Monday, April 28, 2008

Why is human head hair so long?

Head hair is much longer than hair elsewhere on the body. This lengthening has involved several evolutionary changes: faster rate of growth, longer growing phase, increased density, and greater resistance to physical damage (Khumalo, 2005; Loussouarn et al., 2005). The multiplicity of these changes is consistent with sexual selection: the selective pressure seems to have acted on an overall visual effect rather than on one incidental factor. In some non-human primates, head hair has lengthened for apparently similar reasons, perhaps because visual attention tends to focus, as in humans, on the face and its surrounding frame (Darwin, 1936 [1888], p. 906).

Head hair has lengthened only in those human populations that have lived in the temperate and Arctic zones, including some that have back-migrated to the tropical zone, e.g., Austronesians in Southeast Asia and Oceania, Amerindians in the tropical New World. 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).

This point seems to be lost on those who think that hominids acquired long head hair at a very early date. Advocates of the 'Aquatic Ape Hypothesis', for example, believe that head hair lengthened during a putative aquatic phase of human evolution when an infant would have to hang on to its mother's hair while in the water "and if the hair floated around her for a yard or so on the surface [the infant] wouldn't have to make so accurate a beeline in swimming towards her" (Morgan, 1972, p. 36). Yet such lengthening could not have occurred until modern humans had begun spreading out of Africa—some 50,000 years ago at the earliest. This same point is also lost on those who argue that long head hair improved mating success among ancestral humans because it signified health and hence mate quality (Hinsz et al., 2001; Mesko and Bereczkei, 2004). Fine. But does health matter less in the Tropics?

In the literature, this lengthening of head hair outside the tropical zone is often ascribed to relaxation of natural selection: short frizzy hair helps dissipate body heat and thus loses its adaptive value in colder climates. But why would a colder climate cause head hair—and only head hair—to lengthen to such an extent and over such a short span of evolutionary time? If we turn to the sexual selection model, we see that non-tropical climates do alter selective pressures, but indirectly so—by altering human demography. When early modern humans left the Tropics, they entered an environment that reduced the supply of mateable men, thus making them a limited resource. This new environment intensified sexual selection of women and would have favored physical traits that retain visual attention, particularly from men.

Why would sexual selection favor longer-haired women? We know that most societies consider a greater amount of head hair to be an appropriate female characteristic (Synnott, 1987). Even in sub-Saharan Africa, where people are naturally shorthaired, women have traditionally lengthened their head hair with vegetable fiber, sinew, or hair from relatives, apparently to enhance their beauty (Bernolles, 1966; Sieber and Herreman, 2000). Head hair is identified with femininity partly because men begin to go bald as early as their 20s, but also because the scalp hairs of women seem to have a higher mean diameter and hence more volume, even in the naturally shorthaired New Guineans (Walsh and Chapman, 1966). It may be that men perceived a greater amount of head hair as more feminine from an early date, and this perceptual bias could have influenced male mate-choice wherever female-female rivalry for mates had become sufficiently intense.


Bernolles, J. (1966). Permanence de la parure et du masque africains. Paris : G.P. Maisonneuve et Larose.

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.

Hinsz, V.B., Matz, D.C., and Patience, R.A. (2001). Does women's hair signal reproductive potential? Journal of Experimental Social Psychology, 37, 166-172.

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

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

Mesko, N., and Bereczkei, T. (2004). Hairstyle as an adaptive means of displaying phenotypic quality. Human Nature, 15, 251-270.

Morgan, E. (1972). The Descent of Woman. London: Souvenir Press.

Sieber, R., and Herreman, F. (2000). Hair in African art and culture. African Arts, 33, 54-69.

Synnott, A. (1987). Shame and glory: a sociology of hair. The British Journal of Sociology, 38, 381-413.

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.

Monday, April 21, 2008

When European skin became white

Modern humans did not begin to lighten in skin color immediately after entering Europe some 35,000 years ago. In fact, these ancestral Europeans remained brown-skinned for tens of thousands of years. This is the conclusion now emerging from studies of skin color loci.

In 2005, a team of Japanese researchers found that the depigmentation of European skin was partly due to a relatively recent allele at the SLC45A2 (AIM1) gene. They dated the allele to c. 11,000 BP and concluded that it had rapidly supplanted the original allele through positive selection (Soejima et al., 2005).

