Evolutionary advantage of naked skin?

[wautd]

Fur can protect you from cold and sunburn, so on first sight I'd expect that naked apes would have a disadvantage compared with their more hairy counterparts. So why did humans evolved into a naked ape?

[Lord of the Abyss]

For one thing, it offers superior cooling, and humans are built for endurance as cursorial hunters; thanks to our naked skin and large amount of sweat glands, an athletic human can easily run until most creatures would be half dead of heat stroke from the heat of their own exertion.

For another - ever notice how much time apes spend picking each other over for parasites? Humans have a much easier time flicking off irritating insects and such.

Also, recall we evolved in the tropics; cold wasn't generally much of a problem, and dark skin protects pretty well from UV as a substitute for fur.

[Spoonist]

We don't know but over the last century or so we have had some vivid speculation. The aquatic ape being one of the most controversial right now.
http://www.scientificamerican.com/artic ... -body-hair
http://merciacollins.hubpages.com/hub/T ... their-Hair

But also note that it doesn't actually need to be a positive trait as long as it is grouped with other positive traits. There are lots of things that as long as they are not actually life-threatening will be passed on. For instance the dark skin of african and australian humans is in stark contrast to apeskin. So if a different system kicks in - melanin then the loss off thick fur isn't bad at all, until you move to less UV parts when you see lighter skin appear again.
Another note is that we are not naked at all, from two perspectives -
1 we still have our fur and it does give a lot of advantages, we just have less off it
2 we protect ourselves and wear clothes, so any disadvantage from not having thick fur is long gone a 100ky ago.

[Ziggy Stardust]

Cooling is a major part of it; eccrine sweat glands essentially take the place of hair follicles (there's only so much space). It simply makes the skin more efficient at dissipating heat. The alternative is having hair and panting to cool down, which many other animals do. Look at other large mammals, like elephants; they have relatively little hair compared to other mammals living in the same environments. As your body mass to surface area ratio increases, naked skin and sweat glands become more energy efficient for controlling body temperature. The reason we still have hair on certain parts of our bodies is that hair acts as a good shield against sunlight (hair on our heads protects our brain from overheating, which is one of many factors that allowed us to become so highly intelligent, and head on our genitals to protect our sperm).

I think there are some who argue that naked skin is also important for health/cleanliness and getting rid of parasites (i.e. the same reason many vultures have no feathers on their heads). That, too, would be a powerful evolutionary force.

[Singular Intellect]

and head on our genitals to protect our sperm).

Wait, how goes getting head protect sperm? Doesn't it often end up getting ingested that way?

On topic, doesn't hair also function as a means of friction reduction between body parts? For example, armpit hair?

[Ziggy Stardust]

Wait, how goes getting head protect sperm? Doesn't it often end up getting ingested that way?

Ha, that was a Freudian slip and a half, wasn't it?

I meant hair on our genitals.

On topic, doesn't hair also function as a means of friction reduction between body parts? For example, armpit hair?

Honestly, the purpose of under-arm hair is still debated. Some people do propose that it has to do with friction. Other propose that it has to do with absorbing sweat/moisture, and is part of the body's ventilation system. However, the fact that is only develops around puberty may indicate that any benefits related to friction/ventilation or auxiliary. Since underarm hair is more similar to pubic hair than that on your head, and develops at the same time, many scientists believe that it serves a purpose sexually, either as a display of sexual maturity (remember, from an evolutionary perspective your body still thinks we are naked) or as some sort of pheromone control system.

And there are others that point out that it could simply be vestigial from when we had hair all over our bodies. The benefits of hairlessness don't specifically promote a naked underarm, but neither is there any sexual selection against it.

[Alyrium Denryle]

We don't know but over the last century or so we have had some vivid speculation. The aquatic ape being one of the most controversial right now.

It is not a controversy. It is one nut with no evidence (how do we know other species have been aquatic? Why, we find them in marine, river, or lake deposits that are not flood deposits, and they have aquatic-adapted skeletal traits), and some internet crazies.

Since underarm hair is more similar to pubic hair than that on your head, and develops at the same time, many scientists believe that it serves a purpose sexually, either as a display of sexual maturity (remember, from an evolutionary perspective your body still thinks we are naked) or as some sort of pheromone control system.

BO is sexy. No, really. Hair absorbs and retains moisture from sweat, which carries pheromones containing MHC markers. The resulting BO is a sexual signal of immune system dissimilarity, with females preferring males with a dissimilar MHC profile.

I dont know if other guys find woman-smell attractive (the research has not been done on men, to my knowledge, and there is the issue of my mate-selection processors being being morphologically female), but there are definitely guys I find more or less attractive based on their baseline "I showered this morning, but it is now five PM" BO. In fact, it is a deal-breaker for me. If I dont like th way someone smells, it is not happening.

