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Sexual Selection, Stasis, Run-away Selection, Dimorphism and Human Evolution

This is cursory and a little simplified (no distinction between intra- and inter- sex selection) on purpose to keep it short (??) and to the point: the evidence of run-away sexual selection in humans.

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Sexual Selection

We have all heard about natural selection and survival of the fittest. Nature red in tooth and claw, etcetera.

There is another aspect of natural selection that is less well known and studied: sexual selection. Of late this seems to be getting more attention due to the problems with assigning species labels to organisms which can interbreed but don't. The reason they don't interbreed is not because they can't but because they don't "want" to -- the other organisms are not sexually attractive.

Obviously sexual selection occurs: in fact it occurs every time mating occurs. This may seem like an oxymoron but consider: if mating outside {group of organisms where interbreeding can occur} was frequent (a) it would be observed and (b) a lot of reproductive energy would be expended in an evolutionary pointless pursuit likely leading to extinction of species that so behaved compared to those that did not.

There is also a study that I remember reading about ~1975 in Canada where male stickleback fish were more likely to (attempt to) mate with objects that looked less and less like female stickleback fish the longer they had gone without mating activity. The study was mocked ("we needed to spend government money to find this out???") and so it was given wide publicity at the time. There is also evidence of this behavior in females too (see State-Dependent Mate-Assessment - [click]): "Mate-choice by females has been shown to be dependent on the state of the female, with females being less selective when limited by time or energy."

Sexual selection simply plays a role in choosing who to mate with and who not to mate with. There are many cases of closely related species that can produce hybrids but don't because they just don't mate.

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Stasis

From (an abundance of) this kind of evidence on can conclude that sexual selection involves actively (whether consciously or not) choosing mates that best represent {the species icon} based on visual, olfactory and behavioral clues, where {the species icon} represents the "ideal mate" not just for the individual but for the population.

If we assume {the species icon} represents average values of features within the population, then in the absence of survival selection pressure within a species population, this active choice mechanism will lead to choosing the more "average" individuals for mates (and excluding the least normal individuals) whenever possible, a process that will essentially guarantee stasis within the species population.

This is observed in many species, and in humans there are several theories on the issue of "beauty" but one of the consistent factors involved is that the more beautiful faces are averaged (see average face-ness - [click]) -- more on this in humans later.

Mature populations without survival stress and without any other reason to {change\evolve} would more or less stabilize around an {average individual icon}.

Of course these individuals would also be healthy and well fit to the ecological niche they occupy, but the reason they are allowed to reproduce is because they are chosen as sexually suitable mates.

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Run-away Selection

If we assume that {the species icon} does not represents an average value for any one (or more) {choice feature(s)} within the population, then in the absence of survival selection pressure within a species population, this active choice mechanism will lead to choosing the more "extreme" individuals displaying the desired feature(s) for mates (and excluding the more normal individuals) whenever possible, a process that will essentially guarantee transformation within the species population over time, until a point is reached where it can be taken no further. Mature populations without survival stress and without any other reason to {change\evolve} would trend towards an {extreme individual icon}, even to the point where species survival could be jeopardized: a species "fatal attraction" if you will.

The "run-away" aspect of this mating behavior develops because not only is the selection for individuals with the {choice feature(s)} but it is by individuals that prefer the {choice feature(s)}. Thus, to use a popular example of run-away sexual selection, not only does the male peacock have a large and extremely ornate tail, but the female peacock prefers males with the largest and most extremely ornate tail.

Richard Dawkins in The Blind Watchmaker also discusses an experiment with a long tailed bird where the male's long tail feathers were cut and then glued back with (1) shorter (2) same length and (3) longer tail feathers and then monitored for breeding success compared to (4) unmodified (control) males. There was no difference between group (2) and (4) (ie the effect of the glue process was eliminated as a variable), but the ones with the artificially long tail feathers were selected above the others: the sexual preference was for expression of the feature to an extent not seen within the population. Natural selection prevented the males from developing the longer tails after a certain point had been reached, but female preference was still for even longer tails.

Other studies have shown that long tail feathers (both male and female) do impose a definite survival disadvantage -- see Foraging cost (Behavioral Ecology Vol. 14 No. 4: 451–456 - pdf [click]) for one.

Thus run-away selection -- in this case at least -- caused survival pressure to play a role when it otherwise didn't have to, in selection of mates.

The existence of species exhibiting extreme sexual selection -- peacocks, scissortail flycatchers, etcetera -- means either (a) they are on the way to extinction or (b) the selection also confers an advantage.

The advantage aspect is usually argued that only the most healthy, mature males could exhibit the full display, so selection for this feature is a short-hand for selecting healthy strong individual predisposed for survival. Certainly immature male peacocks do not have a full display (and have low reproduction success) and sickly or malnourished peacocks do not have as full and vibrant a display as their more healthy competition.

The identifying characteristics of run-away sexual selection, then, that differentiate it from normal sexual selection, involve a feature (or features) carried to an extreme that is not needed for species survival (and which may even jeopardize survival), and that may still be selected for if it were possible to undergo further (still continuing?) evolution.

This would be evident in a skewing of the population chosen for mates versus the variation within the population (demonstrating that it is one extreme end of available variations that is consistently chosen), and it would also be evident in comparison to closely related cousins without the features (demonstrating that the features are not needed for survival): all the longer tailed male birds mated, and close cousins do not have a long tail.

