Five hundred and fifty million years ago, in the prehistoric Cambrian Period, the number of species on the planet began to surge (Figure 1.1). From the fossil record, there is evidence that this explosion was driven by the development of light detectors in the trilobite (Figure 1.2). A visual system, even a primitive one, bestows a delightful bounty of fresh capabilities. One can, as examples, spot food, foes, and friendlylooking mates at some distance. Other senses, like smell, enable animals to detect these as well, but not with the accuracy and lightspeed pace of vision. Once the trilobite could see, the hypothesis goes, this set off an arms race that produced the Cambrian explosion: The trilobite’s prey, as well as its predators, themselves had to evolve to survive.