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Only humans have pubic hair. What is the evolutionary advantage in having it?
Many consider pubic hair to be a sort of protection or warmth, however this can only hold true for the vagina, since pubic hair doesn't offer any additional protective advantages to the male urethra. The most prevailing theory is that simply it is one of the odor producing parts of the body and humans (like most life forms in general) are simply aroused by strong smells. Also most odour producing parts are usually accompanied by hair in order to "catch" pheromones. It is notable that those odours are not the direct product of glandular secretions, as those secrations are in fact odorless. It is their combination with bacteria that gives every person a unique smell. This unique smell gives a lot of important information to the canditate sexual partner. Here is an interesting study about odour importance in sexual reproduction as well as the role of MHC(Major Histocompatibility Complex)
If you are interested you can read further for for the Importance of smell in sexual production
Pubic hair provides evolutionary home for gorilla lice
There are two species of lice that infest humans: pubic lice, Pthirus pubis , and human head and body lice, Pediculus humanus . A new article in BioMed Central's open access Journal of Biology suggests one explanation for the separation of the two species.
In the article, Robert Weiss from University College London describes how he was struck by inspiration while pondering the question of why lice would separate into two groups when our ancestors are quite uniformly hairy, "I was having difficulty envisioning a clear separation of habitats between the groin and other parts of our ancient common ancestor. My 'eureka moment' came, appropriately enough, in the shower: although naked apes have pubic hair, surely our hairy cousins don't?"
Pthirus pubis , popularly known as crabs, evolved from the structurally similar gorilla louse, Pthirus gorillae. Interestingly however, while genetic analysis carried out by David Reed at the University of Florida indicates that this split occurred around 3.3 million years ago, humans are believed to have diverged from gorillas much earlier - at least 7 million years ago - suggesting that early humans somehow caught pubic lice from their gorilla cousins. Happily, this may not be as sordid as it sounds. According to Weiss, "Before one conjures up a King Kong scenario, it should be noted that predators can pick up parasites from their prey. The close contact involved in human ancestors butchering gorillas could have enabled Pthirus to jump hosts, rather as bushmeat slaughter practices allowed HIV to invade humans from chimpanzees in modern times."
So, while head lice may be viewed as a 'family heirloom', inherited down the generations as humans have evolved, pubic lice may well be a recent and slightly unwelcome gift from the more hirsute branch of our evolutionary family.
Why do humans have pubic hair?
As we pointed out in a recent question about why are men hairier than women, our ancestors underwent a process of body hair loss that transformed us into a unique «naked ape». However, we are also the only monkey in the planet to cover our naughty bits with an almost ostentatious excess of coarse hair. Why? If, as discussed previously, we believe that body hair loss was evolutionarily advantageous for our ancestors, why didn’t we also loose our pubic hair and why is it so awkwardly peculiar?
Curiously enough, clues to this answer are to be found in the genealogy of a family of extremely unpopular insects, lice. Like all parasites, this wingless nuisance of an insect has coevolved with its hosts, our ancestors, over evolutionary time. For example, lice found in gorillas (genus Pthirus) diverged from those found today in humans and chimps (genus Pediculus) slightly after the common ancestor to chimps and humans, on the one hand, and gorillas, on the other, separated into two different lineages. Similarly, the louse species that we currently battle in our children’s hair (Pediculus humanus capitis) diverged from modern chimp lice just about the time we parted ways with our ape cousins, about 6 million years ago. Since then, and up until between 80,000 and 170,000 years ago, human lice survived refuged in our heads, as our almost naked bodies prevented them from expanding to gorge in other areas of our body. However, between 80,000 and 170,000 years ago (depending on the actual human settlement) humans started wearing clothes, which paved the way for lice to recolonize our bodies, giving rise to human body lice (Pediculus humanus corporis).