Mature populations without survival stress and without any other reason to {change\evolve} would be pulled away from an average value towards an {extreme individual icon}.

Of course these individuals would also be healthy and well fit to the ecological niche they occupy, but the reason they are allowed to reproduce is because they are chosen as sexually suitable mates.

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Dimorphism

Survival of the fittest does not care about the sex of the individual, and for survival of a species to be optimized, both sexes need to survive to reproduce. Thus, at first glance one would expect that both sexes would be very similar in appearance and behavior, but this is not observed.

Sexual specialization is one reason that there is a difference between the sexes: often one sex raises the young and takes on the task of bringing them to a point of being able to take care of themselves. Thus in birds you see bright plumage in males, but dull and camouflaged coloration in females to better hide from predators. This does not, however, explain the bright plumage of the male (especially as there are many bird species where the coloration is the same for both sexes).

Run-away sexual selection easily explains the most extreme sexual dimorphisms in bird coloration and plumage displays, but it is not limited to just the most extreme examples. There are a lot of birds with red coloration, but it is difficult to get "redder" than a cardinal or a scarlet tanager.

There are also species where there is a large difference in {size\weight\strength}, and where the larger sex usually bullies the smaller one (and the smaller immature members of the same sex) in order to dominate sexually: gorillas, sea lions, hyenas, elk, etcetera.

These differences are not due to survival success, seeing as both sexes do survive, but due to sexual success: they are due to sexual selection. Bullies select themselves and deselect those rivals they can chase off, while "pretty" individuals are selected by those that find the individual "pretty" enough.

Run-away sexual selection usually involves a feature in one sex that is not expressed in the other (or that is incompletely expressed), with the result being a marked sexual dimorphism, but this does not mean that all sexual dimorphism is due to run-away sexual selection.

What the sexual dimorphism demonstrates when it is involved with a run-away sexual selection feature, is which sex is being selected for the feature: the peacock sex with the big tail is the sex where the tail is being selected to be big.

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Human Evolution

Do humans exhibit any features that show the effects of run-away sexual selection: features that have no survival advantage, that may impose a survival burden, and where selection continues to push selection towards a skewed end of natural variation within the population?

Elsewhere I have discussed this, and have noted these features as examples of run-away sexual selection:
(a) Long head hair (longer than any other primate)
(b) Music\Dance\Artistic Creativity (more than any other primate)
(c) Sexual signal features (larger than any other primate)
(d) Skin hair thinness (more than any other primate)

I am not alone with this. See The mating mind: human sexual selection [click] for a discussion on the role of sexual selection in the evolution of the human brain size, complexity and ability -- with both pro and con arguments.

Long head hair certainly fits the "template" of run-away selection: it is extremely long compared to our cousins, beyond anything required for survival purposes alone, it would not be as long in immature males\females, it would not be lustrous in sickly or malnourished people, and it could lead to individuals being {noticed\caught\killed} by predators and other competitors. Long hair is still considered a marker of beauty (ie - sexual attraction): look at Fabio to say nothing of wigs that artificially lengthen hair. There is also sexual dimorphism in head hair with male facial hair exhibiting the same propensity for extreme length as the hair on top of both sexes exhibits, and this would indicate that this feature was selected in the males of the species (initially at least). This is also consistent with facial dimorphism in other primates.

Mating songs and dances are also common in other species, often exceeding any purpose that could be construed for survival alone, with the more creative individuals being selected for mating, presumably because creativity in song and dance also converts into creativity in survival. It would also exhibit health and fitness and maturity compared to other less fit competitors. This same behavior is seen in other species, but it is certainly not as well developed in other primates. That {Music\Dance\Artistic Creativity} ability it still selected for is apparent when you look at the superstars in the music world and other arts. There is little sexual dimorphism here (other than pitch of voice and small differences in flexibility and the like), and this could indicate that the ability of both sexes was important in the mating rituals (you also see this in other species, especially those that make long term pair bonding commitments).

Sexual "signal" features, such as round firm female breasts, well shaped buttocks and a large male penis. These features are not fully developed in either sex until puberty has been reached, and thus they signal readiness for mating. These features are important enough to people in general that a whole industries have been developed to cater to them: porn, breast implants, tight fitting pants, pants with pads for the less butt-endowed, etcetera. That these items are still being selected for hardly needs mentioning, but there is one thing worth noting and that is that both male and female features have been selected for augmented development: this is one case where sexual selection is not just from one sex choosing which of the other gets to reach for the brass ring on the mating merry-go-round, as happens in some other species. There is, however, definite sexual dimorphism in the specific features involved, with the abundant female breast and buttock and the male penis.

Skin hair thinness -- the "bareness" of the human ape body is one of it's defining features because it is taken to extremes not seen in any other primate, even compared to sexual signaling patches of bare skin that develop in some species (notably in the buttocks and breast areas of the female while in estrus), or to the "bare" chested gorilla.

This "bareness" is composed of two components: spacing of hair such that bare areas between the hairs are visible, and such {thin\fine}ness of hair that it is hard to see. The body is not really bare, but appears that way because the hair is hard to see.

