Why does human facial and head hair continue to grow?

Why does human facial and head hair continue to grow?

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Many people can grow extremely long head hair and facial hair. Are there evolutionary theories as to why this is the case? It seems like having long hair could be a disadvantage, and extremely long hair seems to be rare in other mammals.

How humans evolved to have head / beard hair that continues to grow longer than other animals is a topic that many anthropologists & biologists are still not sure about & there is no general consensus as to "why" yet.

The three main views that I am aware of however are :

1) Evolution of the "Aquatic Ape." (Ingram, 2000: Morgan 1997; 1982)

  • Infants, in order to hold onto their mothers in the water, would latch onto her hair. Limiting separation from the mother & increasing chances of survivability
  • Longer hair meant that infants / small children would need to swim less in order to get to their mother
  • Believed to be supported even further when you consider that aquatic mammals are almost always hairless, indicating that at one point, humans were highly "aquatic" mammals.

2) No real benefit, but used as a tool for "mate selection." (Darwin, 1871; Cooper 1971)

  • The view held by many of the Darwin school of thought (I do not mean that as a derogatory), is that at first, "hairiness" was sexually attractive, but eventually "hairlessness" became more sexually attractive in most places (i.e. the face to see facial expressions & socialize better; Wong & Simmons 2001)

  • A sign of "virility" & "health" as can be seen in the mate-selection behavior of lions. Which is true even today as human diagnostic material for health (Klevay, 1972).

3) Practical evolutionary benefits for the human species specifically

  • A lot of body heat escapes from the head, probably the most important part of your body. Hair is a good insulator that can keep in heat. This increases survivability in colder climates. (Wong & Simmons 2001; Bubenick 2003). (A disputed but considered credible reason, especially when you compare hair length and types across different regions throughout history)

  • Protection against damaging UV rays (while still permitting adequate Vit.D3 to come through) & some protection from free-radicals or other harmful particles. Because we became bi-pedal, the head was the main area exposed to the sun (as well as some of our back). Extending hair's usefulness to even hot environments, while other body hair became less important with the development of sweat glands (Wheeler 1985).

  • Heightened "Situational Awareness" through "Touch sense." A concept that may seem silly at first but has some evidence to support the theory. Though the hair is not "alive," it is connected to the follicles & your nerves. In a nutshell, it may help to increase "sensory awareness" & "data gathering" of your environment, which would favor longer hair. This would be an asset in survivability (Kardong 2002; 2010; Sabah 1974;

  • Though not a collegiate Journal article, if reasonably credible, this small article is an interesting case for supporting hair & "Touch sense" in "recent history" & in combat-survival :

I personally lean 90% "Evolutionary benefits" & 10% "Mate selection" as to reasons why humans' hair continues to grow longer than other animals. Beard & head.

Hope this info was a little helpful. There are copious amounts of other respectable articles on each point, but I only referenced a few.

The hair doesn't keep growing!

It's important to notice that there isn't a set point for the actual hair length. The hair doesn't know that it's been cut for example. Rather, there's a set point for time. A single strand of hair, in humans, on the scalp, will be in the growth phase for several years. The growth cessation takes a few weeks, and then the rest phase will last for another couple of months, until finally the hair strand is shed.

I don't think there's a natural advantage for hair of as long a length as many humans can grow it, it may just be little or not at all disadvantageous. The selective advantage, if there is one, probably emanates from sexual selection, similar to why lions grow manes. A display of health and surplus of resources. Hair has a pretty central role in beauty ideals. In some cultures, women even cover up their hair.

There are examples of traits that are disadvantageous to survival in the organism's environment, but shows advantages when it comes to reproduction, to a degree that the genes do not become extinct. I don't remember the name, so take it for what you will, but there's a fish species where the males are either small and stealthy, or they are very flashy. The females prefer the flashy males, but at the same time those males have a harder time avoiding getting eaten by other, larger fish. Thus both phenotypes are passed on to the next generation of fish.