This «lousy» yet interesting family photo does not only reflect a common evolutionary phenomenon, such as parasite-host coevolution, it also features an actor that has provided very useful information regarding the origin of our pubic hair. Namely, a peculiar insect that shelters within the most intimate hair of some unfortunate humans, the pubic hair louse. As it turns out this species (Pthirus pubis) belongs to the gorilla lice group, which specializes in coarse hair, and not to the chimp/human group as would be expected if it had evolved within our common ancestor. What all this suggests is that Pthirus re-colonized humans about 3.5 million years ago, coinciding with the appearance of a new type of (conveniently) coarse hair in our bodies.
Reproduced from Weiss, 2009, adapted from Reed et al., 2007.
In short, lice evolution suggests that our «exaggerated» (in comparison to pubic hair in other monkeys) and coarse mat of pubic hair appeared after we transformed into a naked ape, and therefore most likely to serve a different function than the rest of our body hair. As to which one, the enigma stands unscathed. The fact that pubic hair appears during puberty along with it’s flamboyant aspect has led some anthropologists to speculate that it could function as a sexual ornament involved in the transmission of sexual pheromones (sex-attractant chemicals). The human perineum (region between the anus and the genitals) is rich in apocrine glands, a type of sweat gland involved in the release of sex pheromones in many mammals that is furthermore frequently accompanied by tufts of hair that trap pheromones, hence facilitating its detection by members of the opposite sex. In humans, apocrine glands increase in size and become active during puberty (precisely when pubic hair grows), which inevitably suggests a similar function to that studied in other mammals. Unfortunately, there is (yet) no firm evidence to support this claim. There is one thing we do know, recent studies seem to show that fashion’s modern warfare against pubic hair may have unforeseen consequences. The incidence of infections by pubic lice are decreasing but, in turn, increased skin to skin contact during sexual intercourse is favouring the transmission of certain sexually transmitted diseases. Think of that the next time you decide to brandish a razor.
Pau Carazo. Marie Curie researcher at the Department of Zoology at Oxord University.
Reed , D. L. Light , J. E. Allen , J. M. and J. J. Kirchman , 2007. «Pair of Lice Lost or Parasites Regained: the Evolutionary History of Anthropoid Primate Lice». BMC Biology, 5:7. DOI: <10.1186/1741-7007-5-7>.
Weiss , R. A., 2009. «Apes, Lices and Prehistory». Journal of Biology, 8:20. DOI: <10.1186/jbiol114>.
What is the purpose of pubic hair?
Pubic hair serves several purposes, including disease prevention and friction reduction.
Whether a person chooses to remove none, all, or some of their pubic hair is a personal choice. Although the media, sexual partners, and societal “norms” can sometimes influence this choice, it should be a personal one.
Removing the pubic hair has some risks and potential side effects, but most are mild.
Read on to learn more about why humans have pubic hair, the benefits of having pubic hair, and some safe ways to remove it if a person chooses to do so.
Share on Pinterest One purpose of pubic hair is to reduce friction during sex.
Researchers theorize that pubic hair serves three main purposes for the human body. These include:
- reducing friction during sex
- preventing bacteria and other microorganisms from transmitting to others
- maintaining the optimal temperature for the genitals
Other theories as to the purpose of pubic hair include trapping pheromones. However, most well-controlled scientific studies have not shown any compelling evidence for this.
This ties in with a theory about pubic hair and puberty. Because pubic hair appears during puberty, it is often a physical sign of sexual maturity and may once have served as a visual cue for prospective mates.
The primary benefit of pubic hair is its ability to reduce friction during sexual intercourse.
The skin in the area around the genitals is very sensitive. Pubic hair can naturally reduce friction associated with the movements during sexual intercourse and other activities wherein chafing may occur.
Pubic hair can also help stop bacteria and other microorganisms from entering the body. Specifically, it can help trap dirt and pathogens that may enter the body through the vagina or penis.
