As reported in New Scientist, the switch from smell to sight in sex signaling occurred around 20 million years ago. In order to see the world in full color, two copies of a color vision gene are needed – one red, one green. These genes reside on the X chromosome; thus female monkeys with their two X chromosomes have long had full color vision, while males with their single X chromosome have not and suffered from color-blindness. All male New World monkeys remain color blind today, as do a significant number of human males – for example, an estimated 7% of European men.

About 23 million years ago, though, Old World monkeys (which eventually evolved into chimps, orangs and people) gained a second color vision gene. Suddenly these males could now see the world as their females had seen it all along. Boys will be boys, of course, even mutant ones; so this new super-power of male color vision meant a new approach to mate selection was quickly adapted. Rather than pheromones, female reproductive fitness and fertility could be signaled by “sexual swellings” and gaudy, colorful patches of skin. The lipstick industry soon followed.

“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.

This newfound male appreciation for female skin tones worked so well that the genes supporting pheremone reception apparently just faded away. 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.” His paper on this research, “Evolutionary deterioration of the vomeronasal pheromone transduction pathway in catarrrhine primates” appears in the Proceedings of the National Academy of Sciences online.

Thus if it were not for a real-life X-Men saga millions of years ago, then today scratch-and-sniff porn would be the norm…

For those with a feeling of deja vu…this post originally appeared on SciScoop on Tue Jun 17, 2003 and is the first of a series of PAST BLASTS republished to update the original where possible and to acquaint new readers with some of the almost 3000 stories in the searchable SciScoop archives.