From gills to gonads, you’re more like a fish than you may believe

We all know that humans evolved from a common ancestor of apes. But if you really want to understand how and why the human body works the way it does, you need to look further up the evolutionary tree.

To fish.

Neil Shubin, an evolutionary biologist at the University of Chicago, has written a new book called, Your Inner Fish — A Journey Into the 3.5 Billion Year History of the Human Body. He also hosted a three-part PBS series called Your Inner Fish. He says fish, sharks, even worms and jellyfish, help explain why, for example, our inner ear is made up of three tiny bones or why we get hernias.

Shubin’s enthusiasm about his subject is infectious. Before you know it, you may find yourself discussing the evolution of the inner ear with friends or hear yourself saying the word “gonads” in polite conversation.

Shubin says he relies on the power of scientific discovery as a storytelling device. “People venturing into the unknown and exploring and discovering always has a human aspect to it,” he says.

But what do modern humans have to do with fish? A lot, Shubin says.

“[I]f you look at the Tree of Life,” Shubin explains, “inside every part of our body, every organ, every cell, every gene, we have one entire branch of the history of life which is knowable. It’s knowable by fossils, by comparing DNA and so forth.”

Take the human arm, for example. “If we trace them all the way back through mammals, through reptiles and amphibians, all the way down to fish,” Shubin says, “they compare to the paired fins of fish — which we can connect from the fossil record, from developmental biology, multiple lines of evidence.”

And that's just the beginning.

“If you trace our jaws and ear bones back,” Shubin says, “it’s very clear that they relate to the gill structures in fish. Again, we know [this] from multiple lines of evidence — the fossils, as well as the DNA. Many of the muscles and nerves and bones that we’re using to communicate right now … are derived from the gill structures of sharks and fish — and that seems so utterly strange. But when we unravel … that story, with the evidence behind it, it becomes very powerful.”

“If you think about just the basic mechanism by which we turn our heads and know ourselves in space,” he continues, “the eye is coordinated to where we are in space. As you move your head, your eye can remain fixed in gaze, and that's happening because of the communication between your brain and the muscles in your eye. It's wonderfully complex and it’s also wonderfully ancient. [T]he inner ear is indeed one of the most ancient parts of our body.”

Ever wondered about hiccuping — and why it happens? Shubin says this, too, is an example of our ancient link to fish. In this case, tadpoles.

“When you hiccup,” Shubin explains, “that 'hic' is a sharp inspiration, and then the closure of your glottis and your epiglottis — that’s what causes the 'hic.' [It] comes about through a complex, but stereotyped, pattern of activity of nerves and muscles of our pharynx and the back of our throat and in our chest and so forth.”

What drives that physical response, Shubin adds, is a mechanism in our central nervous system called a central pattern generator. It turns out that the central pattern generator is part of the normal breathing of tadpoles, which have both lungs and gills. Tadpoles use a version of a hiccup to push water across their gills, not into their lungs.

And then there's the not-entirely-pleasant topic of hernias. The human-fish connection with hernias is one of Shubin’s favorites.

“If you look at what happens during development in a shark or a fish,” he explains, “the gonads actually begin high up … close to the heart, above the kidneys. Then they [stay] there. What happens in a human is we begin our development with the gonads in a fish position. As we develop and grow, they descend. In females, they descend to the level of the uterus; but in males, they descend and actually push out of the body wall to form this little sac, the scrotum.”

“This traveling of the gonads,” he continues, “creates a weird loop in the male plumbing that…is involved with the development of a weakness in the male body wall, both during development and later in life. So, the male propensity for certain kinds of hernias relates [to] starting your development like a fish and ending your development like a mammal.”

Shubin says he's constantly surprised by the power of paleontology to explain why creatures look the way they do. New findings are happening all the time, he says.

“I guess what really attracts me to the field … is that this is knowable history, these events that happened billions of years ago. We can produce evidence to show how they happen and I am always pinching myself when this happens, and it's always like the first time when I see these things,” he says.

Shubin says there are a lot of fossils still to be discovered. “We are launching a whole new set of expeditions this summer, to a new time period,” he says. “There are whole parts of the world that have yet to be really explored. We have new technologies that change how paleontologists work.”

He's "really stoked” about his next project. He and his team are going to northern Ellesmere Island — near Greenland, a few hundred miles from the North Pole — to an area of unexplored rock about 400 to 500 million years old. It's expected to provide the earliest look at fish evolution.

“This rock was originally about ten degrees north of the equator,” he explains, “so what you have captured in these rocks is a warm, watery world of shallow, near-shore seas. It’s the perfect environment to find some of these early vertebrates or early fish. … It’s the exciting part, he says. We’re really doing some basic exploration.”

This story is based on an interview by our partner Science Friday, a weekly radio show and website covering science, technology and other cool stuff.