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Why don't we have hair on palms?

Why don't we have hair on palms?


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We have hair all over the body except palms.
What is the biological reason behind this?


The main functional reason is we need to be able to grip things with our hands (and feet, which are also hairless), and hair would interfere with that. Physiologically, the epidermis in these parts of the body is very thick and highly keratinized, and when combined with the thick underlaying layer of dermis, this results in skin that does not support the growth and maturation of hair follicles.


Think Fast: Do Humans Have Hair on the Undersides of Their Arms?

At first glance, the underside of a human arm may look hairless. But a closer inspection will reveal that tiny, colorless hairs cover it like soft peach fuzz.

That's because modern human beings (Homo sapiens) are covered with hair — it's just difficult to see, said Yana Kamberov, an assistant professor of genetics at the University of Pennsylvania.

"We are actually very hairy," Kamberov told Live Science. For instance, our foreheads, ears and, yes, even the undersides of our arms, are covered with tiny hairs called vellus hairs, she said. The only places without hair on the outer human body are the palms, soles of the feet, lips and nipples, Kamberov said. [Why Does Hair Turn Gray?]

Essentially, humans are just as hairy as chimpanzees, according to research comparing hair density between the two species, she said. But whereas chimps are covered with scruffy, black hair that's easy to see, most human hair is less visible because it's minuscule and colorless.

About 2 million years ago, an adaptation led the genus Homo to miniaturize its body hair, Kamberov said. In addition, Homo underwent an adaptation that increased its number of eccrine sweat glands — the glands that most mammals have only on their palms and the soles of their feet.

"The density of those glands exploded, so if you look at the relative density of these glands in a human and a chimpanzee and a macaque, our density is much higher than what you would expect for a primate of our body size," Kamberov said.

These adaptations helped the Homo genus become exceptional long-distance runners, Kamberov said. Most animals need to take breaks during long runs to cool off by panting, Kamberov said. A horse, for instance, can't pant when it's galloping, according to Slate. In contrast, humans can run long distances, even marathons, without having to stop, because we can cool off by sweating with our vast number of eccrine sweat glands.

In addition, if humans had a lot of scruffy hair, as chimps do, the sweat would just coat the hair and not the skin. When most of our body hair is miniaturized, sweat can coat the skin, keeping it cool by making it wet, and then by evaporating off of it, allowing humans to continue walking, trekking or running without overheating, Kamberov said.

That leads to another question, however: Why isn't all of our hair miniaturized peach fuzz?

The answer has to do with puberty, Kamberov said. When humans go through puberty, hormones called androgens trigger some of the tiny vellushairs to "trans-differentiate," or change into terminal hairs that have color, grow longer and cycle, Kamberov said.

It's unclear why some vellus hairs respond to hormones and others don't, she said. The same goes for arm hair — it remains a mystery why the top part of people's arms have terminal hairs and the bottom side does not.

Perhaps those longer hairs are meant to keep the exposed part of the arm warm, Kamberov said.

Another idea is that "it might be an adaptation to minimize friction during arm swing, but that's a wild guess," said Daniel Lieberman, a biological anthropologist at Harvard University.

And still another idea is that the terminal hairs on certain parts of the human body are just leftovers from our long-hair-covered ape ancestors.


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Hair Biology has always interested me and no matter where I look I can never find the appropriate answer to my question. Now I understand that African American hair has a curly follicle, which is what produces the curly hair texture as in many ethnic groups, but how come African Americans are essentially the only ethnic group with coarse curly hair? I don't mean to be rude while asking this question, but it has always been of interest to me. I want to understand the genetics behind it.

-An undergraduate student from Georgia

Having an interest in understanding our differences isn't rude at all!

Hair texture is just one of the many obvious physical differences that exist between ethnic groups. Although hair growth rate, size, shape and texture are unique to every person, we can see trends among groups of people.

While genes are likely to be involved in determining these traits, not much is known yet about the actual ones involved. So what is known about hair type?

Hair follicles are tiny pockets in our scalp out of which our hair grows. As you hinted above, the thickness and texture of our hair depends on the size and shape of these follicles. They help to form and contour our hair as it grows.

Our hair thickness results from a combination of both the size of the follicles themselves and how many of them line our scalp. The size of the follicles determines if the individual hair strands are thick or thin. Large follicles produce thick hairs. Small follicles produce thin hairs. It is that simple!

