On average, your scalp hair grows 0.35 to 0.45 millimeters a day — that’s half an inch per month. Depending on your ancestry (genetics), diet and hormonal state (pregnant women grow hair a bit faster; it’s also thicker and shinier), your hair will grow at a higher or lower rate.
The human body contains roughly 5,000,000 hair follicles, and the function of each hair follicle is to produce a hair shaft. Our early ancestors used to have most of their bodies covered in hair, like our other primate cousins. This served to conserve heat, protect from the sun, provide camouflage and more. Today, however, humans stand out from the 5,000 mammal species because they’re virtually naked, but why is that?
Scientists believe that our lineage has become less and less hairy in the past six million years since we shared a common ancestor with our closest relative, the chimpanzee. Our ape ancestors spent most of their time in cool forests, but a furry, upright hominid walking around in the sun would have overheated. One of the main theories concerning our lack of fur suggests that temperature control played a key role. Bare skin allows body heat to be lost through sweating, which would have been important when early humans started to walk on two legs and began to develop larger brains than their ape-like ancestors. Nina Jablonski, a professor of anthropology at Pennsylvania State University, says there must have been a strong evolutionary pressure to control temperature to preserve the functions of a big brain. “We can now make a very good case that this was the primary reason for our loss of hair well over 1 million years ago,” she said.
“Probably the most tenable hypothesis is that we lost most of our body hair as an adaptation to being better at losing heat from our body, in other words for thermal regulation,” Professor Jablonski said.
“We became very good sweaters as a result. We lost most of our hair and increased the number of eccrine sweat glands on our body and became prodigiously good sweaters,” she told the American Association for the Advancement of Science meeting in Boston.
Besides sweating, losing our furry coat may have also been driven by having fewer parasites infesting our bodies like ticks, lice, biting flies and other “ectoparasites.” These creatures can carry viral, bacterial and protozoan-based diseases such as malaria, sleeping sickness and the like, resulting in serious chronic medical conditions and even death. By virtue of being able to build fires and clothing, humans were able to reduce the number of parasites they were carrying without suffering from the cold at night or in colder climates.
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, while pubes may have been retained for they role in enhancing pheromones or the airborne odors of sexual attraction. The hair on the armpits and groin act like dry lubricants, allowing our arms and legs to move without chafing. Eyelashes, on the other hand, act as the first line of defense against bugs, dust, and other irritating objects. Everything else seems to be superfluous and was discarded. It’s important to note, however, that we haven’t exactly shed our fur. Humans have the same density of hair follicles on our skin as a similarly sized ape. Just look at your hands or feet: they’re covered in hair, but the hair is so thin you can barely make them out.
How hair grows
Hair, on the scalp and elsewhere, grows from tiny pockets in the skin called follicles. Hair starts growing from the bottom of the follicles called the root, which is made up of cell proteins. These proteins are fed by blood vessels that dot the scalp. As more cells are generated, hair starts to grow in length through the skin, passing an oil gland along the way. Emerging from the pit of each of these follicles is the hair shaft itself. By the time it’s long enough to poke out through the skin, the hair is already dead, which is why you can’t feel anything when you get your hair cut.
The hair shaft is made out of a hard protein called keratin. There are three main layers to the hair shaft. The inner layer is called the medulla, the second is the cortex and the outer layer is the cuticle. It is both the cortex and the medulla that holds the hair’s pigment, giving it its color.
Some quick facts about hair:
You’re born with all the hair follicles you’ll ever have – about 5 million of them. Around 100,000 of these are on your scalp.
The hair on your head grows about 6 inches a year. The only thing in the human body that grows faster is bone marrow.
Males grow hair faster than females due to testosterone.
You lose between 50 to 100 strands of hair each day. That’s because follicles grow hair for years at a time but then take a break. Because follicle growth isn’t synced evenly, some take a break (causing the hair to fall out), while the vast majority continue business as usual.
Some follicles stop growing as you age, which is why old people have thinning hair or grow bald.
Everybody’s hair is different. Depending on its texture, your hair may be straight, wavy, curly, or kinky; thick or thin; fine or coarse. These are determined by genetics, which influences follicle shape. For instance, oval-shaped follicles make hair grow curly while round follicles groom straight hair.
