I have just finished reading Neil Shubin’s new book: Your Inner Fish: A Journey Into the 3.5 Billion Year History of the Human Body (2008). Shubin is one of those writers who writes to you as though he is speaking to you.  He manages to keep his sentences short yet friendly while he takes you on a mind-blowing journey from single-celled organisms up to his detailed explorations of human animals. Shubin is Provost of the Field Museum in Chicago, as well as a professor of anatomy at the University of Chicago.

The audience that really needs to read Shubin’s book will actively avoid reading it, of course.  Just think of the frustration that creationists already have with their idea that evolution teaches that “humans descended from monkeys.”  I hear this wrongheaded claim repeatedly and it gets quite tedious explaining to the creationist ignoramuses that no modern believer in evolution believes that humans descended from “monkeys.”  The irony of correcting creationists, however, is that the story of how the Earth’s creatures evolved is actually incredibly more interesting and challenging than the creationist’s simplistic version of evolution.  For instance, human ancestors include not only primates; they include fish too, and reptiles and worms.  Neil Shubin takes us on this awesome journey and there is much to share along the way.

I previously wrote about one of the incredible transitional forms discussed by Shubin, tiktaalik, an ancient fish that crawled out of the water. Tiktaalik, however, is only one of numerous transitional forms.  In fact, if there is a deep lesson to learn from reading Your Inner Fish, it is that every form, every plant and every animal, is literally and truly a transitional form.  Each of the earths living organisms is on a journey from what it used to be, heading toward what it is becoming.  Those who are on this journey include human animals, of course.  We can’t easily see where we are going, but the information provided by fossils, DNA and other objective evidence tells us where we’ve been.

One of the many transitional forms Shubin describes is the trithrledont, part mammal and part reptile, the telltale mark being tiny bumps and ridges on its teeth that included tooth-to-tooth occlusion (70). Shubin doesn’t only work in labs and classrooms.  He had a major part in finding some of the incredible fossils he describes, including trithrledont.   I especially enjoyed his description of how one develops an “eye” for finding fossils.

Over time, I began to learn the visual cues for other kinds of bones: long bones, jawbones and skull parts.  Once you see these things you never lose the ability to find them.  Just as a great fisherman can read the water and see the fish within, so a fossil finder uses a catalog of search images that make fossils seem to jump out from the rocks.

Looking for fossils is hard work, but Shubin reminds us what is at stake:

Early mammals were small.  Very small.  Their teeth were not much more than 2 mm long.  To spot them, you had to be very careful and, more often, very lucky.  If the tooth was covered by a crumb of rock or even a few grains of sand, you might never see it…. occasionally… I’d hit the jackpot and see a deep connection for the first time…. I was seeing some of the first evidence of our pattern of precise chewing, only in a tiny mammal 190 million years old.  The power of these moments was something I’ll never forget.  Here, cracking rocks in the dirt, I was discovering objects that could change the way people think.  That juxtaposition between the most childlike, even humbling, activities and one of the great human intellectual aspirations has never been lost on me.  I try to remind myself of it each time I dig somewhere new.

There are many good lessons in Your Inner Fish.  For example, Shubin tells us about the non-obvious connections between teeth, breasts, feathers and hair: they all developed from skin (79).  He describes how worms are yet another good example of a transitional form. Evolutionarily speaking, it is from worms that we got our heads (96).  But don’t go thinking that we have evolved completely beyond the simple forms of worms.  In utero, each of us initially becomes a simple tube “with a fold of swelling at the head end and another at the tail” (101).  Those cells can unfold themselves into a large animal only by doing their extraordinary biochemical magic.  Most importantly, those tiny cells need to know how to talk to each other.  Such communication was a complex development that did not occur until halfway through the history of life on Earth.  Where is the halfway point?  Shubin illustrates how long it has taken for the human animal to get where it is:

Take the entire 4.5 billion year history of the earth and scale it down to a single year, with January 1 being the origin of the earth and midnight on December 31 being the present.  Until June, the only organisms were single celled microbes such as algae, bacteria and amoeba.  The first animal with a head did not appear until October.  The first human appears on December 31 (119).

We are reminded by Shubin of the many ways that we are still like single-celled organisms.  Much of our bodybuilding apparatus already exists in single celled placozoans.  Moving further toward the present, Shubin notes many shared characteristics between human bodies and sponges.  Those sponges clearly share some of the human body’s features.  Most significantly, “the sponge has much of the cell adhesion, communication and scaffolding apparatus that we have.  Sponges are bodies, albeit very primitive and relatively disorganized ones.”

These findings lead Shubin to a fascinating discussion of why there are bodies at all.  After all, a body is a very expensive thing to own and operate (137).  Throughout Shubin’s book, it is the simple questions that have the most incredible answers.

If you want to make a bunch of DNA, Shubin is willing to help.  He’ll tell you how to gather DNA in your own kitchen.

I found it fascinating to learn that 3% of the human genome is dedicated to odor detection.  This is the equivalent of over 1000 genes, although 300 these genes are useless in humans.  We carry this nonfunctional DNA baggage around, from generation to generation. Our broken olfactory genes “are left over from mammal ancestors who relied more heavily on the sense of smell to survive.”

Transitional life forms are everywhere one cares to look.  For instance, Shubin details the research of German anatomists Karl Reichert, who determined that the bones which turned out to be the tiny middle-ear bones in mammals, ended up serving as large jawbones in reptiles (160). This is a good illustration of nature-the-tinkerer.

Toward the end of the book, Shubin shares with us a biological law that is so simple and yet “so profound that most of us take it completely for granted.”  He describes this law as the “starting point for almost everything we do in paleontology, developmental biology and genetics.  This biological “law of everything” is this

“Every living thing on the planet had parents.”

It is this dramatic and compelling observation that spurs us to explore the biology and morphology of our non-human ancestors. As Shubin compellingly argues in his book, this journey backwards in time beckons because our bodies are biological time capsules.

It is difficult to think of any story more incredible than the true story that humans descended from fish (181). We are, in fact, “tweaked up fish.”  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 50 times to test a theory of gravity.”

Shubin’s book is a delight to read, even for those who are familiar with many of the findings Shubin discusses.  That is because he has a special talent for communicating what it is like to be a real scientist who gets his hands dirty and yet keeps the focus on the dramatic evidence-based big picture.