Wikipedia and the scientific community
Why have human beings lost most of their body fur, but retained hair in only a few specific areas? In particular, why have we on the most part, retained a full head of hair?
Until recently, it was commonly assumed that head hair protected our brains from the sun. But as Elaine Morgan pointed out, this doesn't explain why some babies, middle-aged men or elderly people often have little hair or no hair at all. Another theory proposes sexual selection: that women have long hair in order to attract men, but that still leaves many questions unanswered, for instance, why can men and young children grow long hair too?
It is worth noting that female head hairs thicken and strengthen during pregnancy, and a woman's hair tends to stay this way during the whole period she is lactating (naturally about 2 to 4 years).
And we know that if hair is never cut, it can keep on growing indefinitely, perhaps down to the floor. Another thing to note is that hair floats, and if it remains unbraided, it spreads out, like a current around the mother.
Baby apes and monkeys that live in the forests can cling to their mother's fur as she carries on finding food, but what happened when our ancestors lost their fur? How could baby cling onto mum, especially when she was in the water, or wet and dripping, with bare naked skin?
Long, strong hair provides a logical solution to the question. Mothers have the primary need to grow long hair but it's not exclusive to them, which is maybe why some men (especially young men) can grow long hair. Grandmothers tend to still have long hair, though thinner than in their youth, though most older men do not. This leads to the question: who benefits from having long hair? The answer appears to be children and women of any age, but especially mothers.
The practicality of having long hair in the water works both ways. Children's hair starts to grow at once, and quickly, and by the time they are a few years old, their hair is long enough to grab - especially useful with boisterous toddlers splashing around in the shallows who may get into difficulties. It's much easier for any adults nearby to grab the youngsters’ hair if they find themselves in deep water or fast currents, slipping out of reach.
Children, while swimming, will reach out and grab the nearest safe object or person when they get tired, usually, the nearest familiar adult. They will reach for the nearest part of a person. As long hair tends to float, weed-like, around neck level when treading water, it’s easier for them to grab on to.
Further discussion (from the AAT group)
Early hominoids (c 25-20 Ma?) evolved some sort of long head hairs in both males & females (scalp + beard) for making the face more impressive, as seen in other monogamous primates (e.g. lion marmosets). Peri-Tethys hominid-pongid ancestors in coastal forests lost body hair (c 18-15 Ma?): our body hair follicles + the accompanying sebaceous gland became smaller (terminal-->vellus follicle). Chimps & gorillas are still born naked, they redevelop upperfur, but still have no underfur.
When Homo began diving more & more (after c 2.6 Ma? begin-Pleistocene? ice ages?), our head hairs got "littoral" (sea-diving) adaptations: we evolved a new sort of follicles in the face, between the shoulders etc. (sebaceous follicles or "acne follicles") with +-invisible hair + very large sebaceous gland. The sebum made our head hairs greasy, so that they glued together & "filled" the gaps between head & torso, making the whole body more streamlined, see in the AAT files my reconstructions of archaic Homo.
Human sebum is squalene-rich, as in beavers & otters, unlike most terrestrial mammals (fully marine mammals have +-no skin glands at all).
Axillar, pubic & peri-anal hair underwent comparable changes, filling the gaps between arms & body, and between the groins, thighs & buttocks,
Women have less sebaceous follicles, and during pregnancy get longer& thicker head hairs, which were not glued together IMO, but (as Elaine
Our evolution from divers to waders (early H.sapiens) was late-Pleistocene (>200 ka?), and we began wearing clothes (hides?), so we didn't redevelop body fur. About half of the men are still bald, which suggests the dive-->wade transition was rel.recent.
The hair discussion actually now covers three topics: long versus curly/kinky hair, preservation of facial hair in males and preservation of pubic & axillary hair.
Preservation of pubic hair also may be understood within the context of the AAT. I am not sure there is any advantage for males, just no advantage for males having a significantly different pubic hair pattern than females; the differences that do exist being the result of different levels of androgen hormones, not actually sex. It's virtually impossible to understand the function of a woman's pubic hair in modern society because we just aren't naked enough. Pubic hair is an inverted triangle with hair hanging down from the point between the legs. It does not extend far posteriorly but is actually more of a curtain. The base of the triangle is a line which reaches laterally on each side to the edge of the groin. When a woman emerges from water, what this triangle actually does is channel water to the point of the triangle where it can drip off rather than be conducted posteriorly over the labia. This allows the female perineum to be dryer and thus less vulnerable to a variety of infections which would at the least interfere with copulation. I came onto this theory after studying the function of the fetlock of horses. The fetlock is a small triangle of hair on the back of the horses leg that hangs down below the hoof. When the fetlock is trimmed away (as is done to some showhorses) the hoof stays moister and is subject to yeast infection or thrush.
These are good points. As far as the males getting older and having a change in hair patterns... first of all, male hair in humans doesn't normally change until after the age of 25 or 30, or even later. At which point our ancestors would have been parents for many years, and in many cases grandparents (assuming that females and males were procreating by the age of 15 or even younger).
I've always speculated that the change in head hair in mature males might be related to identification and as a badge, or marker, to signify an older male. The same way a gorilla who is a silver-back is immediately recognized by other gorillas as an older, dominant male in the extended family group.
In aquatic/wading animals, it makes sense that the marker/signal for an old male would be visible in that part of them that is above water -- the scalp. Baldness or a silver head would be immediately recognizable as a badge of maturity and possible dominance of that male. And only the scalp would be reliable visible in a wading hominid.
Lots of interesting thoughts here, and the strange and unique pattern of human hair is something that should be given more attention and thought.
