Is something afoot with Bigfoot?

 

Purported Yeti scalp at Khumjung Monastery (source). Has DNA been retrieved from it for the Oxford-Lausanne Collateral Hominid Project?

 

Over a year ago, geneticist Bryan Sykes launched the Oxford-Lausanne Collateral Hominid Project:

As part of a larger enquiry into the genetic relationship between our own species Homo sapiens and other hominids, we invite submissions of organic material from formally undescribed species, or “cryptids”, for the purpose of their species identification by genetic means. (Sykes, 2012)

The aim was to retrieve DNA from alleged remains of Yeti, Bigfoot, and the like. At the time, I was skeptical that anything would come of the project, since most remains of this type seem to be of dubious provenance. And then there’s the problem of contamination due to human handling.

So I was surprised to see this update on the project webpage, dated August 2013:

Thanks to all who have contributed samples to the project. We have collected and analysed over thirty samples and results are being prepared for publication. Following normal procedure, no results or other information will be available prior to publication, so please do not enquire.

Academics normally don’t like to publish negative results. When I googled the project name, I came across this report about Bryan Sykes meeting with people who had submitted samples of alleged Bigfoot remains: 

Interestingly, Professor Sykes has been visiting North America recently in order to speak with some of the researchers who have submitted samples, and he has also met with US Fish and Wildlife officials at one of their main laboratories located in Melford, Oregon. It has been revealed that Professor Sykes was in California very recently to speak with Justin Smeja, and also to be filmed for a documentary detailing his study which will be released on BBC Channel 4 once the results of the many samples tested by Sykes are published in a scientific journal. (Cooney, 2013)

Something seems to be afoot.  

References

Cooney, J. (2013). Exciting updates on the Oxford-Lausanne Collateral Hominid Project, Bizarre Zoology, June 12

http://bizarrezoology.blogspot.ca/2013/06/exciting-updates-on-oxford-lausanne.html

Sykes, B. (2012). Oxford-Lausanne Collateral Hominid Project, Wolfson College, University of Oxford

https://www.wolfson.ox.ac.uk/academic/GBFs-v/OLCHP

Women and eye color

Inheritance of eye color doesn’t follow a simple Mendelian model. Although the blue-eye allele (C) is less dominant than the brown-eye allele (T), CT heterozygotes aren’t necessarily brown-eyed and CC homozygotes aren’t necessarily blue-eyed. Even TT homozygotes are sometimes blue-eyed. There is also a sex difference, with women having a more diverse palette of eye colors. (source)

Most humans have black hair and brown eyes. Europeans display a much more diverse range of hues, their hair being also brown, flaxen, golden, or red and their eyes being also blue, gray, hazel, or green.

This diversification has gone farther in European women than in European men. According to a twin study, women have a more diverse palette of hair colors, with a greater prevalence of lighter shades, particularly red hair (Shekar et al., 2008).

Women also have a more diverse palette of eye colors, according to a recent study of six SNPs associated with eye color. When both copies of the so-called blue-eye allele are present, the resulting phenotype is more variable in women than in men (Martinez-Cadenas et al., 2013). This translates into a greater range of female eye colors in regions, like northern and eastern Europe, where blue eyes are the single most common phenotype (Walsh et al.,2012). As the study’s authors observe, “in populations with very high blue-eye frequency, such as Iceland or Holland, females show greater proportion of green eyes at the expense of blue eyes” (Martinez-Cadenas et al., 2013). The authors also confirmed earlier findings that eye color doesn’t follow a simple Mendelian model. Although the blue-eye allele (C) is less dominant than the brown-eye allele (T), CT heterozygotes aren’t necessarily brown-eyed and CC homozygotes aren’t necessarily blue-eyed. Even TT homozygotes are sometimes blue-eyed. 

Thus, both hair color and eye color tend to be more diverse in women. There is, however, one difference. With hair color, the “derived” alleles are more fully expressed in women than in men. With eye color, they are less fully expressed. This seems to argue against the theory that hair and eye color diversified through a process of sexual selection that acted more strongly on women than on men. Since blue eyes are the derived phenotype, they should be more fully expressed in women because the female sex is the main target of this selection pressure. Yet the reverse is actually true.

The reason may be physiological. It seems easier to produce new eye colors by modifying the way the blue-eye allele is expressed than by simply creating new alleles. Thus, to produce a range of hues that extends beyond brown and blue, the so-called blue-eye genotype must be more common than the actual incidence of blue eyes. As a result, “more females bearing the ‘blue eye genotype’ (HERC2/OCA2 CC homozygous genotype) end up having green or intermediate eyes” (Martinez-Cadenas et al., 2013).