Certainly there is no survival benefit to it: in every location where humans live they are accompanied by the domesticated dog, which is not at a survival disadvantage with his fur. The extent of apparent bareness of humans, on the other hand, has reached such an extreme condition that it requires compensatory behavior to live in most of those environments: clothes (among others). There are also whole industries devoted to assisting people in further divesting themselves of unwanted hair.

Porn sites show unnaturally bare women, just as ads and magazines promote the extra baring of the female body with creams, surgical procedures and thousands of kinds of razors to remove "unwanted" hair. From this article in USA today:

"More attention is being paid to the leg, which means more attention is being paid to hair removal and the razor and technology," says Marshal Cohen, trend watcher for fashion and retail tracker, The NPD Group. "The whole shaving and hair removal business has just begun. We haven't even gotten to the root of the business."
There is no question that a barer body is {sexier\beautiful\attractive} for the human species when you look at the movies, art, and (of course) porn, particularly for the female. And this isn't just cultural, we have definite sexual dimorphism, particularly in the chest and buttock area, with the female being not only barer but more consistently bare.

In fact the link between enhanced female breast and buttock size and the increased female bareness in these specific areas indicates that these features were selected together: not just breast but bare breast, not just buttock but bare buttock.

There are also several other ape species where sexual readiness is signaled by bare areas on the female ... breast and buttocks. Breasts are also commonly barer when enlarged by producing milk for feeding young. Note that the {hair loss on} and {swelling of} these areas occurs during estrus in the other apes, and thus it is an easily understood path for normal evolutionary operation in selection and development of a feature.

There are also other ape species that {can\do} extend estrus when in the presence of strange males. It is only in the human ape that these features are displayed continuously, much beyond any need for survival or normal needs of a species for reproduction.

The question can also be investigated by what is considered beautiful (women) or handsome (men) and does it show a skewed pattern. This goes back to the average face issue raised above: "average face-ness" was not enough to fully explain the whole pattern in the experiment as there was also a clear bias in the data for youth (see babyface-ness: [click]), and the summary comments on this include these statements (read Summary: [click] for the whole summary):

(1) The results of our study are quite surprising. Compound (i.e. morphed) faces were, on average, regarded as being more attractive than the original faces. The more original faces one compound face consisted of, the higher its assigned attractiveness value. ... on the other hand, the attractiveness of the original faces included in a compound image itself influences the overall attractiveness of the compound face, i.e. the more attractive the original faces, the more attractive the resulting compound face. Thus, not simply the number, but also the attractiveness of the original faces influences the average attractiveness rating of compound faces. This result is in contrast to the 'attractiveness is averageness' hypothesis"

(2) For female faces, it could be shown that babyface attributes - such as large, round eyes, a large domed forehead and small, short nose and chin lead to a rise in attractiveness values. Only very few (9.5%) of the test subjects found the original adult faces most attractive. Most of the test subjects (90.5%) preferred faces with 10%-50% the proportions of the babyface scheme. This means: Even the most attractive female faces can become more attractive when their proportions are altered towards more babyfaceness.

(3) To sum up, our study shows clearly that the most attractive faces do not exist in reality, they are morphs, i.e. computer-created compound images you would never find in everyday life. These virtual faces showed characteristics that are unreachable for average human beings.
{bold mine for emPHASis}

They also found "averaged" but not "babyfaced" beauty applied to males and thus it looks like human beauty involves both and (average individual icon} for many features and a skewed {extreme individual icon} tending towards idealized younger looking sexually mature females. That this also demonstrates the same pattern of run-away sexual selection noted for breasts, buttocks and bareness (children exhibit greater bareness than adults) is not likely to be an accident.

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Human Body Hair

In the above information I introduced a number of features that I felt fit the pattern of run-away sexual selection. Possibly the most contentious of these was the skin hair thinness, or 'apparent bareness' issue at the end. This is because there have been other theories for why human body hair is so different from the other apes, including "bipedal ape" and the "aquatic ape" theories (although both of these theories have been discredited due to their failure to explain all the evidence adequately: most particularly they do not explain the sexual dimorphism that exists between males and females on the degrees of apparent bareness.)

Let's look at this issue in a little more detail and see why human hair is so unique:

From Human Thermoregulation and Hair Loss [click]

Body hair is one of the most important features of the mammal thermoregulatory system. Hair can act as an important heat retention and heat prevention device in mammals. By trapping a layer of dead air against the skin, a layer of hair can act as an extremely efficient insulation, reducing the rate of convective heat loss to the environment. However, this exact same system acts as a way to prevent heat gain from the environment by the same principle; by using this layer of dead air to reduce the rate of convective heat gain from the environment to the skin. Besides insulation, the layer of hair on mammals is important in reducing the radiation from direct and indirect sunlight, and can thus act to reduce heat gain from the environment in two ways.

If loss of hair was an important variable in thermoregulation then we would expect {evolutionary pressure \ natural selection} to show a broad trend of hair thickness variations that could be correlated with the need to {retain\dissipate} heat.

We do see this. From the same source, here discussing the need of larger bodies to {retain less \ dissipate more} heat due to the increase in volume as the cube but skin area as the square of a size dimension:

The obvious solution to this situation is decreased body hair with increasing body size, which is exactly what is seen in anthropoids. When the number of hair follicles present in species per unit of area is compared with body size, all primates (including humans) fit along a regular log linear regression line, along which the density of hair per unit of area decreases as body size increases. Species like chimpanzees and gorillas have relatively fewer hair follicles per unit area of skin compared to the smaller monkeys. Humans fall along this line, and have a relative hair density almost the same as seen in chimpanzees, gorillas and orangutans.

