May 22, 2008 — -- Vestigial organs are parts of the body that once had a function but are now more-or-less useless. Probably the most famous example is the appendix, though it is now an open question whether the appendix is really vestigial. The idea that we are carrying around useless relics of our evolutionary past has long fascinated scientists and laypeople alike.
This week we tackle vestigial organs in a feature article that looks at how the idea has changed over the years, and how it has come under attack from creationists anxious to deny that vestigial organs (and hence evolution) exist at all. To accompany the article, here is our list of the five organs and functions most likely to be truly vestigial.
Rodents and other mammals secrete chemical signals called pheromones that carry information about their gender or reproductive state, and influence the behaviour of others. Pheromones are detected by a specialised sensory system, the vomeronasal organ (VNO), which consists of a pair of structures that nestle in the nasal lining or the roof of the mouth. Although most adult humans have something resembling a VNO in their nose, neuroscientist Michael Meredith of Florida State University in Tallahassee has no hesitation in dismissing it as a remnant.
"If you look at the anatomy of the structure, you don't see any cells that look like the sensory cells in other mammalian VNOs," he says. "You don't see any nerve fibres connecting the organ to the brain." He also points to genetic evidence that the human VNO is non-functional. Virtually all the genes that encode its cell-surface receptors - the molecules that bind incoming chemical signals, triggering an electrical response in the cell - are pseudogenes, and inactive.
So what about the puzzling evidence that humans respond to some pheromones? Larry Katz and a team at Duke University, North Carolina, have found that as well as the VNO, the main olfactory system in mice also responds to pheromones. If that is the case in humans too then it is possible that we may still secrete pheromones to influence the behaviour of others without using a VNO to detect them.
Though goose bumps are a reflex rather than a permanent anatomical structure, they are widely considered to be vestigial in humans. The pilomotor reflex, to give them one of their technical names, occurs when the tiny muscle at the base of a hair follicle contracts, pulling the hair upright. In birds or mammals with feathers, fur or spines, this creates a layer of insulating warm air in a cold snap, or a reason for a predator to think twice before attacking. But human hair is so puny that it is incapable of either of these functions.
Goose bumps in humans may, however, have taken on a minor new role. Like flushing, another thermoregulatory mechanism, they have become linked with emotional responses - notably fear, rage or the pleasure, say, of listening to beautiful music. This could serve as a signal to others. It may also heighten emotional reactions: there is some evidence, for instance, that a music-induced frisson causes changes of activity in the brain that are associated with pleasure.
Around the sixth week of gestation, six swellings of tissue called the hillocks of Hiss arise around the area that will form the ear canal. These eventually coalesce to form the outer ear. Darwin's point, or tubercle, is a minor malformation of the junction of the fourth and fifth hillocks of Hiss. It is found in a substantial minority of people and takes the form of a cartilaginous node or bump on the rim of their outer ear, which is thought to be the vestige of a joint that allowed the top part of the ancestral ear to swivel or flop down over the opening to the ear.
Technically considered a congenital defect, Darwin's point does no harm and is surgically removed for cosmetic reasons only. However, the genetics behind it tells an interesting tale, says plastic surgeon Anthony Sclafani of the New York Eye and Ear Infirmary in New York City. The trait is passed on according to an autosomal dominant pattern, meaning that a child need only inherit one copy of the gene responsible to have Darwin's point. That suggests that at one time it was useful. However, it also has variable penetration, meaning that you won't necessarily have the trait even if you inherit the gene. "The variable penetration reflects the fact that it is no longer advantageous," Sclafani says.
A structure that is the object of reduced evolutionary pressure can, within limits, take on different forms. As a result, one of the telltale signs of a vestige is variability. A good example is the human coccyx, a vestige of the mammalian tail, which has taken on a modified function, notably as an anchor point for the muscles that hold the anus in place.
The human coccyx is normally composed of four rudimentary vertebrae fused into a single bone. "But it's amazing how much variability there is at this spot," says Patrick Foye, director of the Coccyx Pain Service at New Jersey Medical School in Newark. Whereas babies born with six fingers or toes are rare, he says, the coccyx can and often does consist of anything from three to five bony segments. What's more, there are more than 100 medical reports of babies born with tails. This atavism arises if the signal that normally stops the process of vertebrate elongation during embryonic development fails to activate on time.
Most primates have wisdom teeth (the third molars) but a few species, including marmosets and tamarins, have none. "These are probably evolutionary dwarfs," says anthropologist Peter Lucas of George Washington University, Washington DC. He suggests that when the body size of mammals reduces rapidly their jaws become too small to house all their teeth, and overcrowding eventually results in selection for fewer or smaller teeth (International Congress Series, vol 1296, p 74). This seems to be happening in Homo sapiens.
Robert Corruccini of Southern Illinois University in Carbondale, says the problem of overcrowding has been exacerbated in humans in the past four centuries as our diet has become softer and more processed. With less wear on molars, jaw space is at an even higher premium, "so the third molars, the last teeth to erupt, run out of space to erupt", he says. Not only are impacted wisdom teeth becoming more common, perhaps as many as 35% of people have no wisdom teeth at all, suggesting that we may be on an evolutionary trajectory to losing them altogether.
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