Other recent studies

A recent paper has confirmed that European eye color diversified through some kind of selection pressure, and not random factors like genetic drift or founder effects. Blue-eye alleles show a very strong signal of selection (Donnelly et al., 2012). Another study, however, has failed to find any preference for blue eyes over other colors, an indication that all eye colors are at selective equilibrium, at least for the German population under study. This finding may be related to the already high frequency of blue eyes in that population:

Perhaps the frequency of eye colors plays a role. In most countries, blue eyes are less prevalent than other eye colors and may have the image of something special and more valuable. If this assumption is true, brown eyes should be preferred in countries where the majority of the population has blue eyes. (Gründl et al, 2012).

In this case, sexual selection is frequency-dependent, shifting to whichever eye color is least frequent. Eventually, an equilibrium is reached where color novelty is in balance with other characteristics, such as color brightness, that may increase sexual attractiveness.

This last finding shows the opportunistic nature of sexual selection. When too many of one sex have to compete for mating opportunities with too few of the other sex, there will be selection for any traits that increase mating success. In many cases, these traits will hyperstimulate a mental algorithm that is used for sex recognition. In other cases, hyperstimulation will simply involve use of bright or novel colors that can better engage visual attention and remain longer in memory.  

References

Donnelly, M.P., P. Paschou,  E. Grigorenko, D. Gurwitz, C. Barta, R-B. Lu, O.V. Zhukova, J.-J. Kim, M. Siniscalco, M. New, H. Li, S.L.B. Kajuna, V.G. Manolopoulos, W.C. Speed, A.J. Pakstis, J.R. Kidd, and K.K. Kidd. (2012). A global view of the OCA2-HERC2 region and pigmentation, Human Genetics, 131, 683–696.

http://europepmc.org/articles/PMC3325407

Gründl, M., S. Knoll, M. Eisenmann-Klein, and L. Prantl. (2012). The blue-eyes stereotype: Do eye color, pupil diameter, and scleral color affect attractiveness? Aesthetic Plastic Surgery, 36, 234–240.

http://link.springer.com/article/10.1007/s00266-011-9793-x#page-1

Martinez-Cadenas, C., M. Pena-Chilet, M. Ibarrola-Villava, and G. Ribas. (2013). Gender is a major factor explaining discrepancies in eye colour prediction based on HERC2/OCA2 genotype and the IrisPlex model, Forensic Science International: Genetics, 7, 453–460.

http://europepmc.org/abstract/MED/23601698

Shekar, S.N., D.L. Duffy, T. Frudakis, G.W. Montgomery, M.R. James, R.A. Sturm, & N.G. Martin. (2008). Spectrophotometric methods for quantifying pigmentation in human hair—Influence of MC1R genotype and environment, Photochemistry and Photobiology, 84, 719–726.

https://genepi.qimr.edu.au/contents/p/staff/shekaretal2008b1.pdf

Walsh, S., A. Wollstein, F. Liu, U. Chakravarthy, M. Rahu, J.H. Seland, G. Soubrane, L. Tomazzoli, F. Topouzis, J.R. Vingerling, J. Vioque, A.E. Fletcher, K.N. Ballantyne, and M. Kayser. (2012). DNA-based eye colour prediction across Europe with the IrisPlex system, Forensic Science International: Genetics, 6, 330–340.

http://www.fsigenetics.com/article/S1872-4973(11)00144-X/abstract

Facial contrast and femininity

 

How to hyperstimulate a sex-recognition algorithm. Women have higher luminous contrast between their facial skin and their lip/eye color. This contrast effect is influenced not only by degree of lightness but also by degree of redness (source).

Women are fairer in complexion because their skin has less melanin and less blood (Edwards and Duntley, 1939). Male castrates similarly look pale (Edwards et al., 1941). For this reason, relative lightness or darkness of skin seems to be key to sex recognition. People especially focus on the luminous contrast between facial skin and the color of the lips and eyes (Russell, 2009; Russell, 2010; see also Dupuis-Roy et al., 2009):

The luminance contrast between the eyes and the surrounding skin and the lips and the surrounding skin has been termed ‘facial contrast’. Female faces have greater facial contrast than male faces, and facial contrast plays an important role in sex classification and the perception of masculinity and femininity and also attractiveness (Porcheron et al., 2013)

This contrast effect has influenced the development of cosmetics. Different societies, sometimes independently of each other, have invented ways to increase facial contrast by darkening the lips and eye area and, conversely, by lightening facial skin (Russell, 2009; Russell, 2010).

