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

by Neil Shubin

If the odd course of our nerves is a product of our fishy past, the hiccup itself is likely the product of our history as amphibians.

tadpoles have a central pattern generator in their brain stem so that an inspiration is followed immediately by a closing glottis. They can breathe with their gills thanks to an extended form of hiccup. The parallels between our hiccups and gill breathing in tadpoles are so extensive that many have proposed that the two phenomena are one and the same. Gill breathing in tadpoles can be blocked by carbon dioxide, just like our hiccups. We can also block gill breathing by stretching the wall of the chest, just as we can stop hiccups by inhaling deeply and holding our breath. Perhaps we could even block gill breathing in tadpoles by having them drink a glass of water upside down.

The descent of the testes. During growth, the testes descend from the gonads’ primitive position high up in the body. They end up lying in the scrotum, which is an outpocket of the body wall. All of this leaves the body wall of human males weak in the groin area.

Mitochondria carry this bacterial past inside of them: with an entire genetic structure and cellular microstructure similar to bacteria, it is generally accepted that they originally arose from free-living microbes over a billion years ago. In fact, the entire energy-generating machinery of our mitochondria arose in one of these kinds of ancient bacteria. The bacterial past can be used to our advantage in studying the diseases of mitochondria—in fact, some of the best experimental models for these diseases are bacteria.

in 2001, elegant analyses of yeast (including baker’s yeast) and sea urchins won the Nobel in medicine for increasing our understanding of some of the basic biology of all cells.

What do billions of years of history mean for our lives today? Answers to fundamental questions we face—about the inner workings of our organs and our place in nature—will come from understanding how our bodies and minds have emerged from parts common to other living creatures. I can imagine few things more beautiful or intellectually profound than finding the basis for our humanity, and remedies for many of the ills we suffer, nestled inside some of the most humble creatures that have ever lived on our planet.