Then last year, at the Annual Meeting of the American Association of Physical Anthropologists, a molecular anthropologist at the University of Arizona, Heather Norton, presented evidence that Europeans have a similarly recent allele at another skin color gene, SLC24A5. The new allele is dated to 12,000 – 3,000 BP. As she stated during her talk: "The [evolution of] light skin occurred long after the arrival of modern humans in Europe." (Norton & Hammer, 2007).

The challenge now will be to narrow the time window. Did the change happen during the last ice age? This would be before 10,000 years ago. Or did it happen later, after hunting and gathering gave way to agriculture? This would be 7,400 to 5,900 years ago on the North European plain, where European skin is whitest (Bogucki, 1989).

Both datings are consistent with the data currently available. The corresponding explanations, however, differ greatly. If European skin whitened 7,400 to 5,900 years ago, the cause may have been a diet less rich in vitamin D and, hence, selection for lighter skin to facilitate vitamin D synthesis. This is the explanation favored by a write-up in Science:

Either way, the implication is that our European ancestors were brown-skinned for tens of thousands of years--a suggestion made 30 years ago by Stanford University geneticist L. Luca Cavalli-Sforza. He argued that the early immigrants to Europe, who were hunter-gatherers, herders, and fishers, survived on ready-made sources of vitamin D in their diet. But when farming spread in the past 6000 years, he argued, Europeans had fewer sources of vitamin D in their food and needed to absorb more sunlight to produce the vitamin in their skin. Cultural factors such as heavier clothing might also have favored increased absorption of sunlight on the few exposed areas of skin, such as hands and faces, says paleoanthropologist Nina Jablonski of PSU in State College. (Gibbons, 2007)

This explanation falls apart, however, if European skin whitened earlier, such as during the last ice age. The cause may then have been sexual selection, i.e., stronger female-female competition for male mates because of limited polygyny and high male mortality (Frost, 2006, see earlier posts here and here). Preference for lighter-skinned women is attested in a wide range of traditional, premodern societies (van den Berghe & Frost, 1986). Under conditions of intense sexual selection of women, the selective advantage of lighter-skinned women would have entailed a relative depigmentation of the entire population, both women and men.


Bogucki, P. (1989). The Neolithic Mosaic on the North European Plain. Updated paper originally delivered at the Society for American Archaeology meeting in Atlanta, Georgia.

Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103.

Gibbons, A. (2007). American Association Of Physical Anthropologists Meeting: European Skin Turned Pale Only Recently, Gene Suggests. Science 20 April 2007:Vol. 316. no. 5823, p. 364 DOI: 10.1126/science.316.5823.364a

Norton, H.L. and Hammer, M.F. (2007). Sequence variation in the pigmentation candidate gene SLC24A5 and evidence for independent evolution of light skin in European and East Asian populations. Program of the 77th Annual Meeting of the American Association of Physical Anthropologists, p. 179.

Soejima, M., Tachida, H., Ishida, T., Sano, A., and Koda, Y. (2005). Evidence for recent positive selection at the human AIM1 locus in a European population. Molecular Biology and Evolution, 23, 179-188.

van den Berghe, P.L., and Frost, P. (1986). Skin color preference, sexual dimorphism and sexual selection: A case of gene-culture co-evolution? Ethnic and Racial Studies, 9, 87-113.

Tuesday, April 15, 2008

More on Rohrmann et al.

In a previous post on human variation in testosterone levels, I discussed the Rohrmann et al. study and its methodological flaws. This study tested black Americans, white Americans, and Mexican Americans in three age classes: 20-44; 45-69; and 70+. It found that even in the youngest age class the race differences were minor, with the Mexican Americans having the highest testosterone levels.

The main flaw here is that the black-white difference in testosterone levels shrinks after 24 years of age, is gone by the early 30s, and seems to reverse at older ages. So it should not show up in the average t levels of 20 to 44 year-olds. As for the Mexican Americans, their t levels were higher probably because they were younger on average. The median age of the 20-44 year-olds was 31.8 for the whites, 29.5 for the blacks, and 28.6 for the Mexican-Americans. This age disparity is probably understated by the use of medians rather than means, given the youthful age pyramid of the Hispanic-American population.

But another thing bothers me with this study. The 45-69 year old black men had higher t levels (5.62) than the 20-44 year old black men (5.35). This is more than a bit odd. Testosterone declines with age and the large number of participants rules out random sampling error.

Nor could recruitment bias be responsible. The authors of this study did not recruit their participants. In fact, they never even saw them. They used serum samples that the National Center for Health Statistics had earlier collected as part of its Third National Health and Nutrition Examination Survey (NHANES III). The authors state they used 1,479 samples that were still available out of an initial total of 1,998.