Equally important to our hair thickness, the number of follicles on our scalp determines the actual number of hairs crowning our head. Lots of hairs equal thick hair. Sparse hair equals thin hair. On average, our heads are covered with over 100,000 follicles!

Our hair texture can range anywhere from pin-straight to extremely curly. Follicles that are round in cross-section give rise to straight hair. Those out of which curly hair grows are oval. Very tightly coiled hair is due to the nearly flat, ribbon-like structure of the follicles. This hair texture is very common in people of African ancestry.

Not only is African hair wiry, it is also frequently coarse. So why is this?

African hair produces plenty of protective oils, called sebum. In fact, African hair actually produces more oils than Caucasian and Asian hair. However, due to the tight curls, the oil fails to spread evenly along the hair fiber.

Without lubrication, the fibers become very dry. This causes the brittle strands to flake and roughen, resulting in hair that is coarse to the touch. Very curly hair from all ethnic groups often lacks the silky smoothness of straight hair. This may due to the same reason, but to a lesser extent.

The brittleness of African hair adds to the illusion that it cannot be grown long. The tight curls create stresses at each turn in the hair fiber. The hair strands become weak and fragile, making them prone to breakage. As a result, tightly coiled hair tends to stay quite short. So is this hair quality genetic?

There are two strong reasons why we would expect African hair texture to be genetic. Firstly, the texture is universal in Africans, while nearly absent from other ethnic groups. Secondly, it is consistently passed down to the children in each new generation.

Despite this, I could not find any identified gene shown to be responsible. Of course, that doesn't mean that a gene isn't involved! Scientists have just not found it yet. However, we may be able to pull clues from rare occurrences of non-Africans with a similar coarse hair texture.

You may think that coiled hair is unique to those of African ancestry, but it is not. It is, however, quite rare in other races. So rare, in fact, that when it is seen in Caucasians and Asians it is called a syndrome. Woolly Hair Syndrome.

Described in much the same way as African hair, woolly hair is characterized by dry, tightly spiraled fibers. You may be wondering if it initially arose from the mixing of different racial gene pools. That is not thought to be the case.

Since Woolly Hair Syndrome is so infrequent there is little reliable information about it. The actual causative gene or genes have not been singled out yet. However, the syndrome does run strongly in families.

When the exact gene causing a syndrome is not known, scientists look at how the trait is passed along in families. It appears that most cases of Woolly Hair are inherited dominantly. This means only a single copy of the "woolly" version of the gene is needed, passed down from either the father or the mother.

It may be possible that the gene responsible for Woolly Hair in non-Africans contributes to the coarse texture of African hair as well. If this were the case, the "non-woolly" version of the gene is virtually exclusive to Caucasians and Asians. This would explain the silky hair common among these ethnicities.

Likewise, the "woolly" version is nearly exclusive to Africans. Its high prevalence could be explained by the fact that most Africans are carrying two copies of the dominant gene. This would assure that the coarse hair texture is maintained in the population.

Whether the gene responsible for Woolly Hair in Caucasians causes the similar hair texture seen in Africans is hotly debated. Differences have been noted. For example, the curls of Africans tend to lie as separate ringlets, while the curls of woolly-haired Caucasians tend to merge.

This model also raises questions regarding the hair texture of children of mixed race. Using this model we might expect kids with one African gene and one Caucasian gene to have the dominant African hair texture. This does not always appear to be the case. An "intermediate" texture is often seen.

As time passes, genetics will certainly bring to light the reason behind many of our ethnic differences. When that day comes, there may then be a more definitive answer to your question.


Thermoreception

Thermoreception is the process of determining temperature by comparing the activation of different thermoreceptors in the brain.

Learning Objectives

Describe the various types of receptors used for thermoreception: Krause end bulbs, Ruffini endings, free nerve endings

Key Takeaways

Key Points

  • Thermoreceptors can include: Krause end bulbs, which detect cold and are defined by capsules Ruffini endings, which detect warmth and are defined by enlarged dendritic endings and warm and cold receptors present on free nerve endings which can detect a range of temperature.
  • The cold receptors present on free nerve endings, that can be either lightly-myelinated or unmyelinated, have a maximum sensitivity at