A Brief History of Human Hair

Human beings evolved from primates (monkeys), which are mammals, just like us. It&rsquos important to remember that all mammals are covered in body hair (mostly in the form of fur). Humans, however, aren&rsquot covered in fur for a number of reasons, namely that it makes it harder to cool the body. Our ancient ancestors, traveling long distances (perhaps while persistence hunting) in the hot summer sun, would have needed a way to cool their bodies as they moved. Sweating is the most effective form, which happens in the skin, which fur blocks. Therefore, humans evolved to fill their mobile needs by losing the fur over the course of generations.

That being said, primates and other mammals do experience thinning of the hair or hair loss, and in some primate species, they experience something akin to balding.

Why does the hair on your arms stay short, while the hair on your head can grow very long?

Each hair on your body grows from its own individual hair follicle. Inside the follicle, new hair cells form at the root of the hair shaft. As the cells form, they push older cells out of the follicle. As they are pushed out, the cells die and become the hair we see.

A follicle will produce new cells for a certain period of time depending on where it is located on your body. This period is called the growth phase. Then it will stop for a period of time (the rest phase), and then restart the growth phase again.

When the hair follicle enters the rest phase, the hair shaft breaks, so the existing hair falls out and a new hair takes its place. Therefore, the length of time that the hair is able to spend growing during the growth phase controls the maximum length of the hair.

The cells that make the hairs on your arms are programmed to stop growing every couple of months, so the hair on your arms stays short. The hair follicles on your head, on the other hand, are programmed to let hair grow for years at a time, so the hair can grow very long.

Animals that shed have hair follicles that synchronize their rest phase so that all of the follicles enter the rest phase at once. This way, all of the hair falls out at one time. A dog that sheds will lose its hair in large clumps. Many animals can also switch the coloring agent in the hair follicle on and off -- so in the summer, the hair is pigmented ­brown with melanin (see How Sunburns and Sun Tans Work to learn about melanin), but in the winter it is not pigmented, leaving the hair white.

To learn more about hair and related topics, try the links on the next page.

Ask evolution: Why do men have beards?

We mammals are all very hairy, there’s no denying that. But compared to our close primate cousins, chimpanzees and gorillas, humans are practically bald. What noticeable hair we do have is mainly reserved for our scalp and, once we go through puberty, our groins and our armpits.

Our minimally hirsute nature is puzzling, but there’s a pricklier question hiding just below our eyes: why do men, but not women, typically have so much hair on their faces?

The hormonal picture

Everyone has facial hair, male or female. But the facial follicles of (biological) men usually produce thicker, darker hairs that can sometimes end up as substantial beards if not trimmed back regularly.

Turns out this happens in men because facial hair follicles respond aggressively to testosterone, the sex hormone abundant in men.

Because of this hormonal difference, the beard, or facial hair more broadly, is a classic secondary sexual characteristic — a trait that differs between the sexes, but does not directly contribute to reproduction like, for example, the genitals do.

There are many examples of such characteristics in humans: on average, men have beards and thicker body hair, while women have breasts and wider hips.

Sexy beards

Beards are not even unique to humans. “Facial hair exaggeration is not uncommon in mammals — lions famously have manes — and a lot of the primates have facial hair embellishments, like a greying of the facial hair,” says Professor Rob Brooks, an evolutionary biologist at the University of New South Wales, who studies evolution and sex.

How The Human Face Might Look In 100,000 Years

We've come along way looks-wise from our homo sapien ancestors. Between 800,000 and 200,000 years ago, for instance, rapid changes in Earth climate coincided with a tripling in the size of the human brain and skull, leading to a flattening of the face. But how might the physiological features of human beings change in the future, especially as new, wearable technology like Google Glass change the way we use our bodies and faces? Artist and researcher Nickolay Lamm has partnered with a computational geneticist to research and illustrate what we might look like 20,000 years in the future, as well as 60,000 years and 100,000 years out. His full, eye-popping illustrations are at the bottom of this post.

Lamm says this is "one possible timeline," where, thanks to zygotic genome engineering technology, our future selves would have the ability to control human biology and human evolution in much the same way we control electrons to power our world today. (For a critique of the scientific proposals behind Lamm's hypothesis, see this post by Forbes reporter Matthew Herper.)

