University of Michigan evolutionary biologist Jianzhi “George” Zhang argues that the development of the ability to see colors in our primate ancestors led to the loss of the ability to respond to sexual pheromones.

Zhang believes that a significant gene duplication made the difference and that it happened sometime between 23 million years ago and the split of the New World and Old World primates about 35 million years ago. An ancestor of the Old World primates (humans, chimps, gorillas, orangutans, gibbons, baboons and guerezas) developed a second copy of the red/green color-vision gene, which resides on the X chromosome. Female New World monkeys have full color vision because females have two X chromosomes that harbor both red and green color vision genes. But males only have one X chromosome, so New World males have only one copy of either the red or green gene, and that leaves them color-blind. After the red/green gene duplication in the Old World family however, even the males got color vision too.

Once humans could see in color the visual inspection of a potential mate yielded far more useful information and at a greater distance than was the case with scents. As a result of natural selection color-seeing primates came to have neuronal wiring that caused them to place much more importantce on appearance in mate choice. In Zhang's view it is therefore not coincidental that around the time human males developed the ability to see color humans also lost the ability to respond to pheromones:

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.”

The FuturePundit blog focuses on the future. This report is about events that took place tens of millions of years ago in our our evolutionary past. So how does this discovery about the history of human sexual evolution figure into the human future? In a couple of ways:

• It will eventually become possible to fix the genes involved in the pheromone pathway and make humans have a sexual reaction to pheromones.
• Drugs will be able to be developed that will work like pheromones. It is likely that by taking apart the steps of the pheromone signalling pathway in primate species which still have functioning pheromone systems it will be possible to identify portions of the pathway that still work in humans because they are far enough downstream that they have been preserved for other uses. Drugs aimed at enzymes and genes in the downstream portion of the pheromone response may be able to elicit a pheromone reaction in humans.

Many changes that happened in our evolutionary past will not be lost to us forever once it becomes possible to do genetic engineering to ourselves and our progeny. If we want to recover lost functionality or behavioral tendencies it will eventually become possible to do so. I would go so far as to predict that there will eventually be cultish groups who will pursue biological nostalgia fads to make themselves more authentic and less modern by giving themselves features associated with our pre-human ancestors. These faddists of the future will use biological technology to take the back-to-nature movement to a whole 'nuther level.