Key Terms

  • thermoreceptor: a nerve cell that is sensitive to changes in temperature
  • somatosensory: of or pertaining to the perception of sensory stimuli produced by the skin or internal organs
  • epineurium: the connective tissue framework and sheath of a nerve which bind together the nerve bundles, each of which has its own special sheath, or perineurium

Thermoreception

Thermoception or thermoreception is the sense by which an organism perceives temperatures. The details of how temperature receptors work are still being investigated. Mammals have at least two types of sensors: those that detect heat (i.e., temperatures above body temperature) and those that detect cold (i.e., temperatures below body temperature). A thermoreceptor is a sensory receptor or, more accurately, the receptive portion of a sensory neuron that codes absolute and relative changes in temperature, primarily within the innocuous range. The adequate stimulus for a warm receptor is warming, which results in an increase in their action potential discharge rate cooling results in a decrease in warm receptor discharge rate. For cold receptors, their firing rate increases during cooling and decreases during warming. The types of receptors capable of detecting changes in temperature can vary.

Types of Thermoreceptors: Capsule Receptors

Some of the receptors that exhibit the ability to detect changes in temperature include Krause end bulbs and Ruffini endings. Krause end bulbs are defined by cylindrical or oval bodies consisting of a capsule that is formed by the expansion of the connective-tissue sheath, containing an axis-cylinder core. End-bulbs are found in the conjunctiva of the eye, in the mucous membrane of the lips and tongue, and in the epineurium of nerve trunks. They are also found in the penis and the clitoris hence, the name of genital corpuscles. In these locations, they have a mulberry-like appearance, being constricted by connective-tissue septa into two to six knob-like masses.

Krause end bulb: A drawing of a Krause end bulb receptor which can detect cold.

The Ruffini endings, enlarged dendritic endings with elongated capsules, can act as thermoreceptors. This spindle-shaped receptor is sensitive to skin stretch, contributing to the kinesthetic sense of and control of finger position and movement. Ruffini corpuscles respond to sustained pressure and show very little adaptation. Ruffinian endings are located in the deep layers of the skin where they register mechanical deformation within joints as well as continuous pressure states.They also act as thermoreceptors that respond for an extended period in case of deep burn, there will be no pain as these receptors will be burned off.

Ruffini endings: A drawing of a Ruffini ending receptor which can detect warmth.

In addition to Krause end bulbs that detect cold and Ruffini endings that detect warmth, there are different types of cold receptors on free nerve endings.

Types of Thermoreceptors: Free Nerve Endings

There are thermoreceptors that are located in the dermis, skeletal muscles, liver, and hypothalamus that are activated by different temperatures. These thermoreceptors, which have free nerve endings, include only two types of thermoreceptors that signal innocuous warmth and cooling respectively in our skin. The warm receptors show a maximum sensitivity at

45°C, signal temperatures between 30 and 45°C, and cannot unambiguously signal temperatures higher than 45°C they are unmyelinated. The cold receptors have their maximum sensitivity at

27°C, signal temperatures above 17°C, and some consist of lightly-myelinated fibers, while others are unmyelinated. Our sense of temperature comes from the comparison of the signals from the warm and cold receptors. Thermoreceptors are poor indicators of absolute temperature, but are very sensitive to changes in skin temperature.

The Thermoreceptor Pathway

The thermoreceptor pathway in the brain runs from the spinal cord through the thalamus to the primary somatosensory cortex. Warmth and cold information from the face travels through one of the cranial nerves to the brain. You know from experience that a tolerably cold or hot stimulus can quickly progress to a much more intense stimulus that is no longer tolerable. Any stimulus that is too intense can be perceived as pain because temperature sensations are conducted along the same pathways that carry pain sensations.


Nails

Figure 2. The parts of a finger nail

The fingernail is an important structure made of keratin. The fingernail generally serve two purposes. It serves as a protective plate and enhances sensation of the fingertip. The protection function of the fingernail is commonly known, but the sensation function is equally important. The fingertip has many nerve endings in it allowing us to receive volumes of information about objects we touch. The nail acts as a counterforce to the fingertip providing even more sensory input when an object is touched.

Nail Structure

The structure we know of as the nail is divided into six specific parts: the root, nail bed, nail plate, eponychium (cuticle), perionychium, and hyponychium.

Root The root of the fingernail is also known as the germinal matrix. This portion of the nail is actually beneath the skin behind the fingernail and extends several millimeters into the finger. The fingernail root produces most of the volume of the nail and the nail bed. This portion of the nail does not have any melanocytes, or melanin producing cells. The edge of the germinal matrix is seen as a white, crescent shaped structure called the lunula.

