This is a debated question—especially since it has been difficult to determine the existence of pheromones in humans. Scientists have detected that human eggs release a chemical signal that allows them to “communicate” with sperm; it is thought that some human females respond to pheromones with the signals actually coordinating with the menstrual cycle: one increasing the likelihood of ovulation and the second suppressing ovulation. But human pheromones are evasive. One study suggests that over time, humans have lost some of our pheromones because of our ability to see in color. They suggest that when early “humans” developed color vision about twenty-three million years ago, they stopped producing pheromones to attract the opposite sex. In fact, most primates have a gene called TRP2—thought to signal pheromones—but in humans, it is no longer active.