Uh, why were 519 samples no longer available? That comes to over 25% of the original total. What happened to them?

After some googling, I discovered that another study had used the NHANES-III serum bank for research on a sexually transmitted disease: Herpes Simplex virus type 2 (HSV-2).

The National Health and Nutrition Examination Surveys (1988–1994) (NHANES-III) reported that more than 25% of adults between 30 and 39 years of age were positive on serology for HSV-2 in those years. (Chorba et al., 2007, p. 655)

More than 25% ... Hmm, could these be the missing samples we’re looking for? If so, removing them would have disproportionately depleted the black American component of the study population:

Although HSV-2 infection is increasing among young Caucasians, who have a seroprevalence of approximately 17%, infection is more common among African-Americans, who have a seroprevalence of 45%. (Chorba et al., 2007, p. 656).

The study is written up in Fleming et al. (1997). The ‘Methods’ section describes why some serum samples were no longer available, including a “need to use serum for other tests”:

Of the persons originally selected for NHANES III, 82.5 percent were interviewed, and HSV-2 test results were available for 60.2 percent. The reasons that results were unavailable included the inability to locate the selected subject, refusal by that person to be interviewed or to have blood drawn, unsuccessful venipuncture, the need to use serum for other tests, and the loss of serum samples during transportation, storage, or processing.

When any serum samples tested positive for HSV-2, they were probably set aside for the duration of the study, with a view to further analysis. If, as seems likely, these samples were absent from the NHANES III serum bank when Rohrmann et al. undertook their study, their study population would have had disproportionately fewer participants with a polygynous sexual orientation and with correspondingly high testosterone levels. As Chorba et al. (2007, p. 687) point out, “The major known risk factors for acquiring genital HPV infection include having multiple sex partners and having sex partners who have had multiple partners.”

This data bias would explain Rohrmann et al.’s paradoxical finding of higher t levels in older black Americans. Young black Americans with high t levels were likely to test positive for HSV-2 and to have their samples removed from the NHANES III serum bank.


Chorba, T., Guoyu, T., and Irwin, K.L. (2007). Sexually transmitted diseases. In: Litwin MS, Saigal CS, editors. Urologic Diseases in America. US Department of Health and Human Services, Public Health Service, National Institutes of Health, National Institute of Diabetes and Digestive and Kidney Diseases. Washington, DC: US Government Printing Office, 2007; NIH Publication No. 07–5512 [pp. – 649-695]

Fleming DT, McQuillan GM, Johnson RE, Nahmias AJ, Aral SO, Lee FK, St Louis ME. (1997). Herpes simplex virus type 2 in the United States, 1976 to 1994. New England Journal of Medicine, 337, 1105-11.

Rohrmann, S., Nelson, W.G., Rifai, N., Brown, T.R., Dobs, A., Kanarek, N., Yager, J.D., Platz, E.A. (2007). Serum estrogen, but not testosterone levels differ between Black and White men in a nationally representative sample of Americans. The Journal of Clinical Endocrinology & Metabolism, 92, 2519-2525

Tuesday, April 8, 2008

Questions on skin color, beauty, and sexual selection

A student recently sent me the following questions:

-How does skin color influence beauty from a biological standpoint?

Women are lighter-skinned than men in all human populations. This sex difference seems to be wider in populations that are medium in color and narrower in populations that are very dark or very light in color. It appears at puberty and seems to be related to other changes that the skin undergoes at that time of life.

There are three theories as to why this sex difference exists:

1) Women have become lighter in skin color for the same reason that they have smoother skin, higher-pitched voices, and more 'baby-like' faces. These are all infant characteristics. In other primates, adults become less aggressive and more willing to provide care when they see an infant's physical features. So women may have evolved a lighter skin color and a more child-like physique as a means to reduce male aggression and to increase male care-taking behavior.

2) Men subconsciously use lighter skin as a visual cue for recognizing sexually mature women. Several controlled studies have shown that people can distinguish a man's face from a woman's by complexion alone, even when the picture is blurred and offers no other details. The gender cue seems to be the contrast between facial color and eye/lip color. According to this hypothesis, the sex difference in skin color began accidentally as a side-effect of the influence of sex hormones on skin pigmentation. Women then became even more light-skinned because men tended to select those women who looked unambiguously feminine.

3) Women evolved a lighter skin as a means to increase their production of vitamin D and hence meet their greater need for calcium during pregnancy and lactation.