Nail Bed The nail bed is part of the nail matrix called the sterile matrix. It extends from the edge of the germinal matrix, or lunula, to the hyponychium. The nail bed contains the blood vessels, nerves, and melanocytes, or melanin-producing cells. As the nail is produced by the root, it streams down along the nail bed, which adds material to the undersurface of the nail making it thicker. It is important for normal nail growth that the nail bed be smooth. If it is not, the nail may split or develop grooves that can be cosmetically unappealing.

Nail Plate The nail plate is the actual fingernail, made of translucent keratin. The pink appearance of the nail comes from the blood vessels underneath the nail. The underneath surface of the nail plate has grooves along the length of the nail that help anchor it to the nail bed.

Eponychium The cuticle of the fingernail is also called the eponychium. The cuticle is situated between the skin of the finger and the nail plate fusing these structures together and providing a waterproof barrier.

Perionychium The perioncyhium is the skin that overlies the nail plate on its sides. It is also known as the paronychial edge. The perionychium is the site of hangnails, ingrown nails, and an infection of the skin called paronychia.

Hyponychium The hyponychium is the area between the nail plate and the fingertip. It is the junction between the free edge of the nail and the skin of the fingertip, also providing a waterproof barrier.


THE HYPOTHESES

In the long evolutionary history of man, powerful selection factors would have operated on hunter—gatherers who relied on sustained endurance running to hunt middle-sized mobile game.

On a hot savannah plain success would have attended those hunters best able to defer dehydration and resist the extreme thermoregulatory challenge 10 . Adaptations to contain the loss of sweat drops from the body surface would have been crucial. An unstable sweat drop that falls to the ground is a sweat drop wasted, for it will have contributed little to evaporative cooling of the skin surface and its loss will hasten dehydration. The retention of sweat on the skin surface will be encouraged by anything that lowers the surface tension of the sweat, so that it forms a sheet rather than drops. In passing it may be noted that the need to retain sweat on near-vertical surfaces has led to morphological adaptations in addition to the physiological ones. The features of the face may be explained in part by the need for platforms to check, and hair tufts to catch, the descending sweat drops generated from the gland-rich scalp, a specialized heat-dissipating organ. Hence supraorbital tori, flared nostrils, everted upper lip, a chin (a late development in our evolutionary history) and hair remnants such as eyebrows and moustache. Sternal and pubic helical hair serve the same function. Pubic hair extends up the central abdominal gutter in the male to meet and trap the descending sweat drops endomorphic females have no central gutter and the superior surface of the pubic hair is thus horizontal. Above all there must be no sweat drops on the pumping hands, hence there is only an insensible loss of sweat from the palms (unless, strangely, the sweating is prompted by anxiety 2 ).

The ability of the skin to shed water suggests that it is intrinsically hydrophobic with a low surface-free energy. Very pure water has a surface tension of about 72 dynes/cm, which declines to about 30-50 dynes/cm if a finger is inserted into the water (Bangham AD, Personal communication). Though any contaminant might cause this, it may be noted that the surface tension plunges if the finger is first inserted in the ear and that cerumen is only ‘stale dammed sebum’ 11 . It would seem that there is some secretion on the skin that fulfils a surfactant role and discourages drop formation. The somewhat oily secretions from apocrine glands are likely to have emulsifying properties for they have been observed to spread over the skin in a film and usually not to form droplets 12 , 13 .

The sebum from sebaceous glands will also contribute to the surfactant action. Bacterial action on the skin forms free fatty acids from the triglycerides and wax esters found in sebum 7 . Below about 30ଌ the fluid consistency of the sebum fraction changes and it suddenly assumes either a solid or a highly viscous character 14 , but at and above 30ଌ sebum it has a surface tension of about 25 dynes/cm and would thus qualify as a potential emulsifier of sweat.

