Throughout the work, Shubin recurrently compares all creatures to each other, but he heavily emphasizes the comparison of humans to fish. While this may be due to the fact that the title is “Your Inner Fish” which specifically references human and fish interrelations, the constant comparison is made to stress the main theme of the commonality of all creatures on the earth.The major theme of the work is no better stated than by Shubin himself when he says “All animals are the same but different. Like a cake recipe passed down from generation to generation—with enhancements to the cake in each—the recipe that builds our bodies has been passed down and modified for eons. We may not look like sea anemones and jellyfish, but the recipe that builds us is a more intricate version of the one that builds them”. It is because of this belief that Shubin scrupulously seeks out the anatomical resemblances between the creatures of today and the fossils of the past, hoping to find the predecessors that will help to fill in the missing pieces of the vast puzzle of evolution.When he first starts out on his quest to prove the commonality of all animals, Shubin uses paleontology as his primary tool to unearth the ancient connections. Quite early on in his journey he states “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 even a smaller fraction still are ever found, then any attempt to see our past seems doomed from the start”, proving that only so much can be accomplished due to paleontology. Knowing these facts did not hinder Shubin in the slightest, in fact it is through paleontology that he co-discovers the fossil of the “hand fish” properly known as Tiktaalik. The Tiktaalik is an important discovery because it fills the missing puzzle piece of how the transition of limbs from fins to hands was made. Shubin then begins to compare the hand and limb structures of several animals, like the wing of a bat and the hand of a human, and brings the reader’s attention to the homology of these structures. The stark similarities between these limbs and hands are some of the oldest and strongest evidences for evolution.
Every living being on this planet, whether plant or animal, have one thing in common, DNA. Shubin shifts his focus from the fossils of extinct creatures to the DNA of living ones. He takes the comparison of creatures to a new level as he starts with a general observation of the likeness of the limbs of certain creatures and then narrows the search for similarities down to the cellular level and finally hones in on the genes and DNA of creatures. After diving into the biological comparison of DNA, Shubin resurfaces into anatomical sphere and shines a light on teeth, a frequently forgotten part of anatomy, teeth. Teeth are made of a different material than bone that makes them stronger which results in them being preserved better and commonly found when digging for fossils. He goes on to explain that teeth can tell paleontologists the diet of the animal and give insight on how the animal lived. Readers are taken on a ride down chemistry lane when Shubin goes in depth about the molecular composition of teeth, stating that their main component is hydroxyapatite, a super strong inorganic material that makes teeth more durable than even bone.
Zooming out of the mouth and onto the head, Shubin goes on to discuss cranial development and uses the head as a segway into the topic of embryonic development. It is here that he reports the resemblances of the developing fetuses of sharks and human, more specifically the four “arches” that comprise the region where the gills are/would be. He goes on to explain what each arch develops; the third and fourth arches create similar cranial nerve and muscular developments. While each species share the same four arch design, they develop differently depending on the actual organism. Shubin meticulously observes the embryos of several creatures because he believes “Embryos hold the clues to some of the profound mysteries of life”. He also does an intelligent breakdown of the early analysis on embryos of Von Baer and Haeckel, which he uses to discuss the genetics of body development within the embryos of different species.
Humans and most other animals have 5 senses, however the degree, or level, of these sense vary depending on species. Shubin takes the next few chapters to compare the human senses of sight, smell and hearing with those of different animals. He draws the conclusion that while the noses of animals may look different, their mechanisms are fundamentally the same, even when aquatic smelling apparatuses are compared to air smelling ones. A similar approach is taken when he observes the eyes of humans compared to several diverse animals, he notes the genetic likeness that lead to the development of optic tissues; in this comparison Shubin makes sure to note the importance and function of opsin.