These theories are not incompatible with each other. Once this sex difference had become established, for whatever reason (infantile mimicry, physiological side-effect, need for more vitamin D), it could have then become a visual cue for the algorithms that men use to recognize women and assess mate value.

-One theory states that all people came from Africa and migrated to other locations. Why would the migration of those groups lead to such radical differences between races, such as an African's wider nose, a European's fine hair texture, an Asian's almond shaped eyes and an Inuit's body structure?

Many of these physical differences do not seem to involve differences in adaptation to the physical environment. This is especially so for difference in hair and eye color and difference in hair length. It may also be partially true for differences in skin color. The Amerindians have inhabited North and South America from the Arctic to the Tropics for some 12,000-15,000 years, and yet they all have almost the same skin color.

Some people, including Darwin, have attributed these physical differences to the action of sexual selection (as opposed to natural selection by climate, sunlight, etc.). According to this hypothesis, criteria of beauty differ from one human population to the next and, over time, these differences have pushed sexual selection in different directions, with the result that human populations now differ considerably in appearance.

The problem with this hypothesis is that human populations don't seem to differ considerably in beauty criteria. A number of controlled studies have shown that people of varying ethnic/racial backgrounds share similar notions of beauty. In the most comprehensive of these studies, Cunningham et al. (1995) assessed criteria of female beauty among men of Taiwanese, White American, Black American, Asian, and Hispanic backgrounds. All of the subjects perceived a female face to be more attractive when possessing high eyebrows, widely spaced large eyes with dilated pupils, high cheekbones, small nose, narrow face with thin cheeks, large smile, full lower lip, small chin, and fuller hairstyle. There was some variation. The East Asian subjects tended to prefer more immature and inexpressive faces whereas the Black American subjects tended to prefer women with larger buttocks and heavier body build. These differences in beauty criteria seem to be minor and are certainly dwarfed by the differences in physical appearance that Darwin sought to explain.

I have argued that sexual selection has differed among human populations not only in its criteria but also in its intensity with respect to women and men. Intensity of sexual selection is determined by two factors: the overall ratio of men to women and the incidence of polygyny (which affects the ratio of men to women among those who are sexually available). These two factors varied among the different environments that humans inhabited during the hunter-gatherer and agricultural stages of prehistory.

Competition for mates correspondingly varied. In some environments, women competed against other women for a limited number of available men. In others, men competed against other men for a limited number of available women. Female-female competition for available men intensified with increasing distance from the equator. First, the demographic sex ratio became more female-biased because a lower density of game animals lengthened hunting distances and thus increased male mortality. Second, the operational sex ratio became more female-biased because polygyny became less frequent due to the longer winters that restricted food gathering and increased female dependence on male provisioning.

Women thus competed the most for mates in the 'continental Arctic,' i.e., steppe-tundra where wandering herds were almost the sole food supply. Inversely, men competed the most for mates in the Tropics, particularly where year-round agriculture replaced hunting and gathering, thereby allowing women to become primary food producers and freeing men to take more wives.
When too many of one sex compete for too few of the other, the sex in short supply can pick and choose. It can thus favor certain visible characteristics at the expense of others. If demand only somewhat exceeds supply, the characteristics will be those that matter beyond the immediate moment of selection, i.e., that indicate health, youth, or fecundity. Such criteria become relatively less important as demand outstrips supply. The more candidates there are for any one potential mate, the more they must vie for attention and the more success will depend on having characteristics that excite and retain attention.

This is the logic of advertising. 'Visual merchandising' matters most in saturated, highly competitive markets where the consumer is presented with many interesting and otherwise similar choices. When faced with a surplus of choice, an animal, like a consumer, is more easily swayed by eye-catching stimuli. For example, a secondary sexual characteristic may be larger or more vividly colored, thereby hyperstimulating the observer's sex-identification algorithms. Alternately, the visually enhanced feature may be used not for sex identification but for personal identification and communication (e.g., the eyes and other facial features). Either way, one's chances of mating are improved, as are the chances of passing on the eye-catching features.

Variation in mate competition may have influenced the evolution of highly visible and colorful physical traits, like hair length, eye/hair pigmentation, and perhaps even skin color. In particular, it may explain an unusual convergence of color traits in northern and eastern Europe. Within this region, both hair and eye color are highly polymorphic and skin is unusually white, almost at the physiological limit of depigmentation, and much whiter than in other populations at similar latitudes with similar exposure to solar UV at ground level. This 'European exception' constitutes a marked deviation