Thus sebum has three thermoregulatory roles. The first is to coat the straight hair of northern populations and create a water-repellent pelage. Southern populations have non-matting helical scalp hair which serves the double function of reflecting sunlight whilst at the same time allowing the 𠆋reeze-over-body’, generated by running, to penetrate the hair and cool the sweaty scalp. Second, at higher temperatures sebum acts as a surfactant for eccrine secretions. Third, at lower temperatures it repels rain on the exposed skin. It may be postulated, therefore, that the outcome of secretory interactions is for an externally generated fluid, rain, to be projected off the skin in cool wet conditions, whilst in hot conditions the internally generated fluid, eccrine sweat, is encouraged to spread in a film across the skin and to be retained on the surface. This would be a remarkable temperature-dependent switch in function on the part of sebum.


Contents

Like much of the hair on the human body, leg, arm, chest, and back hair begin as vellus hair. As people age, the hair in these regions will often begin to grow darker and more abundantly. This will typically happen during or after puberty. Men will often have more abundant, coarser hair on the arms and back, while women tend to have a less drastic change in the hair growth in these areas but do experience a significant change in thickness of hairs. However, some women will grow darker, longer hair in one or more of these regions.

Chest and abdomen Edit

Vellus hair grows on the chest and abdomen of both sexes at all stages of development. After puberty and extending into adulthood, most males grow increasing amounts of terminal hair over the chest and abdomen areas. Adult women also typically can grow terminal hairs around the areola though in many cultures these hairs are typically removed. [ citation needed ]

Arms Edit

Arm hair grows on a human's forearms, sometimes even on the elbow area, and rarely on a human's bicep, triceps, and/or shoulders. Terminal arm hair is concentrated on the wrist end of the forearm, extending over the hand. Terminal hair growth in adolescent males is often much more intense than that in females, particularly for individuals with dark hair. In some cultures, it is common for women to remove arm hair, though this practice is less frequent than that of leg hair removal.

Terminal hair growth on arms is a secondary sexual characteristic in boys and appears in the last stages of puberty. Vellus arm hair is usually concentrated on the elbow end of the forearm and often ends on the lower part of the upper arm. This type of intense arm vellus hair growth sometimes occurs in girls and children of both sexes until puberty. Even though this causes the arms to appear hairy, it is not caused solely by testosterone. The hair is softer and different from men's arm hair, in texture.

Feet Edit

Visible hair appearing on the top surfaces of the feet and toes generally begins with the onset of puberty. Terminal hair growth on the feet is typically more intense in adult and adolescent males than in females.

Legs Edit

Leg hair sometimes appears at the onset of adulthood, with the legs of men more often hairier than those of women. For a variety of reasons, people may shave their leg hair, including cultural practice or individual needs. Around the world, women generally shave their leg hair more regularly than men, to conform with the social norms of many cultures, many of which perceive smooth skin as a sign of youth, beauty, and in some cultures, hygiene. However, athletes of both sexes – swimmers, runners, cyclists and bodybuilders in particular – may shave their androgenic hair to reduce friction, highlight muscular development or to make it easier to get into and out of skin-tight clothing.

Pubic Edit

Pubic hair is a collection of coarse hair found in the pubic region. It will often also grow on the thighs and abdomen. Zoologist Desmond Morris disputes theories that it developed to signal sexual maturity or protect the skin from chafing during copulation, and prefers the explanation that pubic hair acts as a scent trap. Also, both sexes having thick pubic hairs act as a partial cushion during intercourse. [1]

The genital area of males and females are first inhabited by shorter, lighter vellus hairs that are next to invisible and only begin to develop into darker, thicker pubic hair at puberty. At this time, the pituitary gland secretes gonadotropin hormones which trigger the production of testosterone in the testicles and ovaries, promoting pubic hair growth. The average ages pubic hair begins to grow in males and females are 12 and 11, respectively. However, in some females, pubic hair has been known to start growing as early as age 7.

Just as individual people differ in scalp hair color, they can also differ in pubic hair color. Differences in thickness, growth rate, and length are also evident.

Armpits Edit

Underarm hair normally starts to appear at the beginning of puberty, with growth usually completed by the end of the teenage years.

Today in much of the world, it is common for women to regularly shave their underarm hair. The prevalence of this practice varies widely, though. The practice became popular for cosmetic reasons around 1915 in the United States and United Kingdom, when one or more magazines showed a woman in a dress with shaved underarms. Regular shaving became feasible with the introduction of the safety razor at the beginning of the 20th century. While underarm shaving was quickly adopted in some English speaking countries, especially in the US and Canada, it did not become widespread in Europe until well after World War II. [2] [3] Since then the practice has spread worldwide, some men also choose to shave their armpits.

