Why do humans have continuously growing hair?

Jun 2015
5,730
UK
#1
Men's beards and hair on our heads grows without limit.

But why? what's the evolutionary reason for this?

It cannot be to protect from the cold. We evolved in Africa, which has always been hotter than other continents, and 150,000 years ago and Africans today like all people have continuously growing hair and beards. This hasn't changed obviously as humans left Africa for other continents.

So then, how has it come about? Was it sexual selection?

Or did we need long hair to protect from elements, such as sand or wind, and as long hair could be cut or lost, it had to grow back continuously?
 
Nov 2016
1,531
Indus Valley, Pakistan
#2
Or did we need long hair to protect from elements, such as sand or wind, and as long hair could be cut or lost, it had to grow back continuously?
Yes, I think it has something to do with protection against elements. Thick hair is like having a layer of protection around your body against climate and even physical injury. It was the bodies natural 'clothing'. Our nails similary give protection to the extremities of fingers and toes by creating hardpoints.

And beyond the utility the hair might also provide sexual function. Rich growth might suggest a health. That is why old men dye hair or have transplant surgery in desperate attempt to revisit their youth and sexual appeal.

Although I have no idea where shaving your head bald fit's in.


 

Zeno

Ad Honoris
Jan 2010
13,691
♪♬ ♫♪♩
#3
Men's beards and hair on our heads grows without limit.

But why? what's the evolutionary reason for this?

It cannot be to protect from the cold. We evolved in Africa, which has always been hotter than other continents, and 150,000 years ago and Africans today like all people have continuously growing hair and beards. This hasn't changed obviously as humans left Africa for other continents.

So then, how has it come about? Was it sexual selection?

Or did we need long hair to protect from elements, such as sand or wind, and as long hair could be cut or lost, it had to grow back continuously?
Hair doesn't grow limitless. Extreme exceptions notwithstanding, the hair on our heads grows 2-7 years and then falls out. Eyebrows and body hair usually grows around 30-45 days. Hair is however continuously replaced. (Needless to say this is untrue for head hair in bald people.)

My favorite explanatory theory about the function of hair involves protection from UV from sunlight, thermoregulation and sexual selection.
Most likely early man was a persistence hunter who followed prey over long distances. Humans have more endurance than most other mammals, save wolves (and dogs) which may one of the reasons why we domesticated them. Critical in this is thermoregulation. We have more sweat glands to cool us down than most other mammals , so we don't need to pant or seek out the shade and thus slow down or halt movement. The thick scalp full of hair on our heads serves to protect from the blistering sun which shines from a perpendicular angle on the planet at equatorial latitudes. Moreover a thick scalp full of hair , full of sweat would have a cooling effect on the brain.

Last but not least a thick beard and scalp full of hair are indications of good health and thus sexually attractive.
 
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robto

Ad Honorem
Jun 2014
6,142
Lisbon, Portugal
#4
Yes, I think it has something to do with protection against elements. Thick hair is like having a layer of protection around your body against climate and even physical injury. It was the bodies natural 'clothing'. Our nails similary give protection to the extremities of fingers and toes by creating hardpoints.

And beyond the utility the hair might also provide sexual function. Rich growth might suggest a health. That is why old men dye hair or have transplant surgery in desperate attempt to revisit their youth and sexual appeal.
Than what about people still in their youth but already have grey hair or a balding scalp?
 
Jun 2015
5,730
UK
#5
but then whilst it can protect from the elements, why does it need to grow continuously? what use would homo erectus or homo heidelbergnesis had for 3 foot long beards or head hair?
 

Zeno

Ad Honoris
Jan 2010
13,691
♪♬ ♫♪♩
#6
but then whilst it can protect from the elements, why does it need to grow continuously? what use would homo erectus or homo heidelbergnesis had for 3 foot long beards or head hair?
It's very possible that's just what early humans found attractive. Long hair and thick beards. Not the most satisfying answer, perhaps, but we simply are quite a sexual species.
 
Jun 2016
1,851
England, 200 yards from Wales
#7
The fact that all humans have such hair, but no apes do suggests it evolved between the split between human and ape lineages and the dispersal of modern humans out of Africa.
(Maybe before the spread of earlier humans too, what sort of hair I wonder did Neanderthals or Homo Erectus have?).

A pretty long period of course, but in that time humans moved out onto the open Savanna more, while our ape cousins stayed in the shade - so maybe there is an element of the protection from direct sun involved as Zeno suggested. That same move would be connected with two other differences between apes and humans, the loss of body hair (supposedly to help cooling by sweating) and, as a result of that, the evolution of dark skin to protect the now-exposed skin from UV?

Any of these changes could then be reinforced by sexual selection perhaps.
 
Jun 2016
1,851
England, 200 yards from Wales
#9
You have the same number of hair follicles as any of the great apes.

The not-so-naked ape | The Economist
Naturally, the change was that those follicles grow hair so fine it is hardly visible (in most places), so it doesn't inhibit sweating but also doesn't give much protection from sunlight.

Perhaps the fact that the remaining fine hairs are still useful for the bug-detection suggested in that article is one reason why we haven't lost them entirely?
 
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Lowell2

Ad Honorem
Jun 2014
6,541
California
#10
The primary function appears to be to aid in sweating. Human hair isn't as thick as chmps or gorillas for the same reason that elephants aren't woolly mammoths -- thick hair is an insulator which is not desirable in a warm environment. It is desirable to employ sweat glands in an open vice a forested environment. http://www.journals.uchicago.edu/doi/abs/10.1086/381006 (Genetic Variation at the MC1R Locus and the Time since Loss of Human Body Hair AlanR.Rogers, DavidIltis, and StephenWooding
Department of Anthropology, 270S 1400E RM 102, University of Utah, Salt Lake City, UT 84112 (rogers@anthro.utah.edu) (Rogers and Iltis)/Department of Human Genetics, University of Utah School of Medicine, Salt Lake City, UT 84112 (Wooding), U.S.A. 31 vii 03) also see Function of Body Hair - Why do humans have body hair? | HowStuffWorks
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3250022/
The reduction in density and size of hairs in humans helps in thermoregulation by evaporative loss of heat through sweating. Non-hairy skin will not pose a barrier to evaporation while plenty of hairs would reduce evaporative heat loss.
In human beings, specialized hairs such as eye lashes and hairs inside the nostrils and external ears afford some protection from the environment. Eyebrows prevent sweat from getting into the eyes. Scalp hairs may assist in stabilizing the temperature of the brain. Hairs can also excrete toxic substances like arsenic, and are thus of use in forensic medicines

. The hair follicle plays a role in epidermal homeostasis, wound healing and skin tumorigenesis. Genetic studies have revealed numerous genes involved in follicle formation, growth and cycling. Repigmentation in vitiligo patches often starts around hair follicles.
and as noted in other posts -- it helps in detecting bugs: Human fine body hair enhances ectoparasite detection | Biology Letters
Here, we experimentally test the hypothesis that human fine body hair plays a defensive function against ectoparasites (bed bugs). Our results show that fine body hair enhances the detection of ectoparasites through the combined effects of (i) increasing the parasite's search time and (ii) enhancing its detection.
--some other articles indicate it also helps in detecting ants, spiders, etc on one's body.

As for why there is variation (including baldness) -- probably because relatively, it's a fairly new adaptive gene and sometimes it does too little and sometimes too much -- and also some adaption for varying climates and finally, what the various groups of people considered "pretty" or "handsome" at any given point in time.