To drive this point home, the number of hairs on the human body are precisely what they should be for the human body size. We are not displaced on the scale. There is no special loss of hair required for thermoregulation, and thus there is no special mechanism needed to provide for the loss of hair: no mutation is needed for the explanation of amount of human body hair.

This means one of the factors contributing to apparent bareness, the spacing of hair such that bare areas between the hairs are visible, is due to the body size of humans. Indeed, close-up pictures of chimpanzees, gorillas and orangutans show that the skin is visible through the hair: humans have a normal hair density {number/area} for their body size and surface area.

Let's continue:

The difference between the thick pelage of the Great Apes and humans is not in terms of the density of hair, but in its length and thickness and the production of vellus hair in most humans to the exclusion of terminal hair on the body. Humans are not "hairless", but are merely covered by thinner, smaller and unpigmented hair (Schwartz & Rosenblum 1981; Schultz 1931).

This may seem like an obvious point (although 'underpigmented' might be better than unpigmented: it's not albinoism).

Now to better understand the distinction here, we need to know what vellus hair is and how it differs from terminal hair.

Types of Hair [click]

There are three types of hair:
  • lanugo: fetal hair
  • vellus: replace lanugo hairs in the peripartum period, unmedullated
  • terminal hairs: long, coarse, medullated; typified by scalp and pubic regions

And from Hair Growth, Classification of Hair [click]:

Human hair varies with respect to texture, color, and length.
  • Lanugo hairs cover most of the fetal skin and shed perinatally. These lightly pigmented hairs are fine in texture.
  • Thicker and darker than lanugo hairs, vellus hairs cover most of the glabrous skin surface except palms, soles, palmar and plantar surfaces of fingers and toes, inner aspect of the prepuce, and the glans penis.
  • Darkly pigmented terminal hairs are long and thick and most commonly are located on the scalp, and on the facial areas of men. These hairs can reach a preprogrammed length based on length of the anagen phase. Ultimately, terminal hairs undergo involution and convert to catagen and then to telogen phase. The mechanism and factors that induce the hair to terminate growth and involute are largely unknown.

And from Wikipedia, Vellus hair [click]:

Vellus hair is a very soft and short hair that grows in most places on the human body in both sexes. In Caucasians it is often colourless, or blonde. It is best seen in women and children, as they have less terminal hair to obscure it. Vellus hair is also found in pre-adolescents (Tanner stage 1) as well as in male pattern baldness.

Vellus hair is juvenile hair that is best seen in women? Other than facial hair, is there really that much difference in hair patterns between males and females? See information about a medical 'condition' called Masculine Hair Distribution (in a female) [click]:

This is excessive hair growth in an androgen dependent pattern. It is applied to females who complain of hair growth in the beard area, around the nipples and in a male pattern on the abdomen. Androgens induce the transformation of fine vellus hair into coarse terminal hair.

Female vellus hair that is transformed into terminal hair in a male pattern. As a bad thing.

Put this together with the "baby-faced-ness" and the issue of run-away sexual selection of Human apparent bareness is seen as two results of the same process: juvenile features selected for in the female of the species. And considering that the bareness of the human female could not get much further developed without some mutation to actually decrease the numbers of hairs on the human body (and replace the need for razors and dilapidaries), I would say that it has been carried to the extreme condition that is characteristic of a fully developed run-away mechanism: it has developed as far as it can by current evolution and yet still shows preferences to develop further.

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Enabling Mechanisms

The loss of long thick colorful terminal hair, typical of other apes, in favor of short thin pale vellus hair, as found on most of the human body skin areas, means that the insulation value of that hair has been lost. This insulation protects the individual from both heat and cold, modulating the extremes to provide a more constant "microclimate" for the individual. The loss of such insulation would leave the individual subject to both greater heat gain and greater heat loss without some other mechanism(s) to counteract these trends.

If anyone doubts the ability of insulation to provide sufficient protection from too much heat, then consider that Arabs in the Sahara Desert wear clothes that essentially replace this function of fur - from head to toe.

This is not just the {elongation\oversizing} of {decorative\sex-competitive} features, like tails or antlers, which develop until they reach a point of jeopardizing survival, but the alteration of a feature basic to survival from the start - and in a direction, and to an extreme, that could easily affect survival adversely. The loss of hair presents two simultaneous problems:

Too much exertion on a windless hot sunny day: in the absence of any mechanism to {shield\protect\divest} a body of excess heat, hyperthermia becomes a foregone conclusion, and likely to be fatal if no counteraction is taken. This happens to humans who lose the ability to sweat (or have maxed out their sweat-ability - more below). See Wikipedia - Hyperthermia [click]:

Body temperatures above 40°C (104°F) are considered life-threatening. At 41 °C (106 °F), brain death begins, and at 45°C (113 °F) death is nearly certain. Internal temperatures above 50 °C (122 °F) will cause rigidity in the muscles and, therefore, certain immediate death.