Like skin, hair comes in various colors as determined by the same pigment called melanin. The more melanin in your hair, the darker it will be. As you grow older, your hair has less and less melanin, which is why it fades color and may appear gray.
Hair growth cycle
Follicles have three phases: anagen — growth, catagen — no growth, preparing for rest, and telogen — rest, hair falls out. At its own pace, each strand of hair on your scalp transitions through these three phases:
Anagen. During this phase, cells inside the root start dividing like crazy. A new hair is formed that pushes out old hair that stopped growing or that is no longer in the anagen phase. During this phase, the hair grows about 1 cm every 28 days. Scalp hair stays in this active form of growth for two to six years, but the hair on the arms, legs, eyelashes, and eyebrows have a very short active growth phase of about 30 to 45 days. This is why they are so much shorter than scalp hair. Furthermore, different people, thanks mostly to their genetics, have differing lengths of the anagen period for a given body part compared to other people. For the hair on your head, the average length of the anagen phase is about 2-7 years.
Catagen. About 3% of all the hair on your body this very instant is in this phase. It lasts two to three weeks and during this time, growth stops. During this phase, the hair follicle will actually shrink to 1/6 of its original length.
Telogen. About 6 to 8 percent of all your hair is in this phase — the resting phase. Pulling out a hair in this phase will reveal a solid, hard, dry, white material at the root. On a day-to-day basis, one can expect to shed between 100 to 150 pieces of hair. This is a normal result of the hair growth cycle. When you shed hair, it’s actually a sign of a healthy scalp. It’s when the hair loss is excessive that you should feel worried and contact a doctor.
Why hair only grows to a certain length
Each hair grows out of a follicle and as the hair gets longer and heavier, the follicle eventually can’t hold on much longer and it sheds the hair. But that’s okay: it then starts growing another one. How long you can grow your hair depends on your genetics, and in general, Asians can grow their hair longer than Europeans. This may be surprising for many, but as in all mammals, each of us has a certain hair length beyond which the hair simply won’t grow. Hair length is longest in people with round follicles because round follicles seem to grip the hair better. So, people with straight hair have the potential to grow it longer. Shorter hair is associated with flat follicles. A study published in 2007 also explains why Japanese and Chinese people have thick hair: their follicles are 30% larger than that of Africans and 50% larger than that of Europeans.
In most cultures, women keep their hair longer than men. Cultural rules aside, hair length is actually sexual dimorphic. Generally, women are able to grow their hair longer than males. European males can reach a maximum length of wavy hair to about shoulder length, while the maximum for straight hair is about mid-back length. For European females, wavy hair can usually reach the waist, and straight hair can reach the buttocks or longer.
How to grow your hair faster and longer
While genetics caps your hair length, it is possible to accelerate its growth rate.
1. First of all, your hair growth reflects your general body health. Eat a diet rich in marine proteins, vitamin C (red peppers), zinc (oysters), biotin (eggs), niacin (tuna) and iron (oysters) to nourish strands.
2. If changing your diet isn’t possible, you can try supplements with marine extracts, vitamins, and minerals that nourish your follicles.
3. Besides general health, the next thing you should mind is your scalp health. Use a shampoo that gently exfoliates oil and debris from the scalp as well as a conditioner to moisturize scalp and hair.
4. Trimming is a proven method to grow your hair longer. Although in itself trimming doesn’t promote growth, it does help prevent breakage and, therefore, increases hair length.
Things that actually hurt your hair:
1. Silicone shampoos dry out the hair and degrade it. Blow dryers and flat iron produce similar effects, breaking the hair shafts. Use these products as rarely as possible.
2. UV light bleaches and breaks down hair. When you’re out at the beach, wear a hat to protect your scalp.
3. Salt and chlorine water both soften and dry the hair.
4. Bleaching, dyeing, hair extensions and perms also damage hair.
Tibi is a science journalist and co-founder of ZME Science. He writes mainly about emerging tech, physics, climate, and space. In his spare time, Tibi likes to make weird music on his computer and groom felines.