Facial Edit

Facial hair grows primarily on or around one's face. Both men and women experience facial hair growth. Like pubic hair, non-vellus facial hair will begin to grow in around puberty. Moustaches in young men usually begin to grow in at around the age of puberty, although some men may not grow a moustache until they reach late teens or at all. In some cases facial hair development may take longer to mature than the late teens, and some men experience no facial hair development even at an older age.

It is common for many women to develop a few facial hairs under or around the chin, along the sides of the face (in the area of sideburns), or on the upper lip. These may appear at any age after puberty but are often seen in women after menopause due to decreased levels of estrogen. A darkening of the vellus hair of the upper lip in women is not considered true facial hair, though it is often referred to as a "moustache" the appearance of these dark vellus hairs may be lessened by bleaching. A relatively small number of women are able to grow enough facial hair to have a distinct beard. In some cases, female beard growth is the result of a hormonal imbalance (usually androgen excess), or a rare genetic disorder known as hypertrichosis. [4] Sometimes it is caused by use of anabolic steroids. Cultural pressure leads most women to remove facial hair, as it may be viewed as a social stigma.

Hair follicles are to varying degrees sensitive to androgen, primarily testosterone and its derivatives, particularly dihydrotestosterone, with different areas on the body having different sensitivity. As androgen levels increase, the rate of hair growth and the weight of the hairs increase. Genetic factors determine both individual levels of androgen and the hair follicle's sensitivity to androgen, as well as other characteristics such as hair colour, type of hair and hair retention.

Rising levels of androgen during puberty cause vellus hair to transform into terminal hair over many areas of the body. The sequence of appearance of terminal hair reflects the level of androgen sensitivity, with pubic hair being the first to appear due to the area's special sensitivity to androgen. The appearance of pubic hair in both sexes is usually seen as an indication of the start of a person's puberty. There is a sexual differentiation in the amount and distribution of androgenic hair, with men tending to have more terminal hair in more areas. This includes facial hair, chest hair, abdominal hair, leg hair, arm hair, and foot hair. (See Table 1 for development of male body hair during puberty.) Women retain more of the less visible vellus hair, although leg, arm, and foot hair can be noticeable on women. It is not unusual for women to have a few terminal hairs around their nipples as well. In the later decades of life, especially after the 5th decade, there begins a noticeable reduction in body hair especially in the legs. The reason for this is not known but it could be due to poorer circulation, lower free circulating hormone amounts or other reasons.

Table 1 - Occurrence (%) with Body Hair American Males Aged 14, 16 and 18
Area Age 14 Age 16 Age 18
Pubic 97 100 100
Axillary 40 97 100
Anterior leg 46 90 100
Anterior thigh 30 67 95
Forearm 14 37 80
Abdomen 14 37 75
Buttocks 14 33 50
Chest 3 7 40
Lower back 3 7 20
Upper arms 0 0 10
Shoulders 0 0 0

Ref. Reynolds EL. The appearance of adult patterns of body hair in man. Ann NY Acad Sci 1951:53:576- 584.

Androgenic hair provides tactile sensory input by transferring hair movement and vibration via the shaft to sensory nerves within the skin. Follicular nerves detect displacement of hair shafts and other nerve endings in the surrounding skin detect vibration and distortions of the skin around the follicles. Androgenic hair extends the sense of touch beyond the surface of the skin into the air and space surrounding it, detecting air movements as well as hair displacement from contact by insects or objects. [5] [6]

Determining the evolutionary function of androgenic hair must take into account both human evolution and the thermal properties of hair itself.

The thermodynamic properties of hair are based on the properties of the keratin strands and amino acids that combine into a 'coiled' structure. This structure lends to many of the properties of hair, such as its ability to stretch and return to its original length. This coiled structure does not predispose curly or frizzy hair, both of which are defined by oval or triangular hair follicle cross-sections. [7]

Evolution of less body hair Edit

Hair is a very good thermal conductor and aids heat transfer both into and out of the body. When goose bumps are observed, small muscles (arrector pili muscle) contract to raise the hairs both to provide insulation, by reducing cooling by air convection of the skin, as well as in response to central nervous stimulus, similar to the feeling of 'hairs standing up on the back of your neck'. This phenomenon also occurs when static charge is built up and stored in the hair. Keratin however can easily be damaged by excessive heat and dryness, suggesting that extreme sun exposure, perhaps due to a lack of clothing, would result in perpetual hair destruction, eventually resulting in the genes being bred out in favor of high skin pigmentation. It is also true that parasites can live on and in hair thus peoples who preserved their body hair would have required greater general hygiene to prevent diseases. [8]

