Lamprey are slimy, parasitic eel-like fish, one of only two existing species of vertebrates that have no jaw. While many would be repulsed by these creatures, lamprey are exciting to biologists because they are so primitive, retaining many characteristics similar to their ancient ancestors and thus offering answers to some of life’s biggest evolutionary questions. Now, by studying the lamprey, Caltech researchers have discovered an unexpected mechanism for the evolution of the neurons of the peripheral nervous system—nerves outside of the brain and spinal cord.