Why are humans nearly hairless? And why do some wish to become more so?

At the back of a hairdresser's shop, just off Piccadilly in London, an Irish beautician called Genevieve is explaining what a “Brazilian” is as she practises her art on your correspondent. A Brazilian strip, some are surprised to learn, is nothing to do with Latin American football. Between each excruciating rip, she explains that she is going to remove nearly all my pubic hair, except for a narrow vertical strip of hairs the width of a couple of fingers. This is known colloquially as the “landing strip”.

In only a few years, this form of waxing has gone from the esoteric to the everyday and is starting to rival the ordinary bikini wax in popularity. At the same time the bikini wax is becoming a normal procedure for women of all ages: the youngest person Genevieve has waxed is a 12-year-old girl. Women are styling their pubic hair into hearts, stars and arrows. It is one of the more notable developments in hairdressing since the permanent wave.

The Royal Society outlines Mark Pagel's and Sir Walter Bodmer's theory on human hairlessness. Facial hair becomes an art-form at the World Beard and Moustache Championships.

The agony involved raises the question of why women increasingly feel the need to remove a natural covering of hair. One theory is that they are trying to acquire a prepubescent look in order to please men. The waxers, though, will let you into another little secret which suggests that, even if this is true, it is not the whole story. Some men too, both straight and gay, are waxing their most intimate parts. Ouch.

At a biological level this behaviour seems even odder. Most other mammals seem quite content with a luxuriant growth of fur. The idea of a chimpanzee pulling out the hair on its genital regions is ridiculous. Perhaps waxing is little more than a pseudo-sexual fad: another example of the kind of erotic titivation, such as body piercing and tattooing, that was once popular mainly among sailors, hippies and prostitutes. There is another possibility, though. It could be an extension of a longer-running animal story: humanity's evolution towards near nakedness.

Humans are clearly obsessed with having too much hair. Last year men and women spent $8 billion removing it with razor blades, reports Gillette, which makes the things. Of this, $2 billion was spent by America's 100m men on beard removal. More than 90% of American men over 15 shave about five times a week. But as beards are, biologically, a sexual signal indicating masculinity, why shave them off?

Men have been shaving since antiquity, although the habit really got going only when Gillette replaced the cut-throat razor with the safety razor in 1903. Gus Van Beek, a curator of archaeology at the Smithsonian Institution in Washington, DC, says that Egyptian tomb paintings of men show them without beards, or at least without real beards. When beards are depicted, they are false ones. This is known, says Mr Van Beek, because detached falsies have been found.

Beards may have been considered a disadvantage in hand-to-hand combat, since they can be grabbed. Yet much of the body, or so it is thought, was shaven by the ancient Egyptians. Mr Van Beek says that their razors would have been made first of copper, then of bronze and, much later, of brass. But the ancient Egyptians would not have gone in for the Sphinx, which is another style of pubic wax, named after the completely hairless Egyptian cat.

Great for scouring pots

It is not clear when women began shaving their legs. One idea, almost certainly wrong, is that the fashion began in the 1920s when western women's skirts became shorter. Typically, today's women start shaving at a slightly younger age than men do but they shave an area nine times as large. Although the average male beard has the same number of hairs as a woman's legs and underarms combined (7,000 to 15,000 hairs), the beard is denser and grows much faster. The average American man spends about 33 days of his life removing facial hair. Dry beard hair, says an alarming Gillette fact sheet on shaving, is “extremely abrasive and about as tough as copper wire of the same thickness.”

Though beards and hairy legs may be unwanted, head hair is greatly desired. Many men go to great lengths in their efforts to keep the hair on their heads. Male-pattern baldness, or androgenetic alopecia, is the commonest form of hair loss. By the age of 50, over half of all men are experiencing some thinning or loss of hair at the top or front of their scalp. It is caused by genetics and hormones, specifically the male hormone dihydrotestosterone.

If fat is a feminist issue, then baldness is a male one, according to statistics compiled by members of the American Academy of Cosmetic Surgery. The main reason a man will have cosmetic surgery (apart, that is, from botox injections) is for the transplantation or restoration of his hair. Roughly the same number of American men are having their hair revived as women are having their breasts augmented. In online chat groups, bald men from all over the world discuss their misery and inability to attract a mate (or “gf”). “I'd rather be fat than bald,” bemoans one.