Markus J. Rantala of the Department of Biological and Environmental Science, University of Jyväskylä, Finland, said humans evolved by "natural selection" to be hairless when the trade off of "having fewer parasites" became more important than having a "warming, furry coat". [9]

P.E. Wheeler of the Department of Biology at Liverpool Polytechnic said quadrupedal savannah mammals of similar volume to humans have body hair to keep warm while only larger quadrupedal savannah mammals lack body hair, because their body volume itself is enough to keep them warm. [10] Therefore, Wheeler said humans who should have body hair based on predictions of body volume alone for savannah mammals evolved no body hair after evolving bipedalism which he said reduced the amount of body area exposed to the sun by 40%, reducing the solar warming effect on the human body. [10]

Loss of fur occurred at least 2 million years ago, but possibly as early as 3.3 million years ago judging from the divergence of head and pubic lice, and aided persistence hunting (the ability to catch prey in very long distance chases) in the warm savannas where hominins first evolved. The two main advantages are felt to be bipedal locomotion and greater thermal load dissipation capacity due to better sweating and less hair. [11]

Sexual selection Edit

Markus J. Rantala of the Department of Biological and Environmental Science, University of Jyväskylä, Finland, said the existence of androgen dependent hair on men could be explained by sexual attraction whereby hair on the genitals would trap pheromones and hair on the chin would make the chin appear more massive. [9]

In 1876, Oscar Peschel wrote that North Asiatic Mongols, Native Americans, Malays, Hottentots and Bushmen have little to no body hair, while Semitics, Indo-Europeans, and Southern Europeans (especially the Portuguese and Spanish) have extensive body hair. [12]

Anthropologist Joseph Deniker said in 1901 that the very hirsute peoples are the Ainus, Iranians, Australian aborigines (Arnhem Land being less hairy), Toda, Dravidians and Melanesians, while the most glabrous peoples are the American Indians, San, and East Asians, who include Chinese, Mongols, and Malays. [13] Deniker said that hirsute peoples tend to have thicker beards, eyelashes, and eyebrows but fewer hairs on their scalp. [13]

C.H. Danforth and Mildred Trotter of the Department of Anatomy at Washington University conducted a study using army soldiers of European origin in 1922 where they concluded that dark-haired white men are generally more hairy than fair-haired white men. [14]

H. Harris, publishing in the British Journal of Dermatology in 1947, wrote American Indians have the least body hair, Chinese and Black people have little body hair, white people have more body hair than Black people and Ainu have the most body hair. [15]

Anthropologist Arnold Henry Savage Landor, in 1970, described the Ainu as having hairy bodies. [16]

Stewart W. Hindley and Albert Damon of the Department of Anthropology at Stanford University studied, in 1973, the frequency of hair on the middle finger joint (mid-phalangeal hair) of Solomon Islanders, as a part of a series of anthropometric studies of these populations. They summarize other studies on prevalence of this trait as reporting, in general, that Caucasoids are more likely to have hair on the middle finger joint than Negroids and Mongoloids, and collect the following frequencies from previously published literature: Andamanese 0%, Eskimo 1%, African American 16% or 28%, Ethiopians 25.6%, Mexicans of the Yucatan 20.9%, Penobscot and Shinnecock 22.7%, Gurkha 33.6%, Japanese 44.6%, various Hindus 40–50%, Egyptians 52.3%, Near Eastern peoples 62–71%, various Europeans 60–80%. However, they never completed an Androgenic hair map. [17]

According to anthropologist and professor Ashley Montagu in 1989, Asian people and black people such as the San people are less hairy than white people. Montagu said that the hairless feature is a neotenous trait. [18]

Eike-Meinrad Winkler and Kerrin Christiansen of the Institut für Humanbiologie studied, in 1993, Kavango people and !Kung people of body hair and hormone levels to investigate the reason black Africans did not have bodies as hairy as Europeans. [19] Winkler and Christiansen concluded the difference in hairiness between black Africans and Europeans had to do with differences in androgen or estradiol production, in androgen metabolism, and in sex hormone action in the target cells. [19]