Lamm speaks of "wresting control" of the human form from natural evolution and bending human biology to suit our needs. The illustrations were inspired by conversations with Dr. Alan Kwan, who holds a PhD in computational genomics from Washington University, UPDATE: Lamm's project was also sponsored by the website, where he has a blog.

Kwan based his predictions on what living environments might look like in the future, climate and technological advancements. One of the big changes will be a larger forehead, Kwan predicts - a feature that has already expanding since the 14th and 16th centuries. Scientists writing in the British Dental Journal have suggested that skull-measurement comparisons from that time show modern-day people have less prominent facial features but higher foreheads, and Kwan expects the human head to trend larger to accommodate a larger brain.

Kwan stresses that 60,000 years from now, our ability to control the human genome will also make the effect of evolution on our facial features moot. As genetic engineering becomes the norm, "the fate of the human face will be increasingly determined by human tastes," he says in a research document. Eyes will meanwhile get larger, as attempts to colonize Earth's solar system and beyond see people living in the dimmer environments of colonies further away from the Sun than Earth. Similarly, skin will become more pigmented to lessen the damage from harmful UV radiation outside of the Earth's protective ozone. Kwan expects people to have thicker eyelids and a more pronounced superciliary arch (the smooth, frontal bone of the skull under the brow), to deal with the disruptive effects already observed by present-day astronauts of cosmic rays outside Earth's atmosphere.

The remaining 40,000 years, or 100,000 years from now, Kwan believes the human face will reflect "total mastery over human morphological genetics. This human face will be heavily biased towards features that humans find fundamentally appealing: strong, regal lines, straight nose, intense eyes, and placement of facial features that adhere to the golden ratio and left/right perfect symmetry," he says.

Eyes will seem "unnervingly large" -- as least from our viewpoint today -- and may feature eye-shine enhance low-light vision and even a sideways blink from re-constituted plica semilunaris to further protect human eyes from the disruptive effect of cosmic rays.

There will be other functional necessities: larger nostrils for easier breathing in off-planet environments, denser hair to contain heat loss from a larger head -- features which people may have to weigh up against their tastes for what's genetically trendy at the time. Instead of just debating what to name a child as new parents do today, they might also have to decide if they want their children to carry the most natural expression of a couple's DNA, such as their eye-color, teeth and other features they can genetically alter.

Excessive Borg-like technological implants would start to become untrendy, though, as people start to increasingly value that which makes us look naturally human. That "will be ever more important to us in an age where we have the ability to determine any feature," Kwan says.

Wearable technology will still be around, but in far more subtle forms. Instead of Google Glass and iWatch, people will seek discrete implants that preserve the natural human look - think communication lenses (a technologically souped up version of today's contacts) and miniature bone-conduction devices implanted above the ear. These might have imbedded nano-chips that communicate to another separate device to chat with others or for entertainment.

The bird's eye view of human beings in 100,000 years will be people who want to be wirelessly plugged in, Kwan says, but with minimal disruption to what may then be perceived as the "perfect" human face.

Why does the hair on a dog or cat only grow to a certain length, and why doesn't this happen for people

Actually, it does happen for people. Your hair will not naturally grow beyond a certain length. For the hair on your head, that just happens to be much longer than most dog or cat fur. But if you pay attention to your arms or back or whatever, you'll notice that you have a lot of hair that never seems to get very long, even though you don't ever trim it.

The reason for all this is that hair goes through constant cycles of growing, then falling out, then growing back in again. The length of the cycle is determined by genetics and hormones and limits how long the hair can get before it falls out. You don't notice the cycle because the timing is slightly different for each hair, which means they don't all fall out at once. The cycle length is also different on different parts of your body, which is why your arm or facial hair can't grow as long as your top-of-the-head hair.

So why does our hair turn gray or white?

Dr. Desmond Tobin, professor of cell biology from the University of Bradford in England, suggests that the hair follicle has a “melanogentic clock” which slows down or stops melanocyte activity, thus decreasing the pigment our hair receives. This occurs just before the hair is preparing to fall out or shed, so the roots always look pale.

Moreover, Dr. Tobin suggests that hair turns gray because of age and genetics, in that genes regulate the exhaustion of the pigmentary potential of each individual hair follicle. This occurs at different rates in different hair follicles. For some people it occurs rapidly, while in others it occurs slowly over several decades.