Signs include increasing body temperature (hyperpyrexia), dehydration and lack of sweating, seizures, collapse and decreased consciousness which proceeds rapidly to multi-organ failure and death as the brain 'cooks'.

On the opposite side is the problem of surviving cold windy rainy nights, with activity at a minimum, for then hypothermia becomes a foregone conclusion. This happens to humans that get wet when it is cool and windy. See Wikipedia - Hypothermia [click]:

If body temperature falls below 32 °C (90 °F), the condition can become critical and eventually fatal. Body temperatures below 27 °C (80 °F) are almost uniformly fatal, though body temperatures as low as 14 °C (57.5 °F) have been survived.

If you've ever had your skin turn blue with goose-bumps and your teeth chatter when swimming on a summer day, you've experienced hypothermia (and "goose-bumps" are the retained muscle reaction to cold which made the body hairs stand on end to increase insulation value).

From 90 °F to 104°F is a pretty narrow window for acceptable body temperatures. With a loss of thermal regulation, the results could quickly be fatal unless the development occurred over time and there were enabling mechanisms to allow for a more incremental development of {apparent bareness} in humans.

You can think of this as the "goldilocks problem" - not too hot and not too cold, but just right.

Environment: the Serengeti area in Tanzania, home to Olduvai Gorge, is thought to be one of the ranges of early {hominids\humans}, and from The Serengeti National Park [click]:

With altitudes ranging from 920 to 1,850 metres - higher than most of Europe - mean temperatures vary from 15 degrees to 25 degrees Celsius. It is coldest from June to October, particularly in the evenings.

That's 60°F to 75° (F59°F to 77°F), and the "rainy season" lasts for months. Those temps are the means. From Kenya Country Information [click]:

Extreme temperatures in Nairobi range from 50 degrees to 90 degrees.

In a forested environment those extreme temperatures would be moderated, but the thing to note is that these are all below the normal human body temperature, and at their highest reach the lower limit of concern for human body temperature. 

Current anthropological thinking is that humans developed the fully bipedal gait some 5- 6 million years ago, while still living in a forested environment:

From African Continental Paleoclimate and Hominid Evolution II [click]:

Our glacial simulations indicate that tropical broadleaf forest was not severely displaced by grassland expanding into central Africa, although the outer extent of closed forest decreases, particularly in the north. Our vegetation-climate simulations also indicate that the extent of closed tropical forest during typical interglacials is not represented by today's observed vegetation distributions. Simulated interglacial climate results in expansion of tropical forest from coast-to-coast across much of central Africa.

And from Soil Suggests Early Humans Lived In Forests Instead Of Grasslands [click]:

The analysis was of fossil soils from paleontological sites in the Middle Awash region of Ethiopia’s rift valley, where the remains of a new subspecies of Ardipithecus ramidus have been discovered. They date to the late Miocene period (5.4 million to 5.8 million years ago). Scientists from four institutions report their findings in a pair of papers that appear in the July 12 issue of the journal Nature.

The region where the fossils were found is now a hot, dry semi-desert occupied by nomadic camel herders. At the time the area formed, it was higher in elevation, cooler, wetter and more forested.

The nodules from these late-Miocene hominid fossil sites contain low levels of carbon 13, which is consistent with trees and woody plants. They also contain oxygen isotope ratios that are indicative of a cool, humid climate. "These hominids were living in the forest, despite the fact that grasslands were available," Ambrose said.

Given this environment, substantial {apparent bareness} could develop without jeopardizing survival through overheating. This would indicate that the primary concern in this environment would be to prevent heat loss rather than getting too hot, at least during the initial stages.

A cool shaded environment enables the development of bareness while only having to provide alternate heat retention mechanisms, and not have to provide both retention and dissipation

Heat Retention: from Human Thermoregulation and Hair Loss (click)

The human vascular system has developed reactions to both heat stress and cold stress. The skin has a system of thermal receptors that perceive temperature of the skin and send signals carrying this information to the autonomous nervous system. When the body perceives increased heat loss through the skin, vasoconstriction of the peripheral vascular system occurs to decrease the blood flow (which carries heat from the core to the surface). In humans this vasoconstriction can reduce heat loss by 1/6 to 1/3 depending on the individual and the acclimation of the individual to cold stress. When the body perceives a need for increased heat loss, vasodilation occurs, with increased blood flow to the peripheral vascular system. This vasodilation increases the rate of heat transfer from the core to the surface, and is also an important feature involved with sweating.
We'll get to sweating again later, but it seems that this vascular system would be one of the first mechanism employed, developed within it's limits to enable some level of {apparent bareness}.

The likelihood is that certain areas (breast and buttocks?) were selected for {apparent bareness}, with the rest remaining as before. This would be similar to the gorilla, with the male's "bare" chest area and the female's "bare" breasts, a degree of {apparent bareness} that does not inhibit their survival (although the gorilla has more body mass than the female human, particularly more than an early hominid would have).

 

(1st picture originally from http://www.bristolzoo.org.uk)
(2nd picture originally from http://www.hedweb.com/gorilfam.htm)

add gorillas2.jpg?
(picture originally from http://www.classicsafaris.com/gorillatreks/ultimateprimate.htm)

One advantage of the vascular system is that it works for both heat retention and cooling, thus it is best able to replace both those functions lost with hair reduction, especially for daily fluctuations where a fairly rapid response would be needed. The disadvantage is that it is not as capable of modulating extremes of heat and cold as hair would be. 

