Monday, 5 May 2008

Why are 'we' longhaired?

A reader writes:

It may be that very long hair was selected against in the tropics due to either pests or possibly the greater ill effects of dampness during the rainy season. (In the tropics during the rainy season being too covered up tends to lead to rashes and/or poor healing of blisters and other small wounds.)

This explanation makes sense if the short hair of tropical peoples is the puzzle to be explained. Why are they not like us? How did they become different from us? (Evidently, us means longhaired, melanin-challenged folk).

Well, they didn’t become different from us. We became different from them. We are the evolutionary oddity and we are the ones who need explaining.

In scientific jargon, long head hair is the ‘derived’ characteristic. All ancestral humans started off being shorthaired. Then, somehow, for some reason, and in some populations, their head hair began to grow to remarkable lengths. Why? Isn’t that the evolutionary conundrum?

8 comments:

Average Joe said...

Of course we are the ones who became different. Tropical peoples are short-haired like monkeys and apes.

n/a said...

(1) "Tropical peoples"--even those with frizzy hair--do not share identical hair forms. Khoisan hair differs from Negroid hair, and the hair of Melanesians differs from both. It's hardly obvious any or all of these forms is THE "ancestral" form.

(2) The Great Apes have straight hair. So, yes, frizzy hair is a "puzzle to be explained", as much as any other characteristic in any human group.

(3) Straightness and length are not independent. A round cross-section gives straight hair. An oval cross-section produces kinky, brittle hair.

(4) I believe Coon argued the cross-section is determined by the angle of the hair follicle with respect to the skin, and that angle in turn is related to skin thickness.

I believe others have claimed some groups have dark, thick skin partly as an adaptation against tropical parasites. I don't if they're right or whether their theory has any bearing on differences in hair form. But I suspect lines of thought such as that one will turn out to be more fruitful than one which tries to force fit all data into a preconceived framework where sexual selection is the explanation of choice.

(5) Then there are all the other proposals related to climatic adaptation, which you have summarily dismissed.

It could go either way or both ways--short, frizzy hair is an adaptation to tropical conditions and selection is relaxed elsewhere. Or long hair is an adaptation to cooler climates (Coon pointed out long hair can serve to protect the neck). Or both.

You dismiss this theory with:

"But why would a colder climate cause head hair—and only head hair—to lengthen to such an extent and over such a short span of evolutionary time"

Number one, you're incorrect, since beards are more highly developed in Caucasoids. Body hair as well. But if you mean why didn't body hair return in pre-human quantities, that sort of begs the question of why humans became depilated to begin with. Assuming similar forces were still in place (e.g. humans can wear clothes), your objection is hollow.

(Why do Mongoloids have weakly developed beards? Coon believed Mongoloids were adapted to extreme colds, and, for example, moisture in the breath freezing on beards could lead to selection against facial hair. Europeans are adapted to more moderate conditions.)

n/a said...

The ever thoughtful and objective Peter Frost:

I don't see how long head hair can help to retain body heat. Body heat is best retained by stable pockets of 'dead air'.

Carleton Coon:

Mongoloid hair is the thickest and straightest, as well as the stiffest. Its cuticle has the smoothest surface, its medulla the largest dead air pockets. Of all human hair, it is the one that most closely resembles the hair of the deer family, which affords a maximum of thermal insulation per unit of bulk or weight.

Thermal insulation is also achieved among frizzly- or woolly-haired peoples by matting and thus forming a dead-air pocket over the scalp. A parallel to this is found in the merino sheep of northern Australia. Although the surface of their wool may have a temperature of 190degF in the sun, their body temperature remains well within the limits of normal function. Because human hair of this type rarely falls below the hairline, its presence does not prevent the neck from losing body hear through sweating. Uncut Mongoloid and Caucasoid hair, however, covers the back of the neck completely. And the beards of Caucasoids may also protect the front of the neck. Bald Caucasoid men retain a fringe of hair around the lower zone of the scalp, which bears enough hair to afford the neck some protection.

Variations in human hair make some sense in terms of what we have just learned. The concentration of reddish and dark hair in the foggiest, rainiest parts of Western Europe follows Gloger's rule, as explained in the section on skin color. Blond hair in general, and in particular that kind blond hair which lacks red-pigment cells, reflects 32 percent of light at a 70000 angstrom wavelength, compared to 18 percent reflected by light-brown hair, 15 percent by reddish-brown hair, 12 percent by red hair, 8 percent by dark red, and 1 percent by black hair.

Persons with straight or wavy blond hair have no other substantial protection against the sun's rays to compare with the air chambers trapped in individual Mongoloid hairs and the collective mats of Negroid hair. In Europe the zone of blond hair reaches far eastward from the Baltic to the steppes of southern Russia and even beyond in regions of hot summer sun. In Australia blond hair is concentrated in the hottest deserts. This distribution does no violence to Gloger's Rule, which states that populations of a species living in humid regions tend to have black or red hair, or feathers, whereas those living in arid, open country tend to have tawny hair or feathers. In this as in other respects human beings are just as subject to the laws of nature as other animals.


The Living Races of Man, pp. 238-239

Peter Frost said...

n/a,

I was discussing the lengthening of human head hair. There is variation in hair form among tropical populations, some of which are more derived than others. Head hair seems to have lengthened only in populations that have adapted to non-tropical environments.

If beards evolved as a protection against cold, why don't women have them? And why don't East Asians have them? (Didn't their ancestors live in even harsher Arctic environments?). And if head hair serves only to retain heat, why would it lengthen to the point that it can reach almost down to the waist?


Average joe,

The great apes diverged from our ancestors some 5 to 8 million years ago. I'm not convinced that their hair form or length tells us much about ancestral modern humans who lived 50,000 years ago.

Ishtara said...

Every hairstylist will confirm that nothing reflects the light like straight hair. Straight hair is very shiny (and therefore interesting), while curly and especially frizzly hair reflects more diffuse light and looks literally dull.

So perhaps sexual selection could be behind the change from frizzly/curly to straight, sleek hair, in climate zones where straight hair wasn't selected against for health reasons.

The change in length is harder to justify though. Hair that covers the ears might be an advantage in cold environments, since the ears dispense heat (thinking of the large ears of a fennec fox), and hair insulation would prevent loss of heat.

Males might profit from longer hair in quarrels with other males, since a "mane" visually increases the head size and gives a more intimidating look.

And of course longer hair has a larger shiny / colorful surface, so sexual selection could come into play here too.

Ishtara said...

Every hairstylist will confirm that nothing reflects the light like straight hair. Straight hair is very shiny (and therefore interesting), while curly and especially frizzly hair reflects more diffuse light and looks literally dull.

So perhaps sexual selection could be behind the change from frizzly/curly to straight, sleek hair, in climate zones where straight hair wasn't selected against for health reasons.

The change in length is harder to justify though. Hair that covers the ears might be an advantage in cold environments, since the ears dispense heat (thinking of the large ears of a fennec fox), and hair insulation would prevent loss of heat.

Males might profit from longer hair in quarrels with other males, since a "mane" visually increases the head size and gives a more intimidating look.

And of course longer hair has a larger shiny / colorful surface, so sexual selection could come into play here too.

tod said...

Hair extensions - used by women, I venture to guess, to make themselves more attractive.

Tod said...

What does "Let your hair down" say.

Headscarfs worn by many religions as a sign of piety,the long hair is for their husband alone.