In case you have forgotten, humans have relatively hairless bodies when compared to our other great ape relatives, even to the 5,000 or so other mammalian species out there. This is curious phenomenon — we seem to defy the very classification of being a mammal.
Sure, there’s a lot of variation among human populations. Some of us are hairier than the others, while others of us aren’t. There are even known genetic ‘mutations’ that reverse our hairlessness into a ‘werewolf syndrome’ which is known in science as hypertrichosis. 
Fajardo Aceves Jesus Manuel of Mexico is a modern day example of someone with hypertrichosis. This condition is caused by a genetic defect that,
“causes the hair growth cycle in victims of this rare disease to run amok. The follicles from which the body hair grows are apparently incapable of switching from the growth phase to the dormant phase, which normally ends in the new hair falling out and the cycle begining again.”
Hypertrichosis is considered an atavism — or an evolutionary throwback. An atavism is a trait that reappears which had once disappeared generations ago. This happens mostly likely thru a backward mutation but it can also happen thru transposable elements within the genome, i.e., the ‘hairy gene’ was locked away in a region of the genome that was not expressed but then was shuffled and inserted into an expressable region. Other known atavisms occur thru this mechanism. Genes that coded for a previously existing phenotype are often preserved in DNA, even though the genes are not expressed in some or most of the organisms possessing them.
But hypertrichosis is a rare genetic disorder, actually it is very rare. Only about 50 cases of the condition have been documented since the Middle Ages. That indicates there has been a strong positive selection to keep us hairless. But why? Are there any evolutionary explainations as to why humans are hairless?
Well, there are three main stories explanations (?) for why humans lack fur. Scientific American has asked an expert, Mark Pagel, head of the evolutionary biology group at the University of Reading, and he summarizes these hypotheses for us. These hypotheses vary in evidence but they all revolve around the idea of a positive selective pressure to not be hairy. In other words, it may have been advantageous for the human lineage to have become less and less hairy during the six million years since we shared a common ancestor with our closest living relative, the chimpanzee.
The first hypothesis, named the ‘aquatic-ape hypothesis’ considers that way back in the day, like 8 million year ago an apelike ancestor of modern humans had,
“a semiaquatic lifestyle based on foraging for food in shallow waters. Fur is not an effective insulator in water, and so the theory asserts that we evolved to lose our fur, replacing it, as other aquatic mammals have, with relatively high levels of body fat. Imaginative as this explanation is—and helpful in providing us with an excuse for being overweight—paleontological evidence for an aquatic phase of human existence has proven elusive.“
I was taught that our hairlessness initially came about from an adaptation that occurred as apes moved down from the jungles and into the hot savanna. Hairlessness helped control our body temperature when hominids made the transition to a new ecosystem.
“Our ape ancestors spent most of their time in cool forests, but a furry, upright hominid walking around in the sun would have overheated.”
This theory seemed to make a lot of sense, when I was taught this by my professors. However, it had one major flaw. The lack of fur,
“might have made it easier for us to lose heat during the day, [but] we also would have lost more heat at night, when we needed to retain it.”
A recent theory, one that I briefly introduced on Primatology.net back in March, considers that ancestors to modern humans were selected be hairless as a means,
“to reduce the prevalence of external parasites that routinely infest fur. A furry coat provides an attractive and safe haven for insects such as ticks, lice, biting flies and other “ectoparasites.” These creatures not only bring irritation and annoyance but carry viral, bacterial and protozoan-based diseases such as malaria, sleeping sickness, West Nile and Lyme disease, all of which can cause chronic medical problems and, in some cases, death. Humans, by virtue of being able to build fires, construct shelters and produce clothes, would have been able to lose their fur and thereby reduce the numbers of parasites they were carrying without suffering from the cold at night or in colder climates.
Human lice infections, which are confined to the hairy areas of our bodies, seem to support the parasite hypothesis. Naked mole rats, animals that can be described as resembling “overcooked sausages with buck teeth,” also seem to support the theory: They live underground in large colonies, in which parasites would be readily transmitted. But the combined warmth of their bodies and the confined underground space probably negate the problem of losing heat to cold air for these animals, allowing them also to become naked.