Another heat retention system common to many mammals is a thicker layer of subcutaneous fat. This layer exists in all primates, but is most developed in humans. Again, from the  same source on thermoregulation:

The distribution of a thick subcutaneous fat layer in modern humans is another "unique" feature in relation to closely related non-human primates, and should be considered in any theory that invokes a thermoregulatory explanation. However, this is hard to distinguish as either a true problem or a false one, as humans do not have any more points of fat production than other primates, but rather have more fat developed from these individual centers ...

Body fat is located under the dermis in a subcutaneous layer. This fat can have several functions in mammals that may or may not be related to thermoregulation. ... The development of a thick layer of subcutaneous fat may be an adaptation to the loss of body hair and the potential cold stress that the human ancestor would have experienced, or may simply be a feature of the behavioral capacity of humans to get access to food.

In the cool forested environment discussed above, absorbing too much radiant energy from the sun would not be a concern, but radiating too much energy to the environment would be. In this type of environment a thicker subcutaneous fat layer would provide sufficient insulation to enable further development of apparent bareness. This would not be fast acting (like the vascular system) but it would be adaptable to seasonal variation, with thicker fat layers in the cooler seasons, thinner in the hotter ones.

This subcutaneous fat layer is one of the arguments for the 'aquatic ape' theory, as aquatic mammals use this to insulate them from the cooler water, but it does not require an aquatic environment so much as a cooler environment. If the environment was consistently cool enough that overheating was not a problem (as the environmental evidence shows above), then the development of the subcutaneous fat layer would provide an incremental increase of insulation to replace an insulation decrease due to a further loss of hair (and still allow the vascular system to deal with day to day variations) without needing an aquatic environment to explain it.

If anyone doubts the ability of these mechanisms to provide sufficient protection from hypothermia (not too cold), then consider that when the Beagle reached the area of Tiera del Fuego at the south end of South America, that the crew were welcomed by nearly naked natives while Darwin and the others froze with the clothes they had on:

From geography.about.com [click]

On one occasion Darwin and crew were near a good size fire and still quite cold, while the aborigines were some distance away and perspiring heavily. Charles noted this with great interest.

The 'bipedal ape' theory postulates that the upright posture is a better radiator of heat while decreasing the area for absorption of heat from the sun. Other studies have disputed the thermal balance calculations, but most of these (pro and con) calculations relate to the high temperatures of a savannah environment. Again, from the same source on thermoregulation: :

A scenario that makes bipedalism a necessary pre-adaptation for the loss of body hair means that bipedalism would have occurred in the absence of the development of the sweating mechanism, and thus the heat dissipation of this proto-biped would not have had the heat dissipative capabilities of a modern human.

When this factor is accounted for, the putative greater heat load capacity of a naked skin of a newly developed biped over denser body hair is negated. Amaral (1996) showed that the thermal stress on a naked biped is up to three times greater at higher temperatures than on a hair-covered skin. In fact, this is exactly the reason that other savanna primates have a dense coat of fur that is even more developed than forest dwelling primates do.

The argument of the 'bipedal ape' theory is that heat loss is enhanced by the upright posture and that the upright posture developed on the savannah as a way to look over the tall grass. But the problems here are (1) we are still in that forest environment (2) we haven't developed the mechanism for sweating yet (3) we don't need to have cooling enhanced, and (4) the biggest problem of all (ibid):

Sweating is a thermoregulatory mechanism of modern humans that effectively removes body heat through evaporation. It becomes extremely effective in the absence of heavy body hair, and actually can be maladaptive in the presence of heavy hair cover.

Effective sweating requires as little hair cover as possible, as it needs air contact (particularly moving air) over the skin to remove the heated sweat. In an individual with a prodigious sweating mechanism and dense hair cover, the heated sweat will generally be retained by the hair cover and actually begin to act as insulation preventing heat loss, leading to hyperthermia.

In other words, {apparent bareness} had to precede the development of sweating, and it had to happen in an environment cool enough that prevention of overheating was not critical to survival (ibid):

... but the preconditions that would favor the loss of body hair and the development of a sweating mechanism (high environmental or metabolic thermal stress relative to an earlier state) can be shown to occur prior to when hunting can be reasonable inferred as a likely possibility. A radiation of hominids circa 2 mya indicates a shift into new environments (and the capacity to survive in these environments), and an even earlier increase in body size would indicate increased metabolic stress with the expense of less efficient heat loss through convection with the air. These are events that likely would have involved the need for the development of thermoregulatory changes much earlier than any reasonable evidence of hunting.

Rather it would seem that (hyper?) development of {apparent bareness} enabled the evolution of sweating and thus allowed the bipedal savannah hunter to take the stage.

The next question is how did the mechanism of sweating evolve, and especially whether any mutation is required to enable it.

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Sweat

What is it, how does it work and where did it come from?

Once again, from Human Thermoregulation and Hair Loss (click)

The sweat on the surface of the skin is mostly water and has a high specific heat. Heat is removed from the skin by conduction to the sweat until the sweat is either evaporated or sloughed off. Sweating is most effective if the sweat is actually evaporated, since it is removing more heat per unit of sweat being produced.

