, conodonts provide an “inconvenient tooth,” if you will. The first hard hydroxyapatite-containing body parts were teeth. Hard bones arose not to protect animals, but to eat them. With this, the fish-eat-fish world really began in earnest. First, big fish ate little fish; then, an arms