evolution of human hair

Human hair does indeed grow in cycles. I have measured human hair growth rates on the head, legs and face and growing hair tends to grow at about 0.4 mm/day on all these sites. The growing phase of hair is anagen and head hair tends to stay in anagen 3-5 years. I don’t have the paper here but I recall work for L’Oreal measuring the length of time hair stayed in anagen in several people over several years and the longest number I recall was 5.5 years. In histology it is easy to tell an anagen hair from a resting (telogen) hair. Anagen hairs are rooted well below the skin but telegen hair roots move well up into the middle of the skin. Hairs that are shed during combing are virtually all telogen hairs and mostly from follicles that have new anagen hairs starting to grow. Most people shed about 35 hairs per day. The 100 hairs per day number that you see quoted sometimes is derived from a back of the envelope calculation and is not correct. The real measured number is about 35.The density of hair follicles on the human body is about the same as chimpanzees and maybe a little more than gorillas IIRC. Every sebaceous (oil gland) on the body has a hair in it somewhere though they may be very fine and hard to find They can grow as the Ramos Gomez brothers show.
ForbiddenHuman hair is elliptical in cross section and even Asian hair has an ellipticity ratio (major axis/minor axis) of about 1.2. Caucasian hair tends to run from 1.3-1.4 and African hair may have a ratio as high as 1.8 and is not purely elliptical. It usually has a variety of shapes.The factors that control the cycling of hair are not completely understood. The wnt signaling pathway, long known through work on fruit flies is involved. http://www.chembio.uoguelph.ca/educmat/chm736/devsig.htm
http://www.chembio.uoguelph.ca/educmat/chm736/devsig.htm
Researchers Identify Signals that Cause Hair Follicles to Sprout | HHMI and wnt signaling clearly shows up during anagen onset
Reddy et. al. Mech. Devel. (2001) 107 69-82.There is a good article on hair growth in the June 2001 issue of Scientific American (pages 71-79)
The hormone that most effects hair growth is dihydro-testosterone (DTH), produced from testosterone by the enzyme 5 alpha reductase. This enzyme plays a key role in the development of sexual dimorphism during fetal development. At puberty DHT triggers fine vellus hairs in the underarms and pubic regions of both sexes and the face of males to become thicker, longer terminal hairs. Later in life the same hormone can trigger terminal hairs on the head to become fine vellus hairs leading to the development of baldness. In males baldness is patterned but in females hair loss is usually diffuse and may not start until after menopause. I have seen women with classic male pattern baldness but it is rare. The animal model for pattern baldness is the stump tailed macaque. Both males and females develop pattern baldness that responds to treatment similarly to humans.
Page not found - Regrowth. Your Source Of Hair NewsFinasteride is a 5 alpha reductase inhibitor originally developed for the treatment of benign prostate hyperplasia. It is used in a lower dose to treat baldness (Propecia).As to why humans evolved relative hairless, there are competing theories. I suspect it has to do with maintaining a proper thermal balance with a bipedal posture and that the development of eccrine sweating to help cool the body also played a role. Eccrine sweating will not be as efficient for cooling the body in a hairy animal. I don’t have any of the papers on that subject here today. I will look for some references later.Added in Edit: One of the few known differences in gene expression between humans and chimps is in hair keratins. The gene for Phi-hHaA is a pseudo gene in humans but functional in chimps and gorillas. SeeLangbein et al. Human type I hair keratin pseudogene phihHaA has functional orthologs in the chimpanzee and gorilla: evidence for recent inactivation of the human gene after the Pan-Homo divergence, Hum Genet. 2001 Jan;108(1):37-42.Randy[This message has been edited by Randy, 02-01-2004][This message has been edited by Randy, 02-01-2004]

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Thank you very much for a lot of interesting info. However, I still have three issues, which I hope you can address in one way or another.
1) While I have seen statements that head hair growth is around what you mentioned, my own experience has defied these studies. Doing the math, I come up with an average maximum hair length of ~30 inches. That is not even 3 feet. It's pretty easy to find pictures of people with hair WELL beyond that length, and I know my girlfriends have had this length.... I should add that it took longer than 5 years to grow that length and so it wasn't that their hair grew faster either.

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There is always a distribution with biological responses. If you measured the height of 50 Chinese selected at random you probably wouldn’t find any that are 7 feet tall and probably not many over 6 feet tall, but then there is Yao Ming.

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I don't want to seem like my anecdotal evidence trumps a study, but when it's pretty obvious hair can grow longer than 3 feet in length, and has grown longer than 5 years, I start having questions about the accuracy of the study. If you have a link to it I would like to see it. And since you seem to have quite a bit of experience with hair, I would ask haven't you seen anyone with hair still growing past 5 years and longer than 30"?

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According to the Guinness Book of World records there is a guy with hair 16 feet long but that is obviously an anomoly.There are a few people who have been able to grow their hair beyond 30 inches. The longest I have ever seen personally was about three feet long on a woman 5 feet tall and she could sit on her braid. I have seen pictures of hair about 4 feet long. Hair 36 inches long will hang nearly to the knees of a women who is 5’3 , so you may be overestimating actual lengths that you have seen.

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2) While your explanation of noneccrine sweating makes some amount of sense (after all hair seems to grow where we sweat a lot so maybe that is a benefit), it does not exlain the real problem I am trying to figure out. I wasn't so concerned why hair was differentially distributed, I am wondering why hair has differential maximum growth lengths across the human body.

