Colour vision ended human pheromone use
The development of colour vision may have led to Old World primates, and hence their human descendants, to lose their ability to detect pheromones, suggests a new genetic study.
Pheromones are highly specific scent molecules that many animals rely upon to find and assess a potential mate. But humans appear to make little, if any, use of pheromone signals, says Jianzhi George Zhang, an evolutionary geneticist at the University of Michigan, Ann Arbor.
Researchers have suggested before that the primates' pheromonal abilities may have fallen by the wayside because they developed colour vision, a better way of selecting mates. "But we establish the timing for when the pheromone signal transduction pathway was shut off," Zhang told New Scientist.
It occurred about 23 million years ago, just before the hominoid superfamily that eventually produced humans branched off. Crucially, the timing approximately coincides with the development of full colour vision in Old World primates, thereby giving a major boost to the theory.
Random decay
Zhang and colleague David Webb used a gene called TRP2 - unique to the pheromone pathway - to track the evolution of the system in primates. In humans, TRP2 has accumulated so many errors in its DNA sequence that is now a "pseudogene" that is no longer active.
It is also a pseudogene in Old World monkeys, such as the baboon and guereza, and apes including the chimpanzee and gorilla. But in New World monkeys, like the tamarin, squirrel monkey and saki, the team found TRP2 was completely functional and bestows a sharp ability to detect pheromones.
A computer simulation of random gene decay estimated the time of TRP2's shutdown at just over 23 million years ago. And this matches the estimated time at which male Old World monkeys developed the full colour vision that their New World cousins still lack.
Gaudy and colourful
In order to see the world in full colour, two copies of a colour vision gene are needed - one red, one green. The genes reside on the X chromosome, so female monkeys have long had full colour vision, while males have not.
But when male Old World monkeys gained a second colour vision gene, about 23 million years ago, it meant a new approach to mate selection was possible. Rather than pheromones, "sexual swellings" - gaudy, colourful patches of skin - could be used to signal female reproductive fitness and fertility.
"We think there might be some advantage to using a vision based signalling system, compared with a pheromonal one, because you can see colour from a distance," says Zhang. With pheromones, he notes, a mate has to close enough for the pheromones to drift through the air and be detected.
Zhang adds that red/green colour blindness is relatively common in humans compared with Old World monkeys and apes - with about seven per cent of male Europeans being colour blind. This might be because the need to very carefully select mates by visual means has become "relaxed" in modern humans, he suggests.
peptides with Epibase® www.algonomics.com
June 25, 2003
Evolution Of Color Eyesight Led To Loss Of Pheromone Response
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.
By Randall Parker at 2003 June 25 01:04 AM Biological Mind
Comments by darwinists and creationists are invited