[Ziggy Stardust]

BO is sexy. No, really. Hair absorbs and retains moisture from sweat, which carries pheromones containing MHC markers. The resulting BO is a sexual signal of immune system dissimilarity, with females preferring males with a dissimilar MHC profile.

I dont know if other guys find woman-smell attractive (the research has not been done on men, to my knowledge, and there is the issue of my mate-selection processors being being morphologically female), but there are definitely guys I find more or less attractive based on their baseline "I showered this morning, but it is now five PM" BO. In fact, it is a deal-breaker for me. If I dont like th way someone smells, it is not happening.

Oh, yeah, smell is a lot more important than most people realize. Completely anecdotally, I almost never use body washes or scented deodorants, just soap and simple unscented anti-perspirant, yet I constantly get compliments from girls about how good I smell. I always try to convince my friends that use cologne that it is a waste of time, but nobody trusts science.

I guess it is possible that underarm hair exists to wick the moisture for the purpose of "creating" the scent. I just don't know a whole lot about human pheromones. I've read the behavioral studies, but I don't know the physiology.

[Parricidium]

The prevailing theory that I know of as to why we're relatively naked: We were filthy as hell.

BO is sexy. No, really. Hair absorbs and retains moisture from sweat, which carries pheromones containing MHC markers. The resulting BO is a sexual signal of immune system dissimilarity, with females preferring males with a dissimilar MHC profile.

To my knowledge, which I admit may not be the most up-to-date, humans have a vomeronasal organ but we cannot detect pheromones. If you can find literature that shows we can, I'd be very interested - I'd love to be proven wrong about this.

People can smell good naturally without it being pheromones.

[Alyrium Denryle]

To my knowledge, which I admit may not be the most up-to-date, humans have a vomeronasal organ but we cannot detect pheromones. If you can find literature that shows we can, I'd be very interested - I'd love to be proven wrong about this.

Pheromone may not be the correct term, but we can pick up the MHC reliably:

That is just a couple. The literature has gotten... impressive these last few years.'


Bhutta, M. F. 2007. Sex and the nose: human pheromonal responses. Journal of the Royal Society of Medicine 100:268-274.

The chemosensory functions of the human nose are underappreciated. Traditional teaching is that the sense of smell detects volatile compounds, which may then allow the identification of substances that may be beneficial or harmful-such as good versus putrefied food. However, increasing evidence from research in other animals suggests that olfaction may serve another and more important purpose, that of mate selection in sexual reproduction; indeed, olfaction may be an essential impetus for evolution.

Cornwell, R. E., L. Boothroyd, D. M. Burt, D. R. Feinberg, B. C. Jones, A. C. Little, R. Pitman, S. Whiten, and D. I. Perrett. 2004. Concordant preferences for opposite-sex signals? Human pheromones and facial characteristics. Proceedings of the Royal Society of London Series B-Biological Sciences 271:635-640.

We have investigated whether preferences for masculine and feminine characteristics are correlated across two modalities, olfaction and vision. In study 1, subjects rated the pleasantness of putative male (4,16-androstadien-3-one; 5alpha-androst-16-en-3-one) and female (1,3,5(10),16-estratetraen-3-ol) pheromones, and chose the most attractive face shape from a masculine-feminine continuum for a long- and a short-term relationship. Study 2 replicated study 1 and further explored the effects of relationship context on pheromone ratings. For long-term relationships, women's preferences for masculine face shapes correlated with ratings of 4,16-androstadien-3-one and men's preferences for feminine face shapes correlated with ratings of 1,3,5(10),16-estratetraen-3-ol. These studies link sex-specific preferences for putative human sex pheromones and sexually dimorphic facial characteristics. Our findings suggest that putative sex pheromones and sexually dimorphic facial characteristics convey common information about the quality of potential mates.

Gangestad, S. W. and R. Thornhill. 1998. Menstrual cycle variation in women's preferences for the scent of symmetrical men. Proceedings of the Royal Society of London Series B-Biological Sciences 265:927-933.

Evidence suggests that female sexual preferences change across the menstrual cycle. Women's extra-pair copulations tend to occur in their most fertile period, whereas their intra-pair copulations tend to be more evenly spread out across the cycle. This pattern is consistent with women preferentially seeking men who evidence phenotypic markers of genetic benefits just before and during ovulation. This study examined whether women's olfactory preferences for men's scent would tend to favour the scent of more symmetrical men, most notably during the women's fertile period. College women sniffed and rated the attractiveness of the scent of 41 T-shirts worn over a period of two nights by different men. Results indicated that normally cycling (non-pill using) women near the peak fertility of their cycle tended to prefer the scent of shirts worn by symmetrical men, Normally ovulating women at low fertility within their cycle, and women using a contraceptive pill, showed no significant preference for either symmetrical or asymmetrical men's scent. A separate analysis revealed that, within the set of normally cycling women, individual women's preference for symmetry correlated with their probability of conception, given the actuarial value associated with the day of the cycle they reported at the time they smelled the shirts. Potential sexual selection processes and proximate mechanisms accounting for these findings are discussed.