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Actually I was talking about eccrine sweating. Non eccrine sweat is apocrine sweating which occurs in association with hair follicles. Horses sweat by apocrine sweating. Eccrine glands are distributed over the whole body including the palms and soles where there are no hair follicles. Humans are not the only animals with eccrine sweat glands but we do seem to be the only ones who use them for major temperature regulation.

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Specifically... even if noneccrine sweating is superior... why would max hair length on the head be so much different than say pubic regions or armpits. It seems that that would become a detriment to the survival of humans before they had the ability to control their hair. Do you have any info suggesting an explanation for this?

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Hair on the head may be there to help protect the head from the sun. It may also have played some role in maintaining body temperature during cool nights since heat loss from the head is higher than from any other area of the body. It is also possible that sexual selection played a role in maintaining head hair for some reason. I am not suggesting that the evolution of our relative hairlessness is well understood. It is still a controversial area of research as I said. I will look up some references on this subject when I have time.

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3) And finally, I was disturbed to see in one of your references that Sonic Hedgehog is now a term in genetics? I can only assume it was named from the video game. Is this true?

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The signaling factors in fruit flies are named for their effects on the fly when perturbed. One of these is known as hedgehog. When the vertebrate factor that is homologous to hedgehog was discovered it was named sonic hedgehog(Shh) apparently after the video game.Medicine & Health - UNSW SydneyRandy

I am going to combine some replies here.With respect to the Celts, Goths and Picts and the length of their hair and beards, where do you think the term barbarian came from?Regarding the L’Oreal studies on the time hair spends in growth and resting cycles the papers are published in the British Journal of Dermatology.Courtois M, Loussouarn G, Hourseau C, Grollier JF. Ageing and hair cycles. Br J Dermatol 1995 Jan;132(1):86-93Courtois M, Loussouarn G, Hourseau S, Grollier JF. Periodicity in the growth and shedding of hair. Br J Dermatol 1996 Jan;134(1):47-54The studies were done on men and not women, probably because of the economic importance of male pattern baldness. They made measurements for 14 years. In the study I have a copy of there were only 10 men in the group. In one of the subjects 28% of the hair stayed in the growing phase (anagen) for up to 5.2 years. By that time the rest of his hair had cycled or was cycling and the time in anagen was less for the all the other subjects. There seem to be a few people who can grow their hair 4 to 5 feet long but most people can’t under any circumstances.Hair length is also discussed here.
http://www.keratin.com/aa/aa010.shtml

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Similarly guys with higher testosterone grow facial hair faster

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Perhaps. Apparently beard hairs do have androgen receptors that stimulate growth that scalp hairs lack. (See Thronton et al.(1998) J. Investigative Dermatology 111:727-732.)
However, the same author has published data showing that a lack of 5 alpha reductase activity is correlated with poor beard growth. (Thornton et al. Clin Endocrinol (Oxf). 1993 Dec;39(6):633-9.
Several years ago I measured beard growth rates in about two-dozen different men (all Caucasian) and never saw significant differences. I seem to remember some reports about testerone productions and 5 O'clock shadow many years ago but I couldn't come up with the science backing up that specific claim.

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As far as I know straight vs. curly (and in between) is the amount of disulfide bonds in the protein.

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Not necessarily. I am not sure there are differences in disulphide bond content between straight and curly hair, though highly curly African American hair is weaker than Caucasian or Asian hair. Hair has two main types of cells, cortical cells that form the central shaft and cuticle cells that protect the outside. Wool that is curly has two different types of cortical cells, ortho and para. This was not thought to occur in human hair. However, J. Alan Swift, who is the guru of hair microscopy, claims that there are two types of cortical cells in curly human hair. By his definitions, which are different than those for wool, the keratin is more tightly packed in paracortical cells. Straight hair has only paracortex and curly hair has both ortho and para.
(Swift JA: Morphology and histochemistry of human hair, in Jolles P, Zahn H, Hocker H (eds), Formation and Structure of Human Hair, Boston, Birkhauser Verlag, 1997, 149-176.)
I recently spoke at a meeting on ethnic hair and skin in Chicago and Bernard from L’Oreal showed data from in vitro hair growth studies indicating that the shape of the hair is programmed into the follicle, which is not necessarily incompatible with Swift’s claims.Regarding the evolution of hairlessness, here is a reference to a paper suggesting that the eccrine sweat gland system in humans evolved along with hairlessness.
Folk GE Jr, Semken HA Jr., The evolution of sweat glands. Int J Biometeorol. 1991 Nov;35(3):180-6.Peter Wheeler has published calculations suggesting that hairlessness evolved both for thermoregulatory reasons and to reduce requirements for drinking water requirements in bipedal hominids in equatorial savannah.Wheeler, P.E. The influence of the loss of functional body hair on the energy and water budgets of early hominids. ( 1992) J. Human Evolution, 23, 379-388Wheeler, P.E. The thermoregulatory advantages of heat storage and shade-seeking behaviour to hominids foraging it equatorial savannah environments. J. Human Evolution(1994) 26, 339-350.Amaral has challenged this interpretation suggesting that hairlessness evolved in a forested environment, perhaps before bipedality.Amaral, L. Q. Loss of body hair, bipedality and thermoregulation. Comments on recent papers in the Journal of Human Evolution, J. Human Evolution (1996) 30, 357-366.Wheeler has of course challenged this reinterpretation.
Wheeler, P.E. The environmental context of functional body hair loss in hominids (a reply to Amaral, 1996) J. Human Evolution 30, 367-371I'd say this is still a pretty open question and the answer may never be known for sure. Right now I find Wheeler's ideas most likely but still pretty speculative.Randy