Color vision may have originated in humans and related primates to spot blushes on cheeks and faces pale with fear. Whereas birds' and bees' color receptors are evenly sensitive across the visible spectrum, two of the three kinds of color photoreceptors in humans and other Old World primates are both most sensitive to roughly 550-nanometerwavelength light. California Institute of Technology neurobiologists suggest that this closeness in sensitivities is optimized toward detecting subtle changes in skin tone because of varying concentrations of oxygenated hemoglobin in the blood. This could help primates tell if a potential mate is rosy from good health or if an enemy is blanched with alarm. Supporting this idea, they say in their upcoming June 22 Biology Letters paper, is the fact that Old World primates tend to be bare-faced and barebottomed, the better to color-signal with.