Human beings' hairy preoccupations are curious because, compared with their closest animal relations, humans have very little hair to begin with. Hair is unique to mammals, and is one of the most obvious and defining characteristics of the group. It may have first evolved when sensory hairs—rather like a cat's whiskers—were multiplied over the body and became a useful insulator. Many scientists believe the evolution of hair is related to the evolution of warm-bloodedness: the ability to maintain a constant internal temperature. This may have given mammals an advantage in their early, nocturnal, environment, when they lived in the shadow of dinosaurs. Mammals have probably had hair for about 200m years, but since hair does not generally fossilise, scientists are not quite sure.

Of the 5,000-plus species of mammal, the only other (mostly) hairless creatures are elephants, rhinoceroses, hippopotamuses, walruses, whales and naked mole rats. It is easy to see why these few animals are not so hairy. Elephants and rhinoceroses are some of the largest of mammals, live in hot places and have trouble staying cool. The others live, at least part of the time, in water—where hair is not very useful—or underground, where temperature does not fluctuate as much as on the surface. Pigs are different. Their relative hairlessness has been bred into them fairly recently, in rather the same way as the unfortunate Mexican hairless dog.

A scientist might argue that humans are not, technically, “hairless”. Many have the same density of hair follicles as an ape of the same body size would have. But human hair is generally fine and short, and so humans look naked compared with their closest animal relations. How bare they are, though, does vary racially—which may explain why one Thai lady has requested that her European boyfriend should have his entire body waxed. But completely hairless human skin is found in only a few areas such as the soles of the feet, the areolae round the nipples, the umbilicus, and the palms and undersurface of the fingers and toes.

Despite the title of Desmond Morris's 1967 book, “The Naked Ape”, scientists do not know when in evolutionary history the “great denudation” took place. Or, for that matter, why. One of the more imaginative theories is that humans were once aquatic apes. This, it is argued, would explain why humans have hair on their heads: since the aquatic ape's head would have to be held out of the water, it would have needed protection from the glare of the sun. The aquatic-ape theory is also used to explain why humans are relatively nimble in water, certainly compared with chimpanzees; and why the hair on the human back points in a direction that would reduce resistance from the water while swimming. But, as is often pointed out, the idea lacks hard evidence.

Cool, man

For many decades, the most popular explanation of hairlessness was that humans lost their hair to keep cool. Too much hair made humans—very active apes—hot, like elephants. Elephants evolved huge floppy ears to radiate heat back into their surroundings. But when hominids moved out of the forests and into the savannah, the same task could be carried out by the entire body, thanks to hominids' upright posture (which exposed less skin to the sun) and their lack of hair.

Unfortunately, as Mr Morris points out, there are problems with this idea, too. One is that no other animals of human size, indeed, no other savannah mammals at all, have shed their fur. Where are the naked lions? Another is that, though bare skin increases the chances of heat loss, it also increases the chances of heat gain, and the risk of damage from the sun. And nakedness makes humans vulnerably cold at night, even in Africa.

Mark Pagel, at the University of Reading, and Sir Walter Bodmer, at the John Radcliffe Hospital in Oxford, have a new idea. They believe that parasites are the key to human hairlessness. Humans, they say, lost their hair in order to reduce the burden of parasites such as fleas and ticks, some of which would have transmitted disease. Early humans probably lived close together in hunter-gatherer groups, in which the rate of parasite transmission was high. Hairless skin was easier to keep clean. Cultural adaptations, such as the use of fire, shelter and clothing, allowed humans to become furless.

What convinces them, they say, is the recent evidence of the great toll that ticks, lice and fleas have on the survival of furry and feathered creatures. Many animals die from parasites, and fleas carry the plague. Other scientists have recently discovered that when foreign species arrive on new shores they typically come with half as many parasites as they had at home. This gives them a huge competitive advantage over the local species, and explains why some become pests.

In 1874 Charles Darwin noted that, in the tropics, hairlessness would help humans to free themselves from ticks and other parasites. He showed some support for the idea in a passage reporting, “It is said to be a practice with the Australians, when the vermin get troublesome, to singe themselves.”

Some even argue that early cave paintings prove that cavemen were removing hairs from their face for similar reasons. At first, it is said, they plucked hair out using a pair of seashells as tweezers, and later they scraped away at it with razors made of flint or horn. Since horn becomes blunt quickly, it may almost be said that stone-age man invented the disposable razor. Others may have singed their facial hair with burning twigs. Why? Perhaps because it became sweaty and dirty, made eating awkward and played host to nits.