Ancient Egypt was an early center for the development of cosmetics (Dayagi-Mendels, 1989). Indeed, the Egyptians “had most of the cosmetic aids which have ever been devised” (Corson, 1972, p.8), including rouge for the lips and cheeks, eyeliner (kohl), eyeshadows, and foundation […] In Mesopotamia, pots of colored paints for the eyes, and rouges for the lips have been found in Sumerian tombs near Ur from 5,000 years ago.

[…] Excavations at Harappa and Mohenjo-daro have found kohl pots and sticks for lining the eyes, as well as red iron oxides and white lead-based compounds that have been surmised to be rouge for the lips and cheeks and foundation for lightening the skin […] Evidence for ancient uses of received cosmetics in the East Asia is less clear, though there is a long history of the use of white face paint and rouge for the lips in China and Japan (Corson, 1972). Overall, there is evidence for the idea that the received style of cosmetics developed in multiple centers of early technology development, and spread outward to peripheral areas such as Europe and Southeast Asia, analogous to the spread of other technologies like agriculture and writing. (Russell, 2010)

This contrast effect may have likewise influenced evolution within our species. The most common human phenotype combines relatively light skin with dark hair and dark eyes. The opposite pattern is biologically possible but rare. In addition, the whitening of European skin might have provided a sufficient degree of luminous contrast for hair and eye color to diversify.

Not just luminous contrast

Facial contrast involves differences not only in luminosity, i.e., lightness/darkness, but also in hue, particularly the redness of the lips versus the lack of ruddiness of adjacent facial skin. Contrast in luminosity and hue declines with age and may provide a means for distinguishing younger from older women:

Several aspects of facial contrast – the luminance and color differences between the facial features and the skin surrounding those features – were found to decrease with age. These included the luminance contrast around the eyes and eyebrows, the red-green (a*) contrast around the mouth and eyes, and the yellow-blue (b*) contrast around the eyes. (Porcheron et al., 2013)

Hue contrast explains why redness—lipstick, red clothing, etc.—seems to enhance femininity even though women are the less ruddy sex. Red colors highlight this lack of ruddiness, thereby stimulating the mental algorithm that people use for sex recognition:

Stephen & McKeegan [26] found that people increase the redness (a*) of the lips to make a female face appear more feminine and attractive. Elliot & Niesta [46] found that pictures of women are perceived by men as more attractive and sexually desirable when they are associated with the color red–whether by the placement of a red border around the picture, or the presence of a red colored shirt on the woman. (Porcheron et al., 2013)

Some questions remain unanswered. Why, for instance, do many women redden their cheeks? Are the cheeks a secondary contrast zone? And what about women from racial groups with dark facial skin?

This last question seems to have given the study’s authors some trouble with the reviewers. Evidently, darker-skinned women still have some facial contrast. They just don’t have as much, probably because they don’t need as much. Dark skin tends to characterize highly polygynous societies; in fact, the correlation between darkness of skin and polygyny rate is greater than the one between lightness of skin and latitude (Manning et al., 2004). In such societies, women are almost sure to get mated and remain so as long as they are fertile. There is thus only weak selection for those who can better attract potential mates and more strongly stimulate sex-recognition algorithms. 

On a final note, it’s no coincidence that the polygyny rate correlates inversely with latitude. In simple tropical societies, especially those with year-round farming, women can more easily provide for themselves and their children without male assistance. Since women cost so little to support, men will tend to seize all mating opportunities. A man will have as many wives as he can get, and not as many as he can support.

 

References

Dupuis-Roy, N., I. Fortin, D. Fiset, and F. Gosselin. (2009). Uncovering gender discrimination cues in a realistic setting, Journal of Vision, 9(2), 10, 1–8. http://journalofvision.org/9/2/10/ 

Edwards, E.A. and S.Q. Duntley. (1939). The pigments and color of living human skin, American Journal of Anatomy, 65, 1-33.

http://onlinelibrary.wiley.com/doi/10.1002/aja.1000650102/abstract;jsessionid=43509EAC6110062BBF5676995DA16C6B.d01t01?deniedAccessCustomisedMessage=&userIsAuthenticated=false 

Edwards, E.A., J.B. Hamilton, S.Q. Duntley, and G. Hubert. (1941). Cutaneous vascular and pigmentary changes in castrate and eunuchoid men, Endocrinology, 28, 119-128.

http://endo.endojournals.org/content/28/1/119.short 

Manning, J.T., Bundred, P.E., and Mather, F.M. (2004). Second to fourth digit ratio, sexual selection, and skin colour, Evolution and Human Behavior, 25, 38-50.

http://www.ehbonline.org/article/S1090-5138(03)00082-5/abstract 

Porcheron, A., E. Mauger, and R. Russell (2013). Aspects of facial contrast decrease with age and are cues for age perception, PLoS ONE 8(3): e57985