Once hairlessness had evolved this way, it may have become subject to sexual selection—being a feature in one sex that appealed to another. Smooth, clear skin may have become a signal of health, like a peacock’s tail, and could explain why women are naturally less hairy than men and why they put more effort into removing body hair. Despite exposing us to head lice, humans probably retained head hair for protection from the sun and to provide warmth when the air is cold. Pubic hair may have been retained for its role in enhancing pheromones or the airborne odors of sexual attraction.”
Will we ever know why hairlessness was selected for in our lineage?
No, just like we may never really know why bipedalism was selected for in our lineage.
But the problem with hairlessness versus bipedalism, is that hairlessness is one of those wild witch hunts that we embark on, one that relies on story telling and a whole lot of what-if’s. It is full of conjecture and not much scientific evidence. Regardless of its flaws, this question remains a very thought-provoking one to me, as someone interested in physical anthropology.
After writing this, I consider hairlessness could have been due to a founder-type effect. Since no supportable physiological significance can be drawn at this time, what if hairlessness became dominant a dominant allele because people who carried the hairless recessive mutant allele replaced people carrying the dominant hairy allele after a population bottleneck?
Again another what-if, however it is a plausible evolutionary possibility… But is it supportable?
No, not at this time… unless we figure out when this allele was really prevalent in our populations after a certain time and correlate its switch from recessive mutant to a dominant after some bottleneck inducing phenomenon, like an epidemic that targeted hairy people more. Until we sequence more paleoDNA from other hominids (mind you we can only extract nuclear DNA from Neanderthals, as of now) this hypothesis is also just as outlandish & unsupported as the aquatic ape one.
24 Comments
June 11, 2007 at 1:45 pm
It seems as if adaptations for hairlessness could have arisen as we began living in closer proximity and in larger groups during the Neolithic. There was an increase in infectious disease during this time, so it seems as if a loss of body hair could have somewhat somewhat offset an individual’s chance of being infected during this transitional period. Of course, this would mean that the loss of body hair was a relatively recent occurrence…
June 11, 2007 at 5:18 pm
Right on Robert, yeah I think the Neolithic coulda been one of the times where hairlessness was selected for. I think you are onto something. The Neolithic transition was a critical shift, and could manifest founder-effect selection.
Again, the problem still remains to find out when did was this mutation introduced into our genome… Until we find out, this and all the other theories are very speculative.
Thanks for commenting!
September 27, 2007 at 2:45 pm
Kambiz, any comment on this?
I’ve an OT question that relates to dinosaur feathers and hominid hair.
When hominid researchers in the field find a skull, what is their priority? To pick and brush it off right? Since we know that feather imprints can be retained if the critter was covered in fine mud (rare but not unusual), shouldn’t skull diggers be looking at the inverse of the skull for hair imprints on the dried mudstone? Of course once the skull’s soft tissues decomposed, the surrounding silts or clays moving inside would destroy some of the features, but the dead hair protein would probably leave an imprint, possibly visible through MRI or micro or nano Xray analysis or something. I for one would like to know the precise hair/beard patterns, as I’m tired of paintings, drawings, models of ancient neandertals, erectoids and apiths with short haircuts, since sapiens have hair that grows a yard long when left uncut.
But then again, some people think our ancestors were like chimpanzees on savannas, where long hair is useless. I do wish the bone & stone folks would check on hair, probably the only soft tissue recognizable after 20k years in the hardening mud.
September 29, 2007 at 9:34 pm
DDeden,
Excellent question. I don’t know of any hominin fossils that told us something about the hair patterns and density.
It could be that people aren’t careful while excavating the remains, but from my experiences, people really slow down and take it easy when they find a precious hominin fossil. I doubt that it has to be due to their priorities because there’s nothing more important than delicately removing a fossil, taking note of the context it’s coming from.
That being said, a lot of why we don’t see fossils with evidence of hair patterns from hominins has to do with the sediment and chemistry the fossil was laid down in. Not every hominin fossil is formed because it settled in ’soft muddy sediment.’ Fossils like famous archaeopteryx one, which show imprints of feathers, are rarities. Only a certain condition would yield such a result. Many fossils are formed in different sediments, with different chemicals, pumus, and even physical dynamics that affect the ability soft tissue is imprinted.