Valerie Anne Randall of the Department of Biomedical Sciences, University of Bradford, said in 1994 beard growth in Caucasian men increases until the mid-thirties due to a delay caused by growth cycles changing from vellus hair to terminal hair. [20] Randall said white men and women are hairier than Japanese men and women even with the same total plasma androgen levels. [20] Randall says that the reason for some people being hairy and some people not being hairy is unclear, but that it probably is related to differing sensitivity of hair follicles to 5α-reductase. [20]

Rodney P.R. Dawber of the Oxford Hair Foundation said in 1997 that East Asian males have little or no facial or body hair and Dawber also said that Mediterranean males are covered with an exuberant pelage. [21]

Milkica Nešić and her colleagues from the Department of Physiology at the University of Niš, Serbia, cited prior studies in a 2010 publication as indicating that the frequency of hair on the middle finger joint (mid-phalangeal hair) in whites is significantly higher than in Black populations. [22]

It has been shown that individuals can be uniquely identified by their androgenic hair patterns. For example, even when one's particular distinguishing features such as face and tattoos are obscured, persons can still be identified by their hair on other parts of their body. [23] [24]


Without a doubt, the human trait that sets us apart the most from the animal kingdom is our extraordinary brain. Humans don't have the largest brains in the world &mdash those belong to sperm whales. We don't even have the largest brains relative to body size &mdash many birds have brains that make up more than 8 percent of their body weight, compared to only 2.5 percent for humans. Yet the human brain, weighing only about 3 pounds when fully grown, give us the ability to reason and think on our feet beyond the capabilities of the rest of the animal kingdom, and provided the works of Mozart, Einstein and many other geniuses. [Brain Facts]

Contrary to popular misconceptions, humans are not the only animals to possess opposable thumbs &mdash most primates do. (Unlike the rest of the great apes, we don't have opposable big toes on our feet.) What makes humans unique is how we can bring our thumbs all the way across the hand to our ring and little fingers. We can also flex the ring and little fingers toward the base of our thumb. This gives humans a powerful grip and exceptional dexterity to hold and manipulate tools with. This is getting off the topic, but what if we all had six fingers?

The human ability to control fire would have brought a semblance of day to night, helping our ancestors to see in an otherwise dark world and keep nocturnal predators at bay. The warmth of the flames also helped people stay warm in cold weather, enabling us to live in cooler areas. And of course it gave us cooking, which some researchers suggest influenced human evolution &mdash cooked foods are easier to chew and digest, perhaps contributing to human reductions in tooth and gut size.


An Evolutionary Quirk

It turns out that this mechanism actually serves some biological purpose — or it did in our animal ancestors, at least. One is for warmth: In wintery climates, piloerection expands the amount of air between a creature's flesh and the cold, offering a thicker layer of insulation. You can't see their skin because of the fur, but if you could, it would look a lot like human skin does whenever there's a chill (e.g., covered in goose bumps).

Another, Dr. Bubenik says, is protection against potential predators. "The hair will stand up in many animals when they feel threatened — in a cat being attacked by a dog, for example," he writes. "The elevated hair, together with the arched back and the sideward position the animal often assumes, makes the cat appear bigger in an attempt to make the dog back off."

Another example of this is in porcupines. At rest, a porcupine's quills lay almost flat against its body, but when the animal's defense mechanism kicks in, the spines stick straight out. This response is useless for humans — we don't have enough hair on our arms and legs to suddenly make us appear larger, and we're also not likely to be in many situations where such a reaction is necessary — but it's one of the many things we inherited from our ancestors.


There is Solid Science Behind Drug Testing

Also known as forensic toxicology, the analysis of controlled substances involves the collection of chemicals that have the legally recognized potential for abuse. They include “street drugs” such as heroin and ecstasy, and prescription drugs like oxycodone.

Drug testing is the most frequent forensic function performed by publicly funded crime laboratories, which analyze biological samples for the presence of toxins present in an individual to determine whether the amount of those substances is above a harmful level. It is used to make inferences on an individual’s death, illness, and mental or physical impairment. Like DNA analysis, the analysis of controlled substances is a mature forensic science discipline and one of the areas with strong scientific underpinnings developed along the lines of classical analytical chemistry.

The NAS report found that there exists an adequate understanding of the uncertainties and potential errors in the analysis of controlled substances due to rigorous scientific testing.