... Eccrine glands are specialized sweat glands that produce large quantities of sweat that is mostly water ...

Vasodilation increases the blood flow from the core to the periphery while increasing the rate of heat loss to the skin, which is then removed through the evaporation of sweat ...

The vascular system opens to transfer heat to the skin (as before), Eccrine glands transfer that heat to water and transports the heated water to the skin surface, where evaporation removes heat energy, cooling the surface (which is then transmitted back to the interior by the vascular system). The system is very efficient for moving heat quickly to the surface where it then depends on evaporation to finish the cycle

Now we need to know what Eccrine glands are and how we got them. Let's continue (ibid: ):

The distribution of these glands along the body follows a very regular pattern. Sebaceous glands are found in association with all or nearly all hair follicles and have no known functional significance in thermoregulation. Apocrine glands are found in the axillary regions (the pubis, the perianal region, and the axillae) in humans, whereas in most non-human primates (excluding gorillas and chimpanzees) they are found throughout the entire body. It is important to note that in the human fetus apocrine glands begin to form all over the body in association with hair follicles, but are mostly resorbed into the body during development. Eccrine glands are found over the entire surface of the body, in both hairy and non-hairy areas, and have no developmental tie to individual hair follicles. In most non-human primates (again excluding gorillas and chimpanzees) eccrine glands are only found on surfaces used in locomotion (the soles of the hands and feet, and among the dermatoglyphics found on places of high friction, like the tails of prehensile species or the knuckle pads of knuckle-walkers.

The original purpose of Eccrine glands is to keep bare areas of skin (and areas likely to build up calluses) soft and supple. Note the exclusions of Chimpanzees (I assume they include Bonobos as "Pigmy" Chimpanzees) and Gorillas, our closest relatives in the ape family. This means that at some point previous to the divergence of Humans, Chimpanzees and Gorillas, a mutation (1) occurred and (2) was fixed, that caused the expression of Eccrine glands on these other body areas. Current thinking is that humans and chimpanzees shared a common ancestor some 5-6 million years ago, and that humans, chimpanzees and gorillas shared a common ancestor some 1 million years earlier:

From {{need title for artricle}}(click)

(this) yields a time and 95% confidence interval of 5.4 +/- 1.1 million years ago (36 nuclear genes) for the human-chimpanzee divergence. Older splitting events are estimated as 6.4 +/- 1.5 million years ago (gorilla, 31 genes) ...

The fact that this mutation shows up in Gorillas and Chimpanzees indicates that it was fixed before the selection for {apparent bareness} occurred. In humans this inherited mutation was opportunistic to the development of a new and effective (within limits) means to deal with overheating of an already {apparently bare} body, and this allowed the expansion of humans into hotter and more open environments as the forests were replaced by savannahs.

There are a couple of draw-backs to the sweating mechanism: (1) If the air is saturated with moisture, then evaporation does not happen and only the less efficient "sloughing" (sweat run-off) occurs (and if there is no air movement then the saturation can be localized, building up around the individual: hot sweaty nights); (2) There is only so much moisture in the body, and it is possible to sweat up to 2 liters of water in an hour:

From Cycling Performance Tips (click)

And this is especially true in cycling where evaporative losses are significant and can go unnoticed even though sweat production and loss through the lungs can easily exceed 2 quarts per hour. To maximize your performance, it is essential that fluid replacement begin early and continue throughout a ride. A South African study comparing two groups of cyclists (one rehydrating, the other not) exercising at 90% of their maximum demonstrated a measureable difference in physical performance as early as 15 minutes into the ride.

From Marathon Physiology (click)

Sweat: During a marathon, sweat production routinely ranges from 1 to 1.5 liters per hour, and can exceed 2 liters per hour for some runners. If you sweat much more than you drink on the run, your blood volume drops, reducing the amount of blood that is pumped with each heart beat. When this occurs, your heart rate increases and eventually your pace will suffer.

And without a means to replenish this water continued exertion will result in depletion of the water reserve (dehydration), and hyperthermia will ensue. In spite of copious water supplies along the route several marathon runners at the recent Greek Olympics succumbed to hyperthermia. Hair (or fur) does not need to be replenished, so this reliance on extra water for sweat can be a disadvantage in some areas.

Pheromones: Apocrine glands also produce some liquid material, and this responsible for the distinctive odors associated with sweat in these areas, odors that may be linked to pheromones. Once more, from the same source on thermoregulation:

The sebaceous glands - as mentioned previously - act as lubricants for the skin and hair, maintaining moisture content and prevent the skin and hair from drying out and cracking. The apocrine glands in humans (and gorillas and chimpanzees) seem to have a sexual function, and produce odors. This occurs through the combination of the sebaceous, apocrine, and eccrine glands. In the axillary areas the sebaceous and apocrine glands produce a constant stream of secretions of high cellular content. When the eccrine glands are stimulated to produce sweat, the sweat mixes with the secretions of the sebaceous and apocrine glands and spreads throughout the axillary area. In the moist microenvironment of these areas, bacteria is abundant on the skin and on the hair, and when given access to the organically saturated sweat, begins to break down the cellular components, with body odor as the result of this process. The sweat excretions themselves are odorless.