The process to make a fossil takes thousands, if not hundreds of thousands of years, depending on the conditions. A lot can happen during that time. And it happens in different places too.
Fossilization is the process where organic compounds in bone is replaced with inorganic minerals. Often, soft tissue, like hair and the skin it grows out of, is sloughed off and removed by natural phenomenon, scavengers, etc. Other times, hair just won’t last that long. You know how thin is, to have it degrade but still yield an imprint is rare. Even the most malleable sediment will fill in and warp.
I guess the best paleoanthropologists can do, is to sample for trace elements, like degraded hair protein since that may last. I don’t think any sort of visualization technique, can, at this time, show us what you’re looking for.
I hope that answers your question.
Kambiz
October 10, 2007 at 3:56 pm
Neotony. I curious about that as an explanation for many of the distinctive traits we see in our fellow primates. It seems to me lately that the hairlessness that we exhibit into our adulthood is somehow allied with the suite of other expressions of neotony, traits that figure prominently in who we were and who we were to become: big heads, thin skin, cartilage growth throughout life, and covered with fat. It’s not so much that we’re hairless but that our follicles never produce hair like a normal maturing animals in the population from which we diverged. Maybe hairlessness was a genetic inevitability as a result of the way our genes work in certain relationships with other genes. So maybe big brains mean that the potential to grow a heavy pelt is proportionately reduced. At some point the lack of a hair is overcome by the emergent concept of manufactured shelter and the enhanced security of communalism. Being able to tolerate ones fellow species members becomes a requisite for success in gaining access to mates. This kind of instinct for social bonding is not uncommon in young animals but later as adults the group becomes “us” and everything else is “out there”. But in another expression of neotony, our ancestors retained their flexible/elastic social instincts into their adult lives, reinforcing the cohesiveness that provides a degree of advantage over other animals with whom they share the habitat’s resources…and as we have continued to these times.
October 20, 2007 at 8:55 am
Why are most native americans/Indians almost hairless, no arm or leg hair, yet they have a full head of hair? Is this typical? Is this a true feature of an indian or native american?
October 21, 2007 at 2:00 am
Bobbie Neu. East Asians generally have very little facial hair and Native Americans are presumed to have at least some East Asian ancestry. Could easily account for it. When I visited USA I met Pueblo Indians who had very sparse beards and moustaches, looked quite a bit like as Ho Chi Minh.
December 19, 2007 at 4:18 pm
The naked mole rat hairlessness as a method to reduce ectoparasites and suggested to parallel the reduction of hair in humans, is not supported. Lice are warm-body-temperature dependent, living between the skin and insulative fur; naked mole rats are virtually cold-blooded (very unlike the human metabolism). Their colonies are known to be very high in CO2 and low in O2, conditions not conducive to insects. Human ancestors were unlikely to have lived under those circumstances sustainably.
Since most terrestrial mammals thrive encased in their fur coats both in tropical and arctic climes despite associated ectoparasites, something else must have caused human body hair reduction concurrent with scalp hair extension and retention of three species of lice.
December 27, 2007 at 1:34 pm
Here! I got another thesis!
Perhaps, with the appearance of human consciousness and therefore a specific self image/identity, our women ancestors found less hairy men more sexy, because these more resembled a physical otherness from nonhuman animals. Other animals were seen as more primitive, somehow of lower status and hence those men which more resembled nonhuman animals because of strong hairyness were also attributed with that lower status. So women at one point, because of their human sense of identity started preferring less hairy men.
Well at least that was the thing which first entered my mind 0,2 (0.2) seconds after reading the headline…
January 2, 2008 at 3:38 pm
Obvious. Skin parasites prosper only in skin patches where there is hair. Selection favors skins that are unfriendly to parasites. Full stop.
January 4, 2008 at 12:16 am
J: Whales have skin parasites yet lack hair.
Orangutans have lots of long body hair yet lack lice, humans have very reduced hair yet have 3 species of lice.
GVF: If hair reduction was caused by females preferring less hirsute males, why do males have more hair than females? And why during puberty do males get the hairiest, if that is the time of most sexual selection? It doesn’t add up. Hair reduction in most aquatic mammals does. Seems obvious to me that human ancestors spent some time day to day in seawater at the tropics, as many people do today.
