Your inner shark, tadpole, jellyfish . . .

As a trained paleontologist and professor of anatomy at the University of Chicago, Neil Shubin discovered that by using comparative anatomy he could teach human anatomy to his students better. Many animals share similar body plans, and through looking at the sometimes less complicated organization in animals we can gain insight into our own development and anatomy. This idea was the groundwork for his book, Your Inner Fish: A Journey into the 3.5 Billion Year History of the Human Body.

As he humorously and meticulously details, our bodies are filled with evolutionary vestiges that speak to our long and complicated history as survivors of 3.5 billion years of evolution. A prime example of this history can be seen by looking at the developing human embryo; human embryos have four arches that resemble the gill slits in fishes. These arches develop into jaw bones, ear bones, the cranial nerves and other critical parts of our head and neck. The arches also have indentations behind them, and in fish it’s these indentations that form the gill slits that allow water to flow over the gills and out. The indentations in humans usually close over, but occasionally they don’t, resulting in babies born with a pouch or cyst on their neck. In some rare cases, babies are even born with a gill bar represented by a little rod where the neck arch used to be.

In the final chapter, “The Meaning of It all,” Shubin discusses some of the vestiges left from our evolutionary past that lead to, among other things, getting sick, obesity, having hernias and interesting physiological phenomena, such as hiccups.

Technically, a hiccup is a muscle spasm of our diaphragm, neck and throat that causes the sudden inhalation of air which is followed by the glottis — the tissue folds located around your vocal chords — closing, thereby making the “hic” sound. If you’re lucky, you might be able stop them right after they start by inhaling deeply and holding your breath — a technique I learned from the book that has proven effective so far. By mastering this technique you can save yourself from having people yell “boo” in your ear, advising you to “drink water upside down” or goading you to engage in any number of other “cures” which no doubt do more to entertain the audience gathered around you than to solve your diaphragm issues.

If you’re wondering what the hiccups have to do with your fishy ancestry, it all has to do with the management of the autonomic nervous system, or the reason why most of us don’t often forget to breathe.

Normally, the brain stem controls the autonomic response of breathing, just as it does in fish. But for fish, instead of controlling the diaphragm it controls the throat and gills. In fish, the nerves at the base of their head surround their gills, while in humans the nerve signals have an extended trip from the brain stem to the muscles in the chest that control breathing. The excessive length of the nerves leaves them open to interference — from things like spicy food or carbonated beverages — which can cause a muscle spasm.

This response originated in amphibians, where the resulting spasm — the hiccup — plays an important role in tadpoles that breathe with their gills, but also have lung structures. When they inhale water to breathe with their gills, they have the immediate autonomic response of a closing glottis to prevent water from entering the lungs. This extended form of a hiccup is what allows the tadpoles to breathe with their gills. Thankfully, only in extreme cases do humans have hiccups that last longer than 60 hics, as our brains usually remind our bodies in a timely fashion that we don’t have gills.

The book is filled with interesting information just like this, detailing everything from how mammary glands are made from a process of tissue interaction that also forms teeth, scales and feathers, to the genetic origins of our eyes and ears. It appears as though the separate genes for our eyes and ears first existed in a sort of “mosaic” single gene found in box jellyfish. This ancient connection has practical applications and can help us understand why birth defects sometimes affect both the inner ear and the eyes.

From my experience, science books can generally occupy two niches; some are so full of facts and jargon that the layperson stands little chance of finishing the book, let alone understanding it, while others offer readers a peek at a more interesting and vibrant world. With a good amount of paleontological adventure and humor to go along with the science Your Inner Fish lies firmly in the latter category and should be picked up by anyone interested in knowing a little bit more about their ichthyological origins.