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.