According to one 2017 study, pubic hair may help reduce the risk of contracting a sexually transmitted infection (STI). However, additional studies are necessary to prove the effect of pubic hair on preventing STIs.
Pubic hair is normal, and the amount of hair in the pubic region varies from person to person. There is no standard for the amount, the thickness, or the area that pubic hair will cover.
People may notice an extreme variation in hair growth due to hormonal changes. For example, a person may notice excessive pubic hair as a result of polycystic ovary syndrome, while others may notice pubic hair loss due to aging.
No, pubic hair is not unhygienic. However, it does trap dirt and sweat, so it can become more pungent than areas of the body that have less hair.
Like other areas of the body, pubic hair does require regular cleaning. A person should wash their pubic area whenever they shower or bathe, just as they would other parts of their body. Keeping it clean can help prevent odor.
In separate studies, 59% of women and 61% of men stated that they groomed their pubic region for hygienic purposes. However, there is currently no scientific evidence to suggest any health benefits associated with removing pubic hair — other than the removal of pubic lice.
Pubic hair grooming and removal are fairly common behaviors among adults. In fact, according to one 2015 study, 95% of the participants had removed their pubic hair at least once in the previous 4 weeks.
According to the same study, 60% of men and 24% of women were more likely to prefer a “hair-free” partner.
People remove their pubic hair for different reasons. Some common reasons include:
- Personal preference: Some people may prefer the look and feel of having no public hair.
- Their partner’s preference: There may have been an implied or explicit request to groom or remove the pubic hair. However, pubic hair removal should be an individual’s choice.
- Increased satisfaction: One 2019 study suggests a correlation between pubic hair removal practices and relationship satisfaction. It also found that women who reported pubic hair removal had enhanced feelings of femininity.
- Preparation for sexual activity: Hair removal may be particularly beneficial for people who engage in oral sex.
- Peer or societal pressure: Some people may conform to the way that society believes grooming should occur. Again, however, this should be a personal choice.
Removing or trimming the pubic hair is a personal choice, though social pressures can sometimes influence this decision.
Pubic hair removal is generally safe, but there are some common side effects. These include:
- small cuts from razors
- potential injury, if using a razor or scissors
- burns from chemical removers
Pubic grooming injuries are surprisingly common. One 2017 survey found that 25.6% of people who groomed this area sustained injuries during or after hair removal.
Also, limited evidence suggests that removing the pubic hair can increase the risk of STIs. However, further research is necessary to determine whether or not removing the pubic hair increases this risk.
There are a few methods a person can try to safely remove the pubic hair at home. A person needs to use caution with whichever method they choose to help prevent injury.
Some ways to remove hair at home include:
- Shaving: This removes the hair but may cause chafing, nicks, itchiness, or rashes.
- Waxing: This removes the hair but is painful and may result in bleeding and irritation.
- Using chemicals: Depilatory creams can remove the pubic hair, but sensitive skin may burn or react badly to the chemicals.
- Trimming: A person can use scissors or an electric shaver to trim and maintain pubic hair.
Whether or not a person removes their pubic hair is a personal decision. Social pressures from peers, partners, or certain media can sometimes influence the decision. However, this should be a personal choice.
Some people may prefer to remove their pubic hair because it makes them feel better about themselves. Other people might remove it to feel more attractive to their partner. It is important to discuss this with a partner, however.
Ultimately, a person needs to determine how they feel about the decision and do what makes them happiest.
Pubic hair plays a role in reducing friction during activities such as sexual intercourse. It also plays a role in preventing dirt and pathogens from entering the genitals.
A person can safely remove their pubic hair if they wish to, but they do not need to.
Removing pubic hair is generally safe, but it can result in injuries such as burns, nicks, and cuts. A person should use caution no matter how they choose to remove their pubic hair.
10 Mysteries of you: Pubic hair
We may be the naked ape, but on one measure of hairiness humans trump all other primates. While most of them have finer hair around their genitals than on the rest of their body, adult humans sport an impressively thick bush of pubic hair.