The striking thing to me is that adolescent humans -- pre-puberty -- have no body hair at all, not pubic hair or facial hair. Both sexes have a rich, thick mass of hair on the scalp, but otherwise are utterly hairless.
is there any other animal that presents that way? When we talk about hair patterns, etc. in humans, we really have two different animals to consider. The adult, and the pre-adult. Is there another creature that exhibits such a sudden change in hair/fur that occurs at the onset at sexual maturity?
Isn't that unique to humans?
I could see it as a signal. It would be difficult to survive though, as a bald male outside all day in a warm climate. Major melanoma risk.
I wonder what the global pattern is? Like: beards aren't all that common globally. A few chin hairs, but those bushy beards seem to be a Northern European thing. In the North it makes sense ... it's insulation for the face, and maybe it came out during the ice age. Women didn't go out hunting in the snow much.
OTOH, head hair was rarer in BOTH sexes during the 1700's and 1800's. I don't know about before that, but head fungus was really common so people lost their hair. Hence the popularity of wigs and head scarves (which made the problem worse). Hair was something of health risk, because it held lice and helped promote fungal infections, in a time and place where bathing wasn't so common. In tropical climates sure, they swam and waded daily (and still do where it's possible). But in North Europe during the Ice Age? Probably not so much.
DHA definitely makes hair healthier. When I stopped eating oily fish for awhile my skin gets all dry too. It's amazing how much difference it makes! Swimming daily helps hair too though. I think it's the difference between ducks and chickens. A chicken can go it's whole life without getting wet, and is totally happy with that. But if a duck doesn't get a chance to dunk it's head in water daily ... it gets eye infections. Even if the water they dunk in is goopy water full of duck poop ... it keeps their eyes healthy. If you give any duck a little bowl of water, the first thing they do is dunk their heads in it.
So humans, if they don't wash their hair a fair bit ... the scalp gets unhealthy and the person goes bald. Give a human a bowl of water in the morning and the human splashes the water all over their face. If that isn't a sign of the necessity of water for humans, I don't know what is.
It is interesting that you apply a sociobiological cause to a physical phenomena. It is complex and very chicken and eggish. Let us say that nature made hair grey as we age because the vitality necessary to uphold pigmentation wains with ongoing "rusting" or oxidation of the body. That is just pure physics...in the same way that iron rusts with time. Now nature may use the "sign/symbol/signal" of that grey hair to alert a social group to many things...dominance, aging DNA, respect, parental figure, and even money-in-the-bank etc... I assume that the social adaptation and behavioral codes arose as a consequence of the eventual "rusting" of our physical form.
Humans have terminal (visible & pigmented hair), vellus (nearly invisible)& sebaceous (acne) follicles. IIRC, growth hormone: terminal follicles of scalp hair, brows, eyelashes, low di-hydro-testosterone (adult women): inferior pubic hair, axillar hair, sebaceous follicles, high DHT in skin (adult men): superior pubic hair, beard, mustache, more body hair, familial baldness (vellus follicles).
It is possible that, because of their location, the sebum of these follicles once kept the neck hair, beard and moustache supple, fatty, waterproof and well-fitting to the body, so that the "manes" were not dispersed by the water while swimming. Sebum and long head-hairs together made the male's neck completely streamlined (Figure 1). The convergent evolution of the integument of male Homo and male Eumetopias (as compared to the primitive Primates or Carnivora) is quite remarkable: both developed body nakedness,(this was based on my incorrect reading of a textbook on mammals, but is wrong I think: adult male Steller's sealions, unlike adult male elephant seals, do have body hair AFAIK --mv) thick SC white fat, extensive superficial venous networks in the limbs, abundant sebum secretion, long coarse neck-hairs and a tendency to nakedness at the top of the skull (and perhaps thermoactive sweat glands, see B)(AFAIK, it's only furseals, not sealions, that have thermoactive sweat glands --mv).
Morgan (2,6) supposed that women's head hairs, growing longer than men's and not kept together by abundant sebum, once enabled the newborn baby to accompany its mother in the water by grasping her hair. This fits in with other data. The baby's SC fat grows rapidly from the 34th foetal week until a few months after birth, so the baby is born with a thick layer of white fat (about 16 % of its body weight, see B). ..."
Seborrheic dermatitis (35) is partly hereditary (caused by Pityrosporum yeasts belonging to our normal microflora), and more common in young adult
Also in the axillar and ano-genital areas, where atypical localizations of acne and seborrheic dermatitis can be found, there possibly existed
A reason why hairs are still present in pubes and pits could be simply, that our aquatic adaptation would be intermediate. Meaning, that these would be the very last regions to shed the fur during the course of evolution. Alister Hardy originally argued, that hair on the scalp would have remained on hominins to protect from sun rays, as the head would be the only part of the body generally above water on a swimming and wading hominin. Elaine Morgan argued, that hair on the scalp would provide especially women with grabbing tissue for the infant to cling onto in the water, leaving the mother's arms free to rummage for food in shallow water, while keeping her head above it.
Similar to how simian young generally cling to the fur of the mother. We would've shared that trait because of mutual simian ancestors, where the young likely also clang to their mother's fur before LCA.In that context, occassional male baldness could simply be because men were more prone not to have a toddler at their scalp, maybe even needed to dive more and deeper for foods, and therefore were more often selected for the increased hydrodynamics of a bald scalp.
I have to say, I'm not a big fan of sexual selection as a big mover, but that could also have played in. And again the analogy to elephants, provided that they are indeed old semiaquatics as well (three times longer than us, in such case); their little remaining hair (if any) are small tusks at the very tip of their tail (which may be comparable to human pubes and pits, because it's an extremity) and sometimes prominent tusks on top of their scalp as well, especially the young.
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