You may not feel much like a shark, fruit fly, or worm, but you share many aspects of your anatomy and physiology with these and all other animals on Earth. All the various bits and pieces of you–organs, bones, nerves, even your genes–show up in different but fundamentally similar forms in other animals, in some cases animals that lived half a billion years ago, revealing how all creatures on Earth, including you, are just variations on a theme.

Neil Shubin is professor of anatomy at the University of Chicago. This feature is adapted from Shubin's book Your Inner Fish: A Journey Into the 3.5-Billion-Year History of the Human Body (Vintage, 2009). Illustrations are by Kalliopi Monoyios.