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0057985 

Russell, R. (2010). Why cosmetics work. In Adams, R., Ambady, N., Nakayama, K., & Shimojo, S. (eds.) The Science of Social Vision, New York: Oxford.

http://public.gettysburg.edu/~rrussell/Russell_SocialVision_cosmetics_chapter.pdf 

Russell, R. (2009). A sex difference in facial contrast and its exaggeration by cosmetics, Perception, 38, 1211-1219

http://public.gettysburg.edu/~rrussell/Russell_2009.pdf 

Great hair ... and how it evolved

Crystal Gayle, American country music singer (source). In humans of Eurasian origin, head hair can grow down to the mid-back and even farther. Long silky hair must have evolved relatively late, certainly no earlier than the last 50,000 years.

 

All of us are born pale, and this infant pallor is striking in otherwise dark-skinned families. Lighter skin also characterizes women, who in this and other ways seem to mimic certain visual, tactile, and auditory aspects of infants (paedomorphic face, soft hairless skin, higher pitch of voice, etc.). Skin tone is, in fact, a key input for sex recognition, particularly the luminous contrast between facial skin and the eyes or lips. This visual cue becomes critical to the mating success of women wherever the supply of mateable men is limited, the result being sexual selection for lighter-skinned women and, thus, a gradual lightening of mean skin color not only for women but for the entire population as well.

Head hair, silkier and longer …

Head hair may have followed a similar evolutionary trajectory. In sub-Saharan Africa:

[…] the majority of African babies are not born with springy tight curls, the African child at birth is either bald or has silky loose curls similar to the Jheri curls. The springy tight curls develop within the first year of life but a few negroid Africans retain their silky hair type for life. (Ajose, 2012)

As ancestral humans spread north and out of Africa, loose silky hair began to persist beyond early childhood and became lifelong. Several changes were involved: faster rate of growth, longer growing phase, increased density, and greater resistance to physical damage (Khumalo, 2005; Loussouarn et al., 2005).

This process seems to have gone further in women, their scalp hairs having higher mean diameter and hence more volume, even in the naturally shorthaired New Guineans (Walsh and Chapman, 1966). Men also tend to lose their head hair, often as early as their 20s. In his review of the literature, Sigler (2011, p. 13) concludes that hair growth rate and final length are somewhat greater in women than in men. It looks as if this hair lengthening was driven by a selection pressure that acted primarily on women.

… but not in all humans

Long silky hair isn’t universal in our species. It exists only in those humans who are native to temperate and arctic regions or in those whose ancestors have back-migrated to the tropics, i.e., tropical Amerindians and Austronesians. Darwin noted "the extraordinary difference in the length of the hair in the different races; in the negro the hair forms a mere curly mat; with us it is of great length, and with the American natives it not rarely reaches to the ground" (Darwin, 1936 [1888], p. 906).

If long silky hair evolved outside the tropics, it must have appeared after ancestral humans had begun to spread out of Africa, some 50 thousand years ago. This point is lost on many paleoanthropologists who attribute this evolutionary change to much earlier events, like the discovery of fire or even a hypothetical aquatic phase of our hominid past.

Whatever the initial cause, there is a consensus, going back to Charles Darwin, that long silky hair is primarily ornamental and due to sexual selection: “we know that long tresses are now and were formerly much admired, as may be observed in the works of almost every poet; St. Paul says, "if a woman have long hair, it is a glory to her” (Darwin, 1936 [1888], p. 906). This view was part of his more general one that “the differences between the races of man, as in colour, hairiness, form of features, &c., are of a kind which might have been expected to come under the influence of sexual selection” (Darwin, 1936 [1888], p. 556).

Darwin believed that sexual selection produced physical differences among human populations through differing notions of beauty. He even raised the possibility that “each race would possess its own innate ideal standard of beauty” (Darwin, 1936 [1888], p. 890). An alternate view, which I favor, is that innate notions of beauty are similar in all humans. To the extent that physical differences among human populations are due to sexual selection, the reason is that this selection pressure has been stronger in some populations than in others (Frost, 2008). 

Why, then, did head hear grow longer as humans left the tropics?

As ancestral humans spread out of the tropics, women found it harder to gather food during the cold season and had to rely more on men to get food for themselves and their children. Polygyny accordingly became costlier for men, with the result that only the ablest hunters could take additional wives. At the same time, men had to roam over larger hunting territories because the density of wildlife was lower. Hunting-related mortality thus rose among young men.

For these two reasons—less polygyny and higher male mortality—the non-tropical zone tended to have a lower ratio of men to women on the mate market. More women had to compete for fewer available men, thus shifting the pressure of sexual selection from the latter to the former. The situation was not unlike that of actresses lining up for a part in a Hollywood movie. When all of the candidates seem perfect for the job, even little details can make a big difference.

