Your Inner Fish: A Journey into the 3.5 Billion-Year History of the Human Body

by Neil Shubin

If you consider that over 99 percent of all species that ever lived are now extinct, that only a very small fraction are preserved as fossils, and that an even smaller fraction still are ever found, then any attempt to see our past seems doomed from the start.

But why live in these environments at all? What possessed fish to get out of the water or live in the margins? Think of this: virtually every fish swimming in these 375-million-year-old streams was a predator of some kind. Some were up to sixteen feet long, almost twice the size of the largest Tiktaalik. The most common fish species we find alongside Tiktaalik is seven feet long and has a head as wide as a basketball. The teeth are barbs the size of railroad spikes. Would you want to swim in these ancient streams? It is no exaggeration to say that this was a fish-eat-fish world. The strategies to succeed in this setting were pretty obvious: get big, get armor, or get out of the water. It looks as if our distant ancestors avoided the fight.

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 race began. Little fish developed armor, big fish obtained bigger jaws to crack the armor, and so on. Teeth and bones really changed the competitive landscape.

One of the joys of being a scientist is that the natural world has the power to amaze and surprise. Here, in ostracoderms, an obscure group of ancient jawless fish, lies a prime example. Ostracoderms are among the earliest creatures with bony heads. Cut the bone of the skull open, embed it in plastic, pop it under the microscope, and you do not find just any old tissue structure; rather, you find virtually the same structure as in our teeth. There is a layer of enamel and even a layer of pulp. The whole shield is made up of thousands of small teeth fused together. This bony skull—one of the earliest in the fossil record—is made entirely of little teeth. Teeth originally arose to bite creatures; later, a version of teeth was used in a new way to protect them.

Teeth not only herald a whole new way of living, they reveal the origin of a whole new way of making organs. Teeth develop by an interaction of two layers of tissue in our developing skin. Basically, two layers approach each other, cells divide, and the layers change shape and make proteins. The outer layer spits out the molecular precursors of enamel, the inner layer the dentine and pulp of the inside of the tooth. Over time, the structure of the tooth is laid down, then tweaked to make the patterns of cusps and troughs that distinguish each species. The key to tooth development is that an interaction between these two layers of tissue, an outer sheet of cells and an inner loose layer of cells, causes the tissue to fold and makes both layers secrete the molecules that build the organ. It turns out that exactly the same process underlies the development of all the structures that develop within skin: scales, hair, feathers, sweat glands, even mammary glands. In each case, two layers come together, fold, and secrete proteins. Indeed, the batteries of the major genetic switches that are active in this process in each kind of tissue are largely similar. This example is akin to making a new factory or assembly process. Once plastic injection was invented, it was used in making everything from car parts to yo-yos. Teeth are no different. Once the process that makes teeth came into being, it was modified to make the diverse kinds of organs that lie within skin. We saw this taken...

When you look into eyes, forget about romance, creation, and the windows into the soul. With their molecules, genes, and tissues derived from microbes, jellyfish, worms, and flies, you see an entire menagerie.

Some groupings are so strong that, for all intents and purposes, we consider them fact. The fish–turtle–polar bear–human grouping, for example, is supported by characteristics from hundreds of genes and virtually all features of the anatomy, physiology, and cellular biology of these animals. Our fish-to-human framework is so strongly supported that we no longer try to marshal evidence for it—doing so would be like dropping a ball fifty times to test the theory of gravity.

Carl Sagan once famously said that looking at the stars is like looking back in time. The stars’ light began the journey to our eyes eons ago, long before our world was formed. I like to think that looking at humans is much like peering at the stars. If you know how to look, our body becomes a time capsule that, when opened, tells of critical moments in the history of our planet and of a distant past in ancient oceans, streams, and forests. Changes in the ancient atmosphere are reflected in the molecules that allow our cells to cooperate to make bodies. The environment of ancient streams shaped the basic anatomy of our limbs. Our color vision and sense of smell has been molded by life in ancient forests and plains. And the list goes on. This history is our inheritance, one that affects our lives today and will do so in the future.

What are the leading causes of death in humans? Four of the top ten causes—heart disease, diabetes, obesity, and stroke—have some sort of genetic basis and, likely, a historical one. Much of the difficulty is almost certainly due to our having a body built for an active animal but the lifestyle of a spud.

The real story is that Apollo 8 is a symbol for the power of science to explain and make our universe knowable. People can quibble over the extent to which the space program was about science or politics, but the central fact remains as clear today as it was in 1968: Apollo 8 was a product of the essential optimism that fuels the best science. It exemplifies how the unknown should not be a source of suspicion, fear, or retreat to superstition, but motivation to continue asking questions and seeking answers.