"Color vision made pheromones unnecessary," Zhang said. As a channel for sexual signaling, color vision works better at a distance than pheromones, Zhang believes. A pheromone attaches to a water molecule, drifts about in the air currents and finally lands on the proper receptor in someone else's nose. The receiver can't immediately be sure who sent it, where it came from or when. But with sexual swelling, everyone in the troop can see precisely when and where the signal is, even at a significant distance.
Sexual swelling occurs in about 10 percent of all primate species, but only in the Old World species of Africa and Asia, which is where humans probably originated, as well.
To test their idea, Zhang's team zeroed in on a human gene called TRP2, which makes an ion channel that is unique to the pheromone signaling pathway. They found that in humans and Old World primates, this gene suffered a mutation just over 23 million years ago that rendered it dysfunctional. But because we could use color vision for mating, it didn't hurt us. In turn, the pheromone receptor genes that rely on this ion channel fell into disuse, and in a random fashion, mutated to a dysfunctional state because they haven't experienced any pressure from natural selection. Zhang calls this process "evolutionary deterioration."
Zhang's paper, "Evolutionary deterioration of the vomeronasal pheromone transduction pathway in catarrrhine primates" appears in the Proceedings of the National Academy of Sciences online (http://www.pnas.org/cgi/doi/10.1073/pnas.1331721100)