Your Inner Fish: A Journey into the 3.5-Billion-Year History of the Human Body

by Neil Shubin

The “inner fish” that Randy found was not a single bone, or even a section of the skeleton. Randy’s inner fish lay in the biological tools that actually build fins. Experiment after experiment on creatures as different as mice, sharks, and flies shows us that the lessons of Sonic hedgehog are very general. All appendages, whether they are fins or limbs, are built by similar kinds of genes. What does this mean for the problem we looked at in the first two chapters—the transition of fish fins into limbs? It means that this great evolutionary transformation did not involve the origin of new DNA: much of the shift likely involved using ancient genes, such as those involved in shark fin development, in new ways to make limbs with fingers and toes.