If you're going to tell a story from the beginning, there's always the question of how far back you should go. In Your Inner Fish, Neil Shubin starts about 375 million years ago and tells the story of how it is that every creature with a body is more closely related to humans than casual observation might suggest.
Shubin is a paleontologist who was teaching and doing research at the University of Chicago when a shortage of professors put him in front of an "Intro to Human Anatomy" class. Although his specialty is ancient fish, he says, "the best road maps to human bodies lie in the bodies of other animals... the bodies of these creatures are often simpler versions of ours."
He traces many of those similarities (our inner fish, bird, dinosaur, even fruit fly) over millions of years by looking at some of the physical and genetic structures we share. A casual observer might not realize that a bird's wing, a human's arm, even the fins of a humpback whale have the same basic blueprint: "one bone connected to two bones connected to a blob of bones connecting the fingers or toes."
Woven through the book is the story of a previously unknown fish/animal hybrid that Shubin and his colleagues discovered, fossilized in northern Canada. They gave it the name Tiktaalik ("large freshwater fish" in the local language). As an illustration on the book's cover shows, it looked a bit like a crocodile at its head and a fish at its tail. It had scales and fins; but unlike a fish, it had a neck, shoulders, elbows and wrists.
Scientists had hypothesized that the earliest fish to venture onto land would have had just the sort of structures found in the Tiktaalik. Shubin shows with a fanciful family tree that there are ways to be certain when particular adaptations or mutations occur in a species. His example: a humorless couple who have a son with a squeaky nose, who has a son with a squeaky nose and floppy feet, who has a son who's a "full bozo" with a squeaky nose, floppy feet and curly hair.
Predictably for a scientist, Shubin is systematic and detailed in making the case for the connections of human beings to other beings — citing evidence from the physiology of our skeletons to our genetic material. A fruit fly is vastly simpler than a human, but for both, certain key genes appear on every strand of DNA in the same order. Information about our heads is at one end, our bodies in the middle and our "tail" (which develops into the base of the human spine) at the other.
A comparison of the similar-looking shark embryos and human embryos also reveals our "inner fish." The bumps on the sides of sharks' heads become specialized jaws and gills; similar bumps in human embryos grow into bones and cartilage in our ears and throat.
Your Inner Fish is an ambitious book, zooming in to the cellular level to explore our DNA and zooming out to look broadly at the entire history of life on the planet. Neil Shubin makes his case in a way that is engaging and approachable, for which we can thank his fortuitous teaching assignment.