March 11, 2008 at 11:10 pm
Yeah why are some better at being hairless than others? Asian/Native American/African. Could friction be part of it? Traveling through tall grass like in Africa. Native Americans are known for being able to quickly cover large ground through thick foliage. Tribes that live in thick jungles are always hairless. I’ve heard men who wear socks a lot sometimes lose hair from the sock line down….
March 17, 2008 at 9:24 pm
CJONES, I’ve been thinking about your comment for sometime now. I don’t mean to offend you by saying it is rather humorous to think about your hypothesis.
To clarify to you and others, the real reason why ’some are better’ at being hairless has to do with regional adaptations to temperature. Hair functions to regulate body heat. In colder areas, people are generally hairier. That’s not always the case, such as the Nepalese and Tibetan populations are relatively hairless. But in areas like the Caucuses,
Hair is also hormonally controlled. As you may have noticed, hair growth increases along with puberty. While all populations undergo puberty, different levels of hormones affect the amount of hair growth.
You may definitely be onto something, but I don’t think thickness of brush has a direct relation to hairiness. The reason why Africans are hairless is largely because most of Africa is hotter and losing hair may have been advantageous for thermoregulation. Asian populations, I dunno what’s going on with their hairlessness… but one of the reasons as why Native Americans are relatively hairless has to do with the fact that they were founded by Asian populations, so they were pre-dispositioned genetically with hairlessness.
Kambiz
March 18, 2008 at 12:18 am
Between 45ka and 30ka, the ancestors of the modern oriental people eg. Koreans, Tibetans, Vietnamese, and the Native Americans, and the Paleo-Siberians (excluding later mixtures) were geographically isolated at Lake Baikal, where they arrived during a warm inter-glacial period by following the flight path of migrating waterfowl from the African Rift and Paratethyan Black-Caspian basin.
They lived at the north end of Lake Baikal along plentiful brackish hot springs at the edge of the Amur tectonic plate, and they maintained a habit of daily bathing and foraging (fish, molluscs, seaweed, reeds, lowland millet grains as well as medium size animals that came for water and salt) in the warm-cool waters there, continuously through the ice age.
They did not have a strong stone tool culture, and probably used snares, traps, pits, weirs. Their huts were probably 1/2 dug into the ground, covered with skins and bark, insulated with reeds including tatami-like mats and furs.
35ka, tectonic shifting opened the Angara river (the only outlet of Lake Baikal) which sent much of the warm water northward into the Yenesei river and the Arctic sea, but the springs still provided warm water locally.
This caused a major diaspora.
The tribe budded off into small expanding mobile bands venturing along connected river basins (Lena, Yana, Yenesei, Angara) and Arctic-Beringia coasts, having partially adapted culturally to colder water and climate with skin boats and improved big game hunting methods and switching from hot baths (retained in Japanese) to sweatlodge/sauna cleansing which depended on firewood fuel rather than hot springs. Because these bands were mobile, running out of fuel was not an issue, unlike sedentary people. Stone tools were made, but bone and ivory were as commonly used.
At the same time in the west, people had adapted biologically by growing longer body hair, they lived along the Medit./Black/Caspian sea and marshes on fish, waterfowl and game thirsting for water and salt, including migratory herds, and quarried for stone in the Caucasus, Alps etc.
After the last glacial maimum, warming induced some Baikal people to expand eastward along the Amur and up/down coasts to Korea/China/Japan (Yayoi), and others to migrate southward inland following flocks to Tibet, Burma and then south China.
So we end up with Lapp, Manchu and Eskimo Inuit people who are relatively hairless biologically well adapted for warm seashores but culturally adapted for tundra plains and arctic coasts; and blonde hairy Norwegians and swarthy Georgians biologically adapted for sub-tropical forests and deltas but culturally adapted for plains and mountain valley herding and fjord fishing in open plank boats.
A bit brief, but that’s the meat of the matter.
March 23, 2008 at 6:25 am
Truly ‘hair raising’! What is more intriguing is the evolutionary perspective of ‘intertriginous hairs’.