One of those details was head hair. Men do notice long silky hair, if only as a visual input for recognition of women or infants, and such noticeability may be a dealmaker in a mate market where women are in excess supply. In such ancestral environments, head hair would have grown longer and longer with each generation.

Was there gene-culture co-evolution?

This lengthening may have been helped by gene-culture co-evolution. In other words, there was a transitional period when women used artificial means to lengthen their head hair. The resulting cultural expectation for longer hair may have favored the mating success of naturally long-haired women.

This hypothesis has support in the apparently ancient time depth of hair lengthening in Africa. As Khumalo (2008) notes:

Africa as the cradle of mankind is likely to have had hair care since the beginning of human existence. Partly because of the oral tradition of passing down history, it is difficult to corroborate evidence of hair care. But probably the earliest form of hair straightening was the molding of hair into shapes using various clays and mud (e.g., indicating the station of a married woman among the Zulu’s). […] Hair was also lengthened with fibers and grasses, much as is done for braids with synthetic extensions nowadays. Although small decorative comb-like structures have been discovered with archeological finds, it is not clear whether original Africans combed their hair or if these implements were purely decorative. Although not written down, fascinating stories of more recent hair care (and hair disasters) are often told by older women about straightening hair using hot stones even before hot combs became available.

West African women invest much time and effort in lengthening their head hair. This isn’t just an obsession of African American women, as some may think: 

“Big hair,” “plenty of hair,” “much hair”—West African communities, including Mende, admire a fine head of long, thick hair on a woman. Both these elements are crucial: thickness and length. Thickness equals increase in the number of individual strands, and the length is proof of strength. Growing such luxuriant hair requires a Mende woman’s patience and care. Because a man’s hair is kept shaved or cut close to the scalp, people say that “men don’t have hair.” Beautiful hair thus is a distinctly female trait; the more of it, the more feminine the woman (Boone,1986, p. 184).

This hairdressing tradition is ancient enough to have spawned myths:

It is known among Mende that all the “water people,” angels, have marvelous hair. The mermaid Tingoi is known by her long, wavy hair and her glamorous habit of dressing it with a golden comb while seated on a rock. A little girl with especially long hair is feared to be in danger of drowning because she will be very attractive to the “water people,” who may think she is one of them and wish her to join them. (Boone, 1986, p. 192)

Among African Americans, women braided and threaded their hair from an early date, whereas men often shaved their heads—an indication that head hair was deemed to be a female characteristic (White and White, 1995).

In sum, hair lengthening seems to be an indigenous tradition whose origins precede not only enslavement in the Americas but also the first contacts with Europeans in Africa. Its purpose has always been to look more feminine … and not “white.”

From artificial to natural

In sub-Saharan Africa, sexual selection was never strong enough to favor women with naturally long hair. This selection pressure would have steadily increased, however, as ancestral humans spread farther north into environments that tended to limit polygyny and boost male mortality.

Head hair may have lengthened during the initial phase of this increasingly intense sexual selection. Eventually, peak intensity was reached as humans spread into the habitable steppe-tundra of northern and eastern Europe—where women had few opportunities for food gathering and where men had to hunt migratory herds of game animals over long distances. It was this later phase that likely produced the diverse palette of hair and eye colors in present-day Europeans, as well as their strange, albino-like skin.

References 

Ajose, F.O.A. (2012). Diseases that turn African hair silky, International Journal of Dermatology, 51 (supp. S1), 12-16

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-4632.2012.05556.x/full 

Boone, S.A. (1986). Radiance from the Waters: Ideals of Feminine Beauty in Mende Art, New Haven and London

http://books.google.fr/books?id=anjOC0zZ6kgC&printsec=frontcover&dq=Radiance+from+the+Waters:+Ideals+of+Feminine+Beauty+in+Mende+Art&hl=fr&sa=X&ei=kTIBUoHfJYfu8ATZ9YGQBw&ved=0CDsQuwUwAA#v=onepage&q=Radiance%20from%20the%20Waters%3A%20Ideals%20of%20Feminine%20Beauty%20in%20Mende%20Art&f=false

Darwin, C. (1936) [1888]. The Descent of Man and Selection in relation to Sex, reprint of 2nd ed., The Modern Library, New York: Random House.

