If you’ve ever been to a Central American forest, you’ve probably heard the hoots and wails of a howler monkey. But these creatures deserve our attention for more than their howls. They turn out to tell us a lot about the evolution of our own senses. We and some of our close primate relatives are remarkable for having powerful color vision. What triggered the evolution of this adaptation some 25 million years ago? Some researchers have proposed that as the global climate cooled, our ancestors were forced to shift from a diet of fruit to leaves. An ability to detect red and green colors would have helped these early primates detect the best leaves to munch on. The descendants of