March 27, 2008 at 1:21 pm
That’s due to hydrodynamics as a selective agent, (barring conflicts with nursing infants both on land and in water, thus the lack of beard and chest hair in adult females). Better hydrodynamics = less macroturbulence = better oxygen and energy conservation. So, as you state in your post, “friction” reduction, especially in dense, viscous seawater during daily diving for seafood. Since children were not primary foragers, there was no strong selective pressure for better hydrodynamics until breeding age, so aside from scalp hair, children have only vellus hair and tend to be less ‘curvy’ compared to adults. Adult body form is more curved, partly due to childbirth requirements, so the crevices contain fluffy hair, what Darwin termed ‘coarse’ hair, while the scalp and body hair may be straight, wavy, curly or nappy).
Coarse hair is nearly the same in all ethnic races, while the other body/mustache/scalp hair is unique to each group. Why? Because coarse hair is most effective in changing water flow from energy-costing macroturbulence to low friction microturbulence. Great apes lack an under-fur found in lesser apes and monkeys.
Coarse hair at joints/crevices, subcutaneous fat at other locales, the head hair having unique requirements (infant grasping, nursing, previous hominoid hair position, lice being factors).
IMO but not certain, this coarse hair in humans is the remainder of the primate under-fur, which was retained for better swimming and diving, but lost in the apes (which never dove).
March 28, 2008 at 11:03 am
This all makes my think of a totaly different mater. Why does it looks like their was only a limmited amout of admixture between cro-magnon and neandertals. I think because neandertals evolved in a temperate to ice-age climate in the northern hemisphere seperate from other human groups since more than 1 million years (dmanisi ?)and thus had thick pelts (no need to ware clothing) making them sexually unattractive to hairless humans. This does not make them less human, only different.
Since we know that neandertals lived as far east as Siberia, it is only waiting untill an ice-mummy neanderthal is found in the permafrost .
March 30, 2008 at 10:26 pm
Adriaan, that’s an interesting idea. Both polar bears and mammoths had fluffy fur pelts and abundant skin fat. I figure Neandertals had lots of skin fat, but I’m not sure about the pelts.
My thoughts on neandertals: During the warmer periods they were probably naked, they probably swam a lot, they probably were quite hairy, maybe they had reddish -brown woolly body hair. Perhaps they group-ambushed various prey at waterside, then removed the whole pelt (say of a bear or moose) from the carcass, then smoked it, melting off any attached fat, killing the lice etc. then simply wearing that as a longsleeved outer garment during the cold months and adding some squirrel pelt mitts and grass-filled boots with twined cordgrass or tendons to keep everything tight and warm. Later simple sewing gave a better fit, cro-magnons may have brought sewing improvements to the area.
March 31, 2008 at 9:54 am
whoaaaaaa you have a lot of hair
April 16, 2008 at 10:48 am
I think a group of early hominids, who we mistook to be ‘barbarians’ were in true fact ‘barberians’, went around kidnapping hairy people and with really lousy scissors gave them bad haircuts so women ran away, preventing them from having babies. With hairless people having a strong sexual selection advantage, barberians began to lose their natural prey and their numbers diminished sharply – which in turn explains why it’s so hard today to find a good barber, especially for under $20.00.
September 27, 2008 at 2:41 pm
I’m way late in this discussion, but I’m curious about two things:
1. Why most hairy men seem to have chest hair but not necessarily hairy backs. Great apes don’t have chest hair but they have hair on other parts of their torso. I know chest hair is related to levels of testosterone, even though that doesn’t guarantee a man will have chest hair, he probably won’t have a hairy chest without enough of it. So why chest hair and not back hair? Maybe so a baby can cling to his chest? Or maybe the sun on his back caused him to overheat and so losing it was an advantage.
2. It seems like a lot of arguments for chest hair have to do with living in cold climates. But do chest hairs really make a man that much warmer? It doesn’t seem like it would.
October 12, 2008 at 2:11 am
No cold climate mammals have body hair patterns like humans, but the walrus has a similar sparse distribution of hairs, not for warmth but for crevice filling, turbulence reduction and waterflow sensation during diving, the same reason humans retained it. The chest is bumpy, so hair fills in the low spots, the back is less bumpy except the tailbone-buttocks area which has some hair for better flow in water.