Havlicek, J. and S. C. Roberts. 2009. MHC-correlated mate choice in humans: A review. Psychoneuroendocrinology 34:497-512.

Extremely high variability in genes of the major histocompatibility complex (MHC) in vertebrates is assumed to be a consequence of frequency-dependent parasite-driven selection and mate preferences based on promotion of offspring heterozygosity at MHC, or potentially, genome-wide inbreeding avoidance. Where effects have been found, mate choice studies on rodents and other species usually find preference for MHC-dissimilarity in potential partners. Here we critically review studies on MHC-associated mate choice in humans. These are based on three broadly different aspects: (1) odor preferences, (2) facial preferences and (3) actual mate choice surveys. As in animal studies, most odor-based studies demonstrate disassortative preferences, although there is variation in the strength and nature of the effects. In contrast, facial attractiveness research indicates a preference for MHC-similar individuals. Results concerning MHC in actual couples show a bias towards similarity in one study, dissimilarity in two studies and random distribution in several other studies. These vary greatly in sample size and heterogeneity of the sample population, both of which may significantly bias the results. This pattern of mixed results across studies may reflect context-dependent and/or life history sensitive preference expression, in addition to higher level effects arising out of population differences in genetic heterogeneity or cultural and ethnic restrictions on random mating patterns. Factors of special relevance in terms of individual preferences are reproductive status and long- vs. short-term mating context. We discuss the idea that olfactory and visual channels may work in a complementary way (i.e. odor preference for MHC-dissimilarity and visual preference for MHC-similarity) to achieve an optimal level of genetic variability, methodological issues and interesting avenues for further research. (C) 2008 Elsevier Ltd. All rights reserved.


Miller, S. L. and J. K. Maner. 2010. Scent of a Woman: Men's Testosterone Responses to Olfactory Ovulation Cues. Psychological Science 21:276-283.

Adaptationist models of human mating provide a useful framework for identifying subtle, biologically based mechanisms influencing cross-gender social interaction. In line with this framework, the current studies examined the extent to which olfactory cues to female ovulation-scents of women at the peak of their reproductive fertility-influence endocrinological responses in men. Men in the current studies smelled T-shirts worn by women near ovulation or far from ovulation (Studies 1 and 2) or control T-shirts not worn by anyone (Study 2). Men exposed to the scent of an ovulating woman subsequently displayed higher levels of testosterone than did men exposed to the scent of a nonovulating woman or a control scent. Hence, olfactory cues signaling women's levels of reproductive fertility were associated with specific endocrinological responses in men-responses that have been linked to sexual behavior and the initiation of romantic courtship.

Olsson, S. B., J. Barnard, and L. Turri. 2006. Olfaction and identification of unrelated individuals: Examination of the mysteries of human odor recognition. Journal of Chemical Ecology 32:1635-1645.

Although several studies have examined the effect of human odor on kin recognition and mate choice, few have focused on the impact of familiarity on recognition of nonrelatives by olfactory cues. As part of a program designed to engage students in scientific research, 53 high school students researched, planned, and implemented a project to analyze the effect of odor on human recognition of, and preference for, friends, sex, and self. A total of 37 students, including friends of their choosing, wore T-shirts for three consecutive nights. During that time, subjects were controlled for exposure to extraneous perfumes, household odors, and other humans. The students were then asked to smell a series of five shirts and evaluate them with respect to pleasantness. Students were also asked to identify the shirts belonging to themselves and their friend, and determine the sex of the person who wore each shirt. Although subjects were unable to distinguish sex by olfactory cues alone, a significant percentage of subjects were able to identify their own odor (51.6%), as well as distinguish the odor cue of their friend (38.7%). Additionally, subjects who could not identify their friend's cue were apt to choose the odor of a member of the opposite sex as their friend. This result was not believed to rely on odor preference as neither individual, friend, nor stranger odors were rated significantly different with respect to odor pleasantness. The ability to recognize friends via odor cues lends credence to the hypothesis that association and familiarity are important aspects of conspecific olfactory recognition in humans. Furthermore, this study augments evidence that olfaction may supplement visual and auditory cues used in human conspecific and kin recognition.


Roberts, S. C., A. Kralevich, C. Ferdenzi, T. K. Saxton, B. C. Jones, L. M. DeBruine, A. C. Little, and J. Havlicek. 2011. Body Odor Quality Predicts Behavioral Attractiveness in Humans. Archives of Sexual Behavior 40:1111-1117.