My husband is a hairy man...

The parasite theory may also help explain why women are less hairy than men. Mates of either sex would have chosen each other because of their lack of hair, argue Mr Pagel and Sir Walter, since this would suggest that the chosen one was likelier to be free of disease. But as men are more likely than women to select mates on their appearance, it may be that the evolutionary pressure driving hairlessness was greater in women than in men.

Humans...lost their hair in order to reduce the burden of parasites such as fleas and ticks, some of which would have transmitted disease

Hairiness is also related to the level of a predominantly male hormone. Hence, being less hairy may be a sexual signal of femininity. The companies that advertise female shaving, waxing and depilatory products often play on the fear that hair is dirty. The message is clear: if you don't want to look like a dirty man in a hygiene-obsessed world, get rid of your body hair. And as the fashion for revealing clothing and microscopic underwear spreads, so too does the desire to show only smooth, naked skin—a desire often reinforced by society. Consider the outcry when Julia Roberts waved to a crowd and revealed, to the horror of many, a hairy armpit.

More and more of the body is on display, and not always pleasurably. For many women, for instance, men's hairy backs and chests are an acquired taste. Smoother certainly usually means easier on the eye. Mr Pagel says he is struck by how many advertisements for women's clothes and scent show them with their backs exposed. This may, conventionally, be thought of as a normal, sexually suggestive, display. But Mr Pagel adds, “We do not normally regard backs as secondary sexual characteristics and so it occurred to me that what these advertisements may be subconsciously displaying is the ‘health’ and ‘fitness’ of the model by revealing a large area of unblemished skin.”

Some evolutionary biologists, though, are bristling at the parasite theory. Robin Dunbar, of the University of Liverpool, is sceptical. Parasites, he says, would have become a problem only when shelters were first established. But hairlessness, he says, evolved before shelters did. Some research suggests it evolved when walking upright became popular among hominids more than 2m years ago.

Mr Dunbar supports the cooling theory, and argues that when humans invaded the open plains, hair loss doubled the distance they could travel on a pint of water. Moreover, the presence or absence of hair clearly affects insulation because hair length changes on animals in different environments: elephants in cold places became the woolly mammoth. And it is now thought that humans started wearing clothes rather recently—work on the genetics and evolutionary origins of clothes lice suggests they first appeared some time between 30,000 and 114,000 years ago—certainly far too late to explain why humans lost most of their hair.

...but I am sexy

Whatever the explanation for the loss of hair, another explanation is needed for why men and women kept dense hair in three places: their heads, armpits and pubes. In particular, those who believe in the parasite theory must explain why humans merely shed some of their hair and not all of it, since the head, armpits and pubic areas are the very regions where human parasitic infections tend to occur.

The answer is sex, of course. David Stoddart, an olfactory biologist with Australia's Antarctic programme, points out that armpit and pubic hair grows just where the major scent glands are to be found. Hair is a means of wafting this scent about. Thus a tuft of hair allows humans, like other animals, to advertise to mates that something of interest is happening on the skin below.

Humans' crowning glory, the hair on their heads, is easier to explain. Again, it is a human characteristic that was shaped by sexual selection. A luxuriant head of hair is, and has always been, desirable in a mate. At least since the days of the Assyrians—between the 17th and seventh centuries BC—hair has been dressed, and has been an important signal for attracting and choosing partners. In Europe in the late 1760s, women's hair rose from the head and took on extraordinary proportions. Fashionable women might dress their hair powdered, draped over wire or basketwork foundations, and crowned with feathers, flowers, baskets of fruit, or even a miniature ship in full sail.

That still leaves beards. The theory here is that sexual selection has kept facial hair in men, presumably because this advertises their male hormones. But why, then, do so many men, in so many cultures, shave them off? Perhaps the fear of parasites is driving some men to be clean-shaven. Maybe the goatee is a compromise between being clean and manly. Or, perhaps, shaving is popular because facial shape in humans is a sexually dimorphic characteristic. Men tend to have squarer jaws than women, and they shave to highlight this. If so, this would explain the trend for emphasising the edge of the jawline with a fringe of hair. But moustaches are a mystery, to evolutionary biologists and to practically everyone else.

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