The sirenians (manatee/dugong) have a similar pattern of sparse hair. Dolphins swim so fast that waterflow turbulence is not reduced by sparse hair, so they lost all their hair. Walruses, manatees, humans swim and dive slow, sparse hair reduces their drag. Women have less body hair because they spent less time diving and more time wading in the warm shallows with the kids and beachcombing.
December 19, 2008 at 3:39 pm
I have heard about high histamine ( Histadelics ) Have lower body hair .
While people with low histamine ( Histapenics )
Have increased body hair.
Now histamine helps stimulate the immune system.
Northern eurpopeans / Northern asians generally have far less body hair than their related southern populations .
( Italians vs Nordics ) (Asian indians vs Chinese)
It is possible that the higher histamine may be helpful to ward off disease in the northern lands ( Although it seems most disease are far more southern in orgins )
Also Histamine increases blood flow , circulation , lowers blood pressure.
It is possible that the higher histamine levels could help northern populations withstand the cooler climates .
( Only problem ) I tend to find almost no data on histamine in ethnic groups . ( The few i have tend to have said african americans have higher histamine )
Which i find strange . . Histamine lowers body hair ( Asians , Northern Europeans ) Check
Histamine lowers blood pressure ( Asians , Northern europeans ) Check
Histamine causes obsessive compulsive disorder .( In my opinions , Asians and Northern Europeans have much more obsessive compulsive tendencies than african / southern groups )
But african americans have more asthma , allergies . ( So that is the true definition of the general traits of histamine )
But there are also multiple histamine receptors and distributions through out the body!
So perhaps asians , and whites have a alteration of histamine that influences a different end of the histamine traits .
I also tend to find almost no data on histamine levels in humans vs apes or other animals .
What i have found is .
Vitamin C defiency causes Exessive Histamine . (Vitamin C is a anti-histamine )
Also Humans are one of the few mammals which can not create our own vitamin c ..
The mammals inclue ( Apes , Guniea pigs , Fruit eating bats , Humans )
All eat alot more fruits containing alot more vitamin c than humans .
So perhaps a Vitamin C defiency has caused ( Humans to lose their hair , Through Exessive Histamine .
( northern populations would have had far less intakes than vitamin c . ( Matches the less body hair in northern populations )
Which also includes the ( Mayans vs the northern plains – eskimo indians )
The mayans are more hairy .
This seems to be a general world wide trait vs ( Northern Vs Southerns )
The only exeption seems to be the ( Hairy ainu of northern japan )
Which does not truely count!
For the ainu are thought to be a tibetian / nepalese transplant into japan ( 6 thousand ? years ago ? or so)
Best evidence of all is the sea mammals .
Sea mammals lack hair . ( They also lack vitamin c!) = Less vitamin c = More histamine = Less body hair .
Nothing in the sea really makes vitamin c . So all sea mammals have almost no existant vitamin c!
It is thought that vitamin c is needed to protect life on the surface ( Not in the seas )
Thus it is interesting that Sea Mammals + Humans are the only ones on earth to not have fur!
Since humans can not make Vitamin C out of carbohydrates ( Like most animals can ) Since we do not eat tons of fruits each day like a monkey!
It is thought we would need to obtain 1 gram of vitamin c a day to saturate our blood with vitamin c ( Like animals which create their own vitamin c make )
This theory explains why Sea Mammals have no fur . Explains why humans lack it . It also explains why the northern populations which lack Fruits in their diets also have less body hair .
The Testosteronee Theory is not true . ( African americans have higher testosterone than whites ) Yet generally have less body hair .
With this said . It is in my personal opinion . That humans began to first lose their fur from a vitamin c defiency . Than later on this was reinforced by natural selection which rewired us to perfer less fur .
Thus we became furless .
January 12, 2009 at 7:20 pm
Sorry Matt, you’ve got so many errors I can’t correct them all. Seaweeds have vitamin C, all anthropoids lost the ability to produce vitamin C including foliavorous monkeys (including the swimming proboscis monkey of Borneo mangrove swamps), many sea mammals have abundant fur and/or vitamin C in their skin, etc.