Frost (2008). Sexual selection and human geographic variation, Special Issue: Proceedings of the 2nd Annual Meeting of the NorthEastern Evolutionary Psychology Society. Journal of Social, Evolutionary, and Cultural Psychology, 2(4),169-191. http://137.140.1.71/jsec/articles/volume2/issue4/NEEPSfrost.pdf 

Khumalo, N.P. (2008). On the history of African hair care: more treasures await discovery, Letter to the Editor, Journal of Cosmetic Dermatology, 7, 231.

http://onlinelibrary.wiley.com/doi/10.1111/j.1473-2165.2008.00396.x/abstract 

Khumalo, N.P. (2005). African hair morphology: macrostructure to ultrastructure, International Journal of Dermatology, 44(Suppl. 1), 10-12.

http://onlinelibrary.wiley.com/doi/10.1111/j.1365-4632.2005.02805.x/abstract 

Loussouarn, G., El Rawadi, C., and Genain, G. (2005). Diversity of hair growth profiles. International Journal of Dermatology, 44(Suppl. 1), 6-9.

Sigler, R. (2011). Our Long Hairitage, Bloomington (Indiana): WestBow Press.

http://books.google.fr/books?id=gUEct0Dxt2IC&printsec=frontcover&dq=our+long+hairitage&hl=fr&sa=X&ei=9SsBUvP0OIaQ9gS-xoDQBg&ved=0CDUQ6AEwAA#v=onepage&q=our%20long%20hairitage&f=false 

Walsh, R.J., and Chapman, R.E. (1966). A study of the quantitative measurement of human head hair fibres, Man, new series, 1, 226-232.

http://www.jstor.org/discover/10.2307/2796348?uid=3737720&uid=2&uid=4&sid=21102539793897 

White, S. and G. White. (1995). Slave Hair and African American Culture in the Eighteenth and Nineteenth Centuries, The Journal of Southern History, 61, 45-76.

http://www.jstor.org/discover/10.2307/2211360?uid=3737720&uid=2&uid=4&sid=21102539793897

Perception of skin color in sub-Saharan Africa

 

Beyoncé Knowles, 2012. Is skin bleaching consistent with indigenous African values? (source)

A Zambian-born sociologist visited his home village with his white American wife and two of their children. Having lost his way, he asked an elderly lady for directions. She gladly told him:

But then she said, addressing his boys in the car, in the Tumbuka bantu African language:

"Monile asungwana, muli uli?" ("Greetings girls, how are you?")

At first, the author was flustered as he assumed the woman could not see very well since the boys were sitting in the car and their full bodies and clothes were concealed from her. The author corrected her. She didn't seem perturbed at all. He thought to himself how could she or anyone not see that these boys had no breasts, were not wearing earrings, blouses, or dresses. His pubescent fifteen year-old had even the beginnings of dark whiskers around his chin. The author dismissed the incident and did not reflect on it again. But a couple of times again, total strangers at a glance, while the two boys were sitting in the vehicle, referred to them in the Tumbuka language as "asungwana" or "those girls". What surprised him was that these comments were not made in a mean way or out of visual perceptual error. (Tembo, 2010, p. 5)

He came back to the United States and began writing up an article about his visit. For this, he had to select a few photos from the hundreds he had taken of men, women, and children from the village. Only then did the answer to his puzzle dawn on him:

His children, who have quite pronounced features of a Sub-Saharan African, but are light skinned compared to many relatives in the village, stood out in all the group photographs. There was nothing unusual about this obvious reality. But then he noticed something very subtle; all women had lighter glowing ambience to their skins than men although both men and women had dark skin tones. Some women had a definite characteristic glow to their lighter dark skin compared to the other women and the men. (Tembo, 2010, p. 6)

Women are in fact lighter-skinned than men throughout the world, although this sex difference is larger in populations of medium color than in those that are very pale or very dark (Frost, 2007; Madrigal and Kelly, 2006; van den Berghe and Frost, 1986). Girls become lighter-skinned than boys from puberty onward, apparently as part of sexual maturation. For one thing, this post-pubescent lightening correlates in girls with the post-pubescent thickening of subcutaneous fat (Mazess, 1967). For another, it correlates with the digit ratio—a marker of the degree of prenatal estrogenization (Manning et al., 2004).

This sexual dimorphism is paralleled by a traditional tendency to associate women with the lighter end of the local spectrum of complexions. Hence, the ideal woman was said to be "white" in Europe and East Asia, "golden" in South-East Asia, and "red" in sub-Saharan Africa.

The term "red" may puzzle non-Africans. It actually means a reddish-brown-orange complexion, which is the lightest color that occurs locally in normal individuals:

[...] in the Tumbuka bantu African language people describe women's beauty saying: "Mwanakazi mswesi ndiye muwemi comene" which translates as "A woman who is red-skinned is most attractive". This is more accurate than what would be the conventional translation: "A woman who is light skinned is most attractive" (Tembo, 2010, p. 10).