There is currently considerable interest in biometric approaches using human odor as a marker of disease or genetic individuality. Body odor is also thought to be used during mate choice to select genetically compatible mates. The idea that body odor reveals information about both genetic identity and genetic similarity is most readily tested by examining odor in twin pairs. However, although this idea can be traced back 130 years to Francis Galton in 1875, most studies using dogs fail to control for shared environmental effects associated with cohabitation. Here we show that odors of identical twins (but not dizygotic twins) can be matched by human sniffers at rates better than chance, even when the twins are living apart. In addition, matching frequencies for identical twin odors were not significantly different from those for duplicate odors from the same individual. These results indicate an important genetic influence on body odor and the potential for developing technologies for human odor printing in relation to underlying genotype.

[Sea Skimmer]

I work outside a lot, the difference between a shirt on and a shirt off is about on par with a 10-12 degree difference in temperature, and that’s with shorts on already. I avoid it because of sunburn and what not, but the difference can be between being able and unable to work in the summer. I can’t imagine how humans could do what they do if we were covered head to toe in fur in hot weather in a time before we have any means to carry water supplies. We'd be stuck just being immobile under shade for a big chunk of the day, which happens to be the way a lot of predator mammals act anyway. Chimps live in the forest and have more or less constant protection from direct sunlight, but humans walking upright are generally thought to have been adapting to more open terrain to start with.

To me the question is more like, why don’t see more mammals with low hair levels in the first place, the advantage in hot climates seem like it should be pretty immense. Protection against minor injury perhaps? Course the advantage humans gain also clearly has limits, since some people have a hundred times the body hair of others.

[Alyrium Denryle]

To me the question is more like, why don’t see more mammals with low hair levels in the first place, the advantage in hot climates seem like it should be pretty immense. Protection against minor injury perhaps? Course the advantage humans gain also clearly has limits, since some people have a hundred times the body hair of others.

A lot of it is camouflage and sexual signals.

[Sea Skimmer]

You could do a lot of that with skin tones though. You would loose signals like hair standing on end for fear reactions, but I don't see a factor like that being very important since it would also make posture more obvious.

[Alyrium Denryle]

You could do a lot of that with skin tones though. You would loose signals like hair standing on end for fear reactions, but I don't see a factor like that being very important since it would also make posture more obvious.

I was tired when I posted last night. Permit me to be more complete.

There are a few "design" considerations to deal with here.

1) sometimes (in fact, most of the the time) you want your signals to be obvious. You want that predator to think you are bigger than you really are. You want that competitor to think the same thing.

2) High temp is not the only part of thermoregulation. Cold matters too. Even in the warm regions of the world, if those regions are dry and thus have little atmospheric heat retention, the temperature can go from 40 degrees C to 12.

3) Locomotion--does hair get in the way?

After these, there is the question of whether or not losing one's hair will help you cool off. It is not always a given. You lose heat due to convection pretty slowly, unless water is involved. This is because your body is basically a big sack of water and its specific heat is much greater than air. The insulating boundary that hair is can help keep you warm when it is cold. High temperature is another story. If the external temperature is warmer than your body temp, you wont be losing heat from simple radiation. Not unless you can get rid of heat faster than the external environment adds it. There are only a few ways to do this. You can increase your body's surface area to volume ratio and use a shit load of capillaries through that extra surface area and permit convection (rather than radiation) to move across that heated skin. This is what elephants do. They even flap their ears to create an artificial breeze. The alternative is to use water. Most mammals pant. Humans sweat. For most mammals, hair does not retain the heat itself. In humans, hair (or a shirt) impedes the evaporation of water directly from the surface of your skin and prevents you cooling off (the water still evaporates. Just not off your skin). So, with hair you not only get overheated but you keep sweating and suffer from dehydration as well. Seems like a pretty strong selective pressure to get rid of hair to me.

[Ziggy Stardust]

2) High temp is not the only part of thermoregulation. Cold matters too. Even in the warm regions of the world, if those regions are dry and thus have little atmospheric heat retention, the temperature can go from 40 degrees C to 12.

And it is worth pointing out that this is the exact environment (dry, large noctural temperature drops) that early humans likely inhabited.

So, with hair you not only get overheated but you keep sweating and suffer from dehydration as well. Seems like a pretty strong selective pressure to get rid of hair to me.

To build on this, the selective pressures that we faced are not necessarily the same other species will face, even in the same environment. There are lots of ways in which animals in hot (or cold) environments thermoregulate that aren't as simplistic as "more hair for cold, less hair for hot". Both physiologically and behaviorally. I can't think of a good example in a hot environment, but my favorite clever thermoregulation example is used by (IIRC) penguins, dolphins, and wolves in extremely cold environments. Their veins and arteries in the extremities are laid out in very specific patterns to take advantage of the properties of countercurrent blood flow. Really cool stuff.