When I started this essay I wondered if our sweat glands had evolved out of the pheromone producing ones, and that this conversion would explain the seeming loss of pheromones in humans. Instead what we see is that the areas that were previously covered with Apocrine glands have diminished to just the armpits and genitals and that they have been replaced by the sweat glands in the other areas, thus reducing the output and diluting it as well. And it could well be possible that the "microenvironment fauna" have evolved and so the odors are no longer similar to what they used to be. It is also possible that during the initial stages of developing sweat glands out of Eccrine glands (by increasing their output), that the early hominids were awash in pheromones and that this drove the species towards the shortened sex cycle and more universal sexual "availability" that is also a characteristic of humans (and Bonobos - those hedonistic "pygmy" Chimpanzees).

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Final Words

The Chimpanzees and Gorillas do not have the sweating mechanism in spite of having the Eccrine glands, and this would indicate that they had too much hair for sweating to be effective (they could not simultaneously evolve less hair and sweat) and thus were stuck in the shadows of the forest. This would also mean that Human {apparent bareness} was fully developed when the savannah encroached on the forests and early hominids began to sweat, and then began to go hunting in the new environments that this adaptation allowed.

The final clue may be in a seeming side comment in this article, Humans March to a Faster Genetic "Drummer" Than Other Primates, UC Riverside Research Says (click)

RIVERSIDE, Calif. (www.ucr.edu) -- A team of biochemists from UC Riverside published a paper in the June 11 issue of the Journal of Molecular Biology that gives one explanation for why humans and primates are so closely related genetically, but so clearly different biologically and intellectually.

It is an established fact that 98 percent of the DNA, or the code of life, is exactly the same between humans and chimpanzees. So the key to what it means to be human resides in that other 2 percent.

The team, which also included Rosaleen Gibbons, Lars J. Dugaiczyk, Thomas Girke, Brian Duistermars and Rita Zielinski, identified over 2,200 new human specific Alu DNA repeats that are absent from the chimpanzee and most likely other primates.

"The explosive expansion of the DNA repeats and the resulting restructuring of our genetic code may be the clue to what makes us human," Dugaiczyk said. “During the same amount of time, humans accumulated more genetic novelties than chimpanzees, making the human/chimpanzee genetic distance larger than that between the chimpanzee and gorilla.”

(bold added for emPHASis)

Note that the {chimp\human} ancestor diverged from the {gorilla} ancestor before the {chimp} ancestor diverged from the {human} ancestor. For there to be more mutations selected for fixation within the human genome means that either survival pressure operating on just humans, or that some process was actively selecting changes. These same changes were fixed in the human genome before humans spread around the world, from DNA variations surprise researchers (click)

Early information from the Human Genome Project indicated that the DNA in the genome of any two individuals is 99.9 per cent identical with the 0.1 per cent variation arising primarily from some three million single nucleotide changes scattered amongst the chromosomes. The new data from the Sick Kids and Harvard groups revealed 255 regions (comprising more than 0.1 per cent) of the genome where large chunks of DNA are present in different copy numbers between individuals ...

So all but ~0.25% of the current genome is shared by all humans, and (neglecting mutations since dispersal) that of the 2% difference between humans and chimps ~85% therefore must have been fixed in the genome before the first bipedal foot left Africa, ~200,000 years ago.

I fully expect most of those Alu repeats to be associated with those {feature\characteristics} where we have a measurable quantitative (more\less of) rather than qualitative (type of) difference: brain size, breast size, buttocks size, penis size, sweat, head hair, bareness.

While the mechanism of run-away sexual selection may be debated for some of these features, this certainly would provide an undeniably active selection mechanism that would result in a genetic pattern similar to that found.

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Conclusions

Apparent bareness had to evolve before sweating could evolve, and it could only evolve while early hominids did not need protection from sever heat stress: while the tropical forest cover was their chosen habitat.

Sweating had to evolve before humans could venture far into the savannah environment in order to deal with the sever heat stress of living in that environment without the protection of {fur\hair}: while the forest was being replaced with savannah but still provided refuge during times of peak heat stress.

Apparent bareness and other features show signs of run-away sexual selection; the genetic code shows sign of active selection at a rate higher than normal natural selection, thus reinforcing the evidence for sexual selection having been an active mechanism in human evolution.

While such an evolutionary process may include specific {feature\direction} aspects, it certainly was not random evolution, but it also certainly was not free of random circumstances (change in climate, change in environment). The actual process was enabled by the ecological conditions that prevailed and by genetic mutations that could be tuned to suit the {sexual\selected} needs, but those {mutations\features} needed to already be available to be acted on. This is the way that selection interacts with natural variation to produce change in species over time.

I also like to think those early hominids had developed one final enabling mechanism that allowed the further spread of the {apparently bare} ape into the world at large: portable shelter, replacement hair, ... proto-clothes. Perhaps capes made of woven grasses or disguises cut from the hides of opportunistic finds. Thus a naked species moves out from a diminishing subtropical garden forest into a hot and (comparatively) barren land, and find they need clothes to shield their nakedness from the unforgiving sun.

Pure speculation, I know, (and not without a certain poetic license), but whatever the role clothes have played in allowing humans to populate the globe, it looks like sex is what has made us human.

Enjoy.

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(Note: this is an essay and as such represents the opinions of the author. You can e-mail comments to me at RAZD8@yahoo.com)

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