It has long been assumed that pubic hair is a remnant of a furrier period in our evolutionary history, and that the real question is why the rest of the body lost its hirsuteness. Earlier this year, though, Robin Weiss of University College London pointed out that our pubic hair clearly became thicker than that on the rest of our bodies at some point in our evolution (Journal of Biology, vol 8, p 20). And this must have happened for a reason. So what drove the evolution of pubic hair?
There’s no accepted explanation, but many potential advantages have been suggested over the years. Perhaps the most popular is that since thicker hair gathers in regions where we have apocrine (scent) sweat glands as well as eccrine (cooling) ones, it may serve to waft odours that signal sexual maturity. It may also act as a visual signal of adulthood, along with growing breasts and widening hips in girls and deeper chests and beards in boys. Various other benefits could have made it worth keeping. A thick bush not only protects the genitals during sex and at other times – reducing chaffing while walking, for example – it also helps keep our most sensitive regions warm and free of draughts. &hellip
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Why do we have pubic hair?
According to board-certified ob-gyn Jacqueline M. Walters, M.D., it does in fact have a purpose. “It provides a cushion against friction that can cause skin abrasion and injury protection from bacteria and other unwanted pathogens, and is the visible result that long-awaited adolescence hormones have kicked in,” she says. “It certainly is nothing to be ashamed of or embarrassed about.”
In fact, there are a few sensual benefits, as well. “The purpose of pubic hair is likely twofold,” explains Alyssa Dweck, M.D., a gynecologist in Westchester County, N.Y., and assistant clinical professor at Mount Sinai Hospital. “It can play a role in regulating temperature but more importantly, it is thought to trap pheromone scent from our natural perspiration, for sexual and partner attractiveness—who knew?”
Other benefits? “Suspected theories, some medical and some not, include that pubic hair prevents dirt and other floating germs from entering the vagina, it keeps our genitals warm, and it’s the perfect cushion during sex, bicycling, and other forms of exercise that put pressure on your vagina,” shares Dr. Ross.
6 Wisdom Teeth
Most people who have teeth&mdashwhich is just about everyone&mdashhave had to come to terms with the painful experience of their wisdom tooth or teeth (as it&rsquos different for everyone) growing out at least once in their life. For those who haven&rsquot, we congratulate you for your luck, but for the other unlucky ones, it&rsquos not the best of experiences. It&rsquos especially bad because they serve seemingly no purpose whatsoever and only end up adding more things in our mouth that could get infected with cavities or such later in life. Most people get them taken out before that happens, but have you ever wondered why they even exist?
The answer is diet. In the old days, our diet comprised all kinds of stuff, like leaves, raw meat, and branches, so our jaw used to be much bigger than what we have today. The wisdom teeth helped us manage our wildly diverse diet, and the bigger jaw made room for the extra set of teeth. Through the years, though, the jaw shrank to accommodate more . . . civilized eating habits, though the wisdom teeth never got the memo.  That&rsquos the reason most of us will make at least one visit in our life to a dentist to deal with a wisdom tooth, and you&rsquove got our ancestors&rsquo way of eating to thank for it.
Pubic hair microbes as a forensic tool
After watching CSI, and with forensic science being more advanced than ever, it’s easy to presume that criminals leave DNA traces everywhere that can help to make a conviction if they are caught.
Human hairs come to mind as a great place to start, however, the majority of samples recovered at crime scenes are shed hairs containing insufficient levels of nuclear DNA, meaning they cannot be used to make an identification. This is because short tandem repeat (STR) analysis is performed on crime-scene DNA, where probes are attached to the sample, then it is amplified in length by a polymerase chain reaction (PCR) to create a DNA fingerprint. Two samples can then be compared to find if there’s a match. With very little or no DNA present, this is not possible.