In a February 2005 Science article (Nishimura, et al.) Harvard scientists proposed that a failure of melanocyte stem cells (MSC) to maintain the production of melanocytes could cause the graying of hair. This failure of MSC maintenance may result in the breakdown of signals that produce hair color.

Mary T. Goldman’s Gray Hair Color Restorer, 1924. National Library of Medicine Digital Collections.

There are other factors that can change the pigmentation of hair, making it lighter or darker. Scientists have divided them by intrinsic (internal) and extrinsic (external) factors:

Intrinsic factors:

Extrinsic factors:

In 2009, scientists in Europe described how hair follicles produce small amounts of hydrogen peroxide. This chemical builds on the hair shafts, which can lead to a gradual loss of hair color. (Wood, J.M et al. Senile hair graying: H2O2 mediated oxidative stress affect human hair color by blunting methionine sulfoxide repair. FASEB Journal, v. 23, July 2009: 2065-2075).

Hair-raising facts:

  • An average scalp has 100,000-150,000 hairs.
  • Hair is so strong that each hair can withstand the strain of 100 grams (3.5 ounces). An average head of hair could hold 10-15 tons if only the scalp was strong enough!
  • Human hair grows autonomously, that is each hair is on its own individual cycle. If all our hair were on the same cycle, we would molt!
  • Hair has the highest rate of mitosis (cell division). An average hair grows 0.3 mm a day and 1 cm per month.

Published: 11/19/2019. Author: Science Reference Section, Library of Congress

Hair whorl and male sexuality

Klar (2004) surreptitiously recorded the direction of hair whorl at a beach near Rehoboth Beach, Delaware that is popular among gay men. Out of 272 men with single whorls, 29.8 percent had counterclockwise whorls. This was a higher proportion than the 9.1 percent counterclockwise he counted in 328 men from malls, stores, and the beach at Atlantic City, most of whom would be straight. This result got a lot of attention in the popular press (France 2007) and is mentioned on a lot of web pages about "gaydar." However, two more rigorous studies did not find a significant difference between gay and straight men in the proportion of counterclockwise whorls. Rahman et al. (2009) and Schwartz et al. (2010) separated gay and straight men based on a questionnaire, not which beach they went to, and determined their whorl type based on close examination, not from a distance. Rahman et al. (2009) found 18 percent of gay and 14 percent of straight men to have counterclockwise whorls, while Schwartz et al. (2010) found 19.7 percent in gay men and 17.2 percent in straight men. In both studies, the observer determined whether the whorl was clockwise or counterclockwise without knowing whether the subject was gay or straight, which may be an important difference between these studies and that of Klar (2004).

How long it takes your body to regrow 19 types of cells and organs, from your skin to your skeleton

Human bodies change and regenerate throughout our lives.

That process is easy to see if you watch babies' limbs grow and their bodies get bigger. It's also obvious when our toenails grow or healthy skin emerges after a burn peels away.

But less obvious systems of regrowth and rebirth in the body continue through adulthood. Dead skin cells constantly rise to the surface of our body, get sloughed off, then are replaced by new stem cells.

Some areas of the body take a long time to refresh themselves — for example, our fat-storage cells shift roughly once per decade, while we get fresh liver cells about once every 300 days.

Of course, your body doesn't simply throw away an entire liver's worth of cells on day 300 and create a brand new set on 301. Instead, it's more of an organic cycle, since liver cells continue to divide and regenerate long after they're mature.

Not every body part regenerates or changes, though. While the body's hairs are in a near constant state of growth, parts of the human brain and head pretty much finish developing at birth (like the lens of the eye that's helping you read this).

Eventually, the tips of our DNA begin to fray as years of wear and tear take their toll on the body — part of the natural aging process.

Here are just a few of the myriad ways that your body regenerates, regrows, and starts anew all the time.

Not all members of the animal kingdom have the same processes of regeneration, of course. Some get wild with their techniques: freaked out geckos can drop their tails and grow new ones, spiders will grow replacement legs after one falls off or breaks, and deer shed their antlers and grow a new rack each year.

Meghan Bartels and Florence Fu contributed to an earlier version of this story.


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