Left and right sides of an organism may develop as symmetrical, albeit mirror, images of each other, but some organs, notably our own hearts and livers, are not mirror images. Palmer reviews how this asymmetry forms the basis for intriguing insights into evolution. Genetically defined asymmetry, whereby the balance generally falls to one side, has arisen multiple times through evolution. The underlying processes suggest beginnings in genetic assimilation as well as mutations. Analysis of heart looping in vertebrates indicates which portions of the signaling cascade are more ancestral than others.