Furthermore, hairs aren’t actually left at crime scenes very often, with research suggesting that male pubic hairs are only transferred to the female genital area in around 4% of sexual assault cases. Finding alternative DNA-based methods would therefore be highly useful to increase the chance of identifying criminals.Research published today in the open access journal Investigative Genetics has proposed that the microbiome of bacteria hosted on our hairs could be used for human identification in forensic investigations. Specifically, they studied scalp and pubic hairs.
The researchers performed metagenomic analysis on 42 samples from seven individuals at three time points. They showed that each harbored unique microbial taxa that could be used to discriminate between individuals, and that pubic hair microbiota could determine between male and female samples based on the species present.
The only difference observed between male and female scalp hairs was the reduced level of transient bacteria on female samples, possibly due to increased ‘grooming activities’.
Interestingly, two of the study participants were a single co-habiting couple, and in one instance at the five-month time point the co-habiting couple’s microbiota were more similar to each other than previously. Interviewing revealed that the couple had sexual intercourse 18 hours prior to the collection of their pubic hairs.
This suggests that an exchange of bacteria had occurred, which the researchers say bodes well for future forensic applications involving sexual crimes. In addition, the authors concluded that bacterial colonies in pubic regions are more stable than scalp microbiota, making pubic hairs a preferred choice to consider further for forensic applications. This study is of course preliminary, and the authors propose that further investigation is warranted to look into the pubic hair microbiome as a forensic tool.
Previous microbiome studies have demonstrated that we all harbor unique bacterial compositions between body sites. It’s also been shown that our surface bacteria can be exchanged through bodily contact. An article recently published in Microbiome showed that a ten-second intimate kiss between individuals can shape the oral microbiota, involving the exchange of approximately 80 million bacteria. More regular kissing can lead to long-term colonization of microbes in the mouth.
While advancing technology in forensic genetics hugely important for improving the conviction rates of criminals, the fact remains that a conviction cannot even be made purely on DNA evidence. Despite this, DNA fingerprinting still remains a hugely important tool in forensic genetics.
DNA fingerprinting was invented by Professor Sir Alex Jeffreys in 1984, through a “eureka” moment when he realised he’d found fully discriminative genetic markers in human blood. As part of a 2012-2013 thematic series on DNA fingerprinting Alec Jeffreys spoke with Investigative Genetics in a podcast about his invention of the technique, amongst other highlights during his scientific career, including the story of the first person to be exonerated of a crime based on DNA evidence.
There have been some past success stories from bringing microbial genetic analysis to the courtroom. A 2013 BMC Biology article describes a case where viral phylogenetics were used to prove the origin of a hepatitis C outbreak. Researchers modelled viral evolution rates to estimate the date of infection, tracking it to ten years before the outbreak and providing a connection between all of the cases.
An anesthetist was later convicted as being responsible for the infection of 275 of his patients with hepatitis C virus. In a BMC Biology commentary, Anne-Mieke Vandamme and Oliver Pybus comment on the importance of molecular phylogenetics as a tool for the forensic investigation of viral transmission.
Investigative Genetics is running an ongoing thematic series of research and reviews on microbial forensics. The series includes new guidelines by Bruce Budowle et al. for validation of high-throughput sequencing and microbial forensics applications. Sign up for alerts to receive updates when further articles are published!
Written by Sam Rose (@rosenovich), Journal Development Editor for Investigative Genetics
What purpose does pubic and armpit hair serve?
There are rarely more complex things than the body systems of living organisms, especially humans, since they are considered to be the most advanced species in terms of intelligence. There are numerous things about our body that still continue to baffle scientists and encourage them to continue their exploration. The presence of armpit hair and pubic hair is one of those mysteries that scientists are trying to unravel. Some outrageous theories have originated over the years, like armpit hair is for babies to hold onto and pubic hair is to act as a cushion during sex, but neither of them makes any sense whatsoever.
The predecessors of modern humans are believed to have had huge amount of hair on their body, covering all parts of their skin. However, it seems to have thinned considerably during the evolutionary process. The scientific world has formed some conclusions about the abundance of hair in the pubic and underarm regions. Some of which make more sense than others. One of the theories is that the hair is to reduce friction when the skin rubs against skin. This is in accordance with the claim that pubic hair is for acting as a buffer during intercourse.
Another reason why hair might be present in our armpits is to protect the skin from bacteria. Armpits are prone to sweating due to their entrapped nature, and armpit hair might be useful for catching most of that perspiration. This ensures that any bacteria that may form in the armpit is mostly concentrated on the hair and not the skin of the armpit, keeping it cleaner. It also helps give the armpits a cushioned feeling, so that the skin doesn’t chaff when the arm is in motion.
However, the most accepted theory is related to pheromones. Pheromones are odorless and oily secretions produced by the apocrine glands, which are indicators of sexual maturity. One fact that backs up this claim is that the underarm and pubic hair starts to grow in adolescent years, an obvious marker of puberty. The apocrine glands become active during the pubescent period as well. Unlike other sweat glands, they are primarily concentrated in the armpit regions and near the genitals. The hair in those regions helps to retain the pheromones and provide a protein-rich environment for their activity. The hair growth is triggered by sexual hormones like testosterone.
The pheromones and the bacterial activity combine to produce a musky scent unique to a person. This scent is attractive to the opposite sex and mates can identify their significant ones with this smell. For some people these scents can be sexually stimulating too. The armpit and pubic hair trap the pheromones, making them stay longer and the scent more prominent. There is also another theory that the pubic hair is to keep the genitals warm. However, this doesn’t seem to be as plausible as the former one. The scientific world is pursuing the matter, hopeful of more definitive results.
Why Humans and Their Fur Parted Ways
One of the most distinctive evolutionary changes as humans parted company from their fellow apes was their loss of body hair. But why and when human body hair disappeared, together with the matter of when people first started to wear clothes, are questions that have long lain beyond the reach of archaeology and paleontology.
Ingenious solutions to both issues have now been proposed, independently, by two research groups analyzing changes in DNA. The result, if the dates are accurate, is something of an embarrassment. It implies we were naked for more than a million years before we started wearing clothes.
Dr. Alan R. Rogers, an evolutionary geneticist at the University of Utah, has figured out when humans lost their hair by an indirect method depending on the gene that determines skin color. Dr. Mark Stoneking of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, believes he has established when humans first wore clothes. His method too is indirect: it involves dating the evolution of the human body louse, which infests only clothes.
Meanwhile a third group of researchers, resurrecting a suggestion of Darwin, has come up with a novel explanation of why humans lost their body hair in the first place.
Mammals need body hair to keep warm, and lose it only for special evolutionary reasons. Whales and walruses shed their hair to improve speed in their new medium, the sea. Elephants and rhinoceroses have specially thick skins and are too bulky to lose much heat on cold nights. But why did humans, the only hairless primates, lose their body hair?
One theory holds that the hominid line went through a semi-aquatic phase -- witness the slight webbing on our hands. A better suggestion is that loss of body hair helped our distant ancestors keep cool when they first ventured beyond the forest's shade and across the hot African savannah. But loss of hair is not an unmixed blessing in regulating body temperature because the naked skin absorbs more energy in the heat of the day and loses more in the cold of the night.
Dr. Mark Pagel of the University of Reading in England and Dr. Walter Bodmer of the John Radcliffe Hospital in Oxford have proposed a different solution to the mystery and their idea, if true, goes far toward explaining contemporary attitudes about hirsuteness. Humans lost their body hair, they say, to free themselves of external parasites that infest fur -- blood-sucking lice, fleas and ticks and the diseases they spread.
Once hairlessness had evolved through natural selection, Dr. Pagel and Dr. Bodmer suggest, it then became subject to sexual selection, the development of features in one sex that appeal to the other. Among the newly furless humans, bare skin would have served, like the peacock's tail, as a signal of fitness. The pains women take to keep their bodies free of hair -- joined now by some men -- may be no mere fashion statement but the latest echo of an ancient instinct. Dr. Pagel's and Dr. Bodmer's article appeared in a recent issue of The Proceedings of the Royal Society.
Dr. Pagel said he had noticed recently that advertisements for women's clothing often included a model showing a large expanse of bare back. ''We have thought of showing off skin as a secondary sexual characteristic but maybe it's simpler than that -- just a billboard for healthy skin,'' he said.
The message -- ''No fleas, lice or ticks on me!'' -- is presumably concealed from the conscious mind of both sender and receiver.
There are several puzzles for the new theory to explain. One is why, if loss of body hair deprived parasites of a refuge, evolution allowed pubic hair to be retained. Dr. Pagel and Dr. Bodmer suggest that these humid regions, dense with sweat glands, serve as launching pads for pheromones, airborne hormones known to convey sexual signals in other mammals though not yet identified in humans.
Another conundrum is why women have less body hair than men. Though both sexes may prefer less hair in the other, the pressure of sexual selection in this case may be greater on women, whether because men have had greater powers of choice or an more intense interest in physical attributes. 'ɼommon use of depilatory agents testifies to the continuing attractions of hairlessness, especially in human females,'' the two researchers write.
Dr. David L. Reed, a louse expert at the University of Utah, said the idea that humans might have lost their body hair as a defense against parasites was a '⟺scinating concept.'' Body lice spread three diseases -- typhus, relapsing fever and trench fever -- and have killed millions of people in time of war, he said.
But others could take more convincing. ''There are all kinds of notions as to the advantage of hair loss, but they are all just-so stories,'' said Dr. Ian Tattersall, a paleoanthropologist at the American Museum of Natural History in New York.
Causes aside, when did humans first lose their body hair? Dr. Rogers, of the University of Utah, saw a way to get a fix on the date after reading an article about a gene that helps determine skin color. The gene, called MC1R, specifies a protein that serves as a switch between the two kinds of pigment made by human cells. Eumelanin, which protects against the ultraviolet rays of the sun, is brown-black pheomelanin, which is not protective, is a red-yellow color.
Three years ago Dr. Rosalind Harding of Oxford University and others made a worldwide study of the MC1R gene by extracting it from blood samples and analyzing the sequence of DNA units in the gene. They found that the protein made by the gene is invariant in African populations, but outside of Africa the gene, and its protein, tended to vary a lot.
Dr. Harding concluded that the gene was kept under tight constraint in Africa, presumably because any change in its protein increased vulnerability to the sun's ultraviolet light, and was fatal to its owner. But outside Africa, in northern Asia and Europe, the gene was free to accept mutations, the constant natural changes in DNA, and produced skin colors that were not dark.
Reading Dr. Harding's article recently as part of a different project, Dr. Rogers wondered why all Africans had acquired the same version of the gene. Chimpanzees, Dr. Harding had noted, have many different forms of the gene, as presumably did the common ancestor of chimps and people.
As soon as the ancestral human population in Africa started losing its fur, Dr. Rogers surmised, people would have needed dark skin as a protection against sunlight. Anyone who had a version of the MC1R gene that produced darker skin would have had a survival advantage, and in a few generations this version of the gene would have made a clean sweep through the population.
There may have been several clean sweeps, each one producing a more effective version of the MC1R gene. Dr. Rogers saw a way to put a date on at least the most recent sweep. Some of the DNA units in a gene can be changed without changing the amino acid units in the protein the gene specifies. The MC1R genes Dr. Harding had analyzed in African populations had several of these silent mutations. Since the silent mutations accumulate in a random but steady fashion, they serve as a molecular clock, one that started ticking at the time of the last sweep of the MC1R gene through the ancestral human population.
From the number of silent mutations in African versions of the MC1R gene, Dr. Rogers and two colleagues, Dr. David Iltis and Dr. Stephen Wooding, calculate that the last sweep probably occurred 1.2 million years ago, when the human population consisted of a mere 14,000 breeding individuals. In other words, humans have been hairless at least since this time, and maybe for much longer. Their article is to appear in a future issue of Current Anthropology.
The estimated minimum date for human hairlessness seems to fall in reasonably well with the schedule of other major adaptations that turned an ordinary ape into the weirdest of all primates. Hominids first started occupying areas with few shade trees some 1.7 million years ago. This is also the time when long limbs and an external nose appeared. Both are assumed to be adaptations to help dissipate heat, said Dr. Richard Klein, an archaeologist at Stanford University. Loss of hair and dark skin could well have emerged at the same time, so Dr. Rogers' argument was 'ɼompletely plausible,'' he said.
From 1.6 million years ago the world was in the grip of the Pleistocene ice age, which ended only 10,000 years ago. Even in Africa, nights could have been cold for fur-less primates. But Dr. Ropers noted that people lived without clothes until recently in chilly places like Tasmania and Tierra del Fuego.
Chimpanzees have pale skin and are born with pale faces that tan as they grow older. So the prototype hominid too probably had fair skin under dark hair, said Dr. Nina Jablonski, an expert on the evolution of skin color at the California Academy of Sciences. ''It was only later that we lost our hair and at the same time evolved an evenly dark pigmentation,'' she said.
Remarkable as it may seem that genetic analysis can reach back and date an event deep in human history, there is a second approach to determining when people lost their body hair, or at least started to wear clothes. It has to do with lice. Humans have the distinction of being host to three different kinds: the head louse, the body louse and the pubic louse. The body louse, unlike all other kinds that infect mammals, clings to clothing, not hair. It presumably evolved from the head louse after humans lost their body hair and started wearing clothes.
Dr. Stoneking, together with Dr. Ralf Kittler and Dr. Manfred Kayser, report in today's issue of Current Biology that they compared the DNA of human head and body lice from around the world, as well as chimpanzee lice as a point of evolutionary comparison. From study of the DNA differences, they find that the human body louse indeed evolved from the louse, as expected, but that this event took place surprisingly recently, sometime between 42,000 and 72,000 years ago. Humans must have been wearing clothes at least since this time.
Modern humans left Africa about 50,000 years ago. Dr. Stoneking and his colleagues say the invention of clothing may have been a factor in the successful spread of humans around the world, especially in the cooler climates of the north.
Dr. Stoneking said in an interview that clothing could also have been part of the suite of sophisticated behaviors, such as advanced tools, trade and art, that appear in the archaeological record some 50,000 years ago, just before humans migrated from Africa.
The head louse would probably have colonized clothing quite soon after the niche became available -- within thousands and tens of thousands of years, Dr. Stoneking said. So body lice were probably not in existence when humans and Neanderthals diverged some 250,000 or more years ago. This implies that the common ancestor of humans and Neanderthals did not wear clothes and therefore probably Neanderthals didn't either.
But Dr. Klein, the Stanford archeologist, said he thought Neanderthals and other archaic humans must have produced clothing of some kind in order to live in temperate latitudes like Europe and the Far East. Perhaps the body lice don't show that, he suggested, because early clothes were too loose fitting or made of the wrong material.
Dr. Stoneking said he got the idea for his louse project after one of his children came home with a note about a louse infestation in school. The note assured parents that lice could only live a few hours when away from the human body, implying to Dr. Stoneking that their evolution must closely mirror the spread of humans around the world.
The compilers of Genesis write that as soon as Adam and Eve realized they were naked, they sewed themselves aprons made of leaves from the fig tree, and that the Creator himself made them more durable skin coats before evicting them. But if Dr. Rogers and Dr. Stoneking are correct, humans were naked for a million years before they noticed their state of undress and called for the tailor.