Looking back at various animals
One of the common motif in the book is the idea that we can learn a lot about humans and how they developed by analyzing the skeletal structure of various animals and fish. The author mentioned a large list of amphibians and fish whose skeletal structure later developed into complex limbs such as the hand.
Looking at the similarities between species
Another common motif in the book is the idea that one should look not at the differences between species but rather at what various species have in common. By analyzing these common features, the author hopes to find an explanation to how the different species came to be and how they developed from a single cell.
Studying animals makes it easier to understand the human anatomy
One of the common motifs in the book is the idea that if a person studies the anatomy of fishes or simpler life forms, then it would also be easier to understand the human anatomy. The reason behind this is that the animal anatomy is similar to the human one but at the same time it is much simpler and thus easier to understand while also not being exceptionally different from the human anatomy.
Like old plumbing
In the fifth chapter, Shubin compares the process through which nerves developed with the action of replacing old plumbing. Through this example, Shubin wanted to stress how sometimes certain things need to disappear for better and more functional elements to take their place.
Comparisons with unlikely animals
Another common motif in the novel is the idea that humans have many things in common with creatures that at a first glance look nothing like us and are the most unlikely beings to have something in common with humans. One such being is the worm mentioned in the fifth chapter and according to Shubin, humans and worms have in common the basic structure of their spine and head.