When Africans speak of a beautiful woman, they may describe her as "red" or even "white," thus seeming to emulate European standards of beauty. On this point, Tembo (2010, p. 12) quotes the lyrics of a Zambian song from the 1950s, "Maggie": 

Ndikonda miyendo yako

Ndifiga yako
Maggie ulinso
Mkazi woyera 

A literal translation would be:

I love your legs

Your figure too
Maggie you are also
A white woman 

Woyera does mean "white" but not in an ethnic sense. Maggie is simply an African woman with a naturally light complexion. Tembo argues that Africans and non-Africans alike have misconstrued this indigenous norm of female beauty as something that European colonialism has imposed. While not condoning skin bleaching, he argues that this widespread practice among modern African women is a logical consequence of indigenous aesthetic values.  

A cultural norm for African women?

This female norm is attested in many other sub-Saharan societies: 

Bambara (Mali)

The Bambara are not unmoved by the beauty of a woman's form; they can distinguish a well-formed body from a malformed one, a pretty woman from an ugly one, and they find a coppery skin more attractive than one of ebony black. (Henry, 1910, p. 217)

Tallensi (Ghana)

In skin colour they vary from black through chocolate brown to bronze, which the natives call "red" (bon-ze'e) and regard as the most attractive bodily hue. (Fortes, 1945, p. 7)

Hausa (Nigeria)

Light skin colour, referred to as "red", ranks high in the Hausa criteria of beauty; many variations of colour, from black to a very light reddish brown are seen. (Smith, 1965, p. 264)

Ibo (Nigeria)

In Ibo culture, however, these yellowish or reddish complexions are considered more beautiful than the darker, 'blacker,' complexions. [...] It is true that, in West Africa, government has for many years been identified with pale-skinned Europeans, but the Ibo evidence suggests that preference for paleness of complexion is indigenous. (Ardener, 1954, pp. 71-72)

Azande (Sudan)

Of the women and girls, some with babies, he kept the most beautiful in Zande eyes, those brightest of eye and clearest of skin and with full breasts, for his couch. (Evans-Pritchard, 1937, p. 60)

Berti (Sudan)

Men and women affirm without any hesitation that men are black, hot and hard and women are white, cold and soft. (Holy, 1988, p. 471) 

Somali (Somalia)

Men appreciate women of good height and stature, with good hips and breasts, and plump but not fat. A reddish tinged skin is thought highly of in preference to a dark dull black. (Lewis, 1962, p. 13)

Masai (Kenya, Tanzania)

Further requirements for being regarded as beautiful are an oval face, white teeth, black gums, a skin color as light as possible ... (Merker, 1910, p. 18)

Rundi (Rwanda, Burundi)

Beauty does not count very heavily, but a man is not displeased if people notice that his wife is attractive and well-fleshed, has a long and narrow nose, a light skin, and is somewhat like a cow. (Albert, 1963, p. 203)

Ganda (Uganda)

There is, in respect of the ordinary negroid complexion, a preference for paleness deeply rooted in the Ganda ideal of beauty. [...] The Ganda concept of skin pigmentation considers light coloured complexions to be differing shades of white. A dark brown skin colour is said to be eruyeru, that is, somewhat white. A really brown-reddish-yellow person is said to be mweru = white, which in comparison would be considered to be blonde; and this in the Ganda aesthetic language is considered as red = myufu, the most perfect skin pigmentation. (Lugira, 1970, pp. 34-35)

Nairobi (Kenya)

In the future the increasing use of skin lightening creams such as "Ambi" may eventually reduce the importance of natural skin color. But whatever the case, in Nairobi of the 1960's, as throughout much of Kenya, the lighter "brown" girls are usually considered to be more beautiful than "black" girls — and the more successful prostitutes are invariably "brown." (McVicar, 1969, p. 242) 

Ila, Lunda, Luvale, and Chokwe (Zambia)

Here too words meaning literally "white" are commonly used to refer to light skins though "red" may also be used. Light skins are admired just as much as is shown to occur among the Ibo, and young girls discussing the possible attractions of various young men have often been heard to emphasize "very black" as a point against someone. In the past at least one attraction of a light skin apart from its intrinsic appeal was the fact that the tattooing stood out against it in strong contrast. Very black skins are not infrequently thought to go hand in hand with inherited witchcraft and a light skin to indicate its absence. Dark-skinned women conscious of their possible disadvantage have been heard to tell men that light-skinned women will be found to be sexually unsatisfying. (White, 1954)

Ngoni (Malawi)

Young men say that what they like in a girl is a light skin colour, a pretty face, and the ability to dance and to copulate well. (Barnes, 1951, p. 30)

Kgatla (Botswana)

[...] the generally admired type is a light-skinned girl of somewhat heavy build, with prominent breasts and large, firm buttocks. (Schapera, 1966, p. 46)

A cultural norm for African infants?

Lighter skin is a norm not only for African women but also for African infants. All humans, in fact, are born pale (Grande et al., 1994; Kahlon, 1976; Walsh, 1964). This pallor is a striking contrast to the darker color of adult Africans. In Kenya, newborn infants are often called mzungu ('European' in Swahili), and a new mother may tell her neighbors to come and see her mzungu (Walentowitz, 2008). Among the Tuareg, children are said to be born "white" because of the freshness and moisture of the womb (Walentowitz, 2008). The cause is often thought to be a previous spiritual life:

There is a rather widespread concept in Black Africa, according to which human beings, before "coming" into this world, dwell in heaven, where they are white. For, heaven itself is white and all the beings dwelling there are also white. Therefore the whiter a child is at birth, the more splendid it is. In other words, at that particular moment in a person's life, special importance is attached to the whiteness of his colour, which is endowed with exceptional qualities. (Zahan, 1974, p. 385)

Another Africanist makes the same point: "black is thus the color of maturity [...] White on the other hand is a sign of the before-life and the after-life: the African newborn is light-skinned and the color of mourning is white kaolin" (Maertens, 1978, p. 41).

What skin color originally meant to humans … and to ancestral primates

In sub-Saharan Africa, perhaps more so than elsewhere, we see the older, non-ethnic way of perceiving skin color, where darker skin means an adult male and lighter skin an infant or an adult female. Facial skin color can also signal whether a woman is younger or older, specifically through the degree of luminous contrast between her face and her lips or eyes (Porcheron et al., 2013). In this older perceptual system, skin color might unconsciously prepare the observer for situationally appropriate behavior, e.g., if the observed person is a woman or an infant, the mental threshold is raised for expression of aggressive impulses and lowered for expression of caring (Guthrie, 1970).

The pale skin of infants seems to be the oldest component of this system, being present even in non-human primates. Among langurs, baboons, and macaques, the skin is pink in newborns and almost black in adults (Jay, 1962). The infant coloration apparently does more than help parents find wayward offspring. As it disappears with age, juveniles no longer arouse the same interest, are less often sought out and held by adult females, and cease to arouse defensive reactions from adults when humans approach (Alley, 1980; Booth, 1962; Jay, 1962). 

Although there are no primate species where the adult female has the infant's pink skin, there are some where fur coloration shows this kind of neoteny. Of the eight primate species where adult males and females differ in coat color, seven are characterized by persistence of the infant's lighter coloration into adulthood among females. Interestingly, 63% of these dichromatic species are monogamous, versus only 18% of all primate species (Blaffer-Hrdy and Hartung, 1979). By retaining a lighter infant-like color, the female might better cope with the riskier social environment of monogamy, which makes her more vulnerable to male aggression and to insufficient provisioning because of longer and more continuous cohabitation.

So why aren’t we all light-skinned?

If lighter skin is perceived as a female trait, even to the point of becoming an unconscious input for sex recognition, wouldn’t it be favored by sexual selection? And since skin color is only partly sex-linked, wouldn’t selection for lighter-skinned women end up lightening both sexes? Humans everywhere would have therefore lightened in color right up to the end point of white skin. So why hasn’t this happened?

There are two reasons: (1) sexual selection of women hasn’t been equally strong everywhere; and (2) this sexual selection has been offset to varying degrees by natural selection for darker skin.  

First, sexual selection of women is weaker in sub-Saharan Africa because of the higher polygyny rate: 20-50% of all marriages in the West and Center and 15-30% in the East and South (Pebley and Mbugua, 1989). With too many men competing for too few women, the pressure of sexual selection is shifted from women to men, and selection is thereby weakened for desirable female traits. This may be why high-polygyny populations in sub-Saharan Africa are visibly darker-skinned than low-polygyny ones (Frost, 2008).

Second, sexual selection for lighter female skin can be offset by natural selection for darker skin—as a means to protect against UV and such adverse effects as sunburn, skin cancer, and loss of folic acid. This is especially true in the tropics. Since UV protection becomes less necessary farther away from the equator, Aoki (2002) has argued that the sex difference in skin color should become correspondingly larger, being less constrained by natural selection. In reality, it’s largest at medium latitudes among populations that are medium in skin color (Frost, 2007; Madrigal and Kelly, 2006). It’s actually smaller in white-skinned northern Europeans. This is probably because of a ‘ceiling effect’: in northern Europe, women are less able to become lighter-skinned than men because both sexes are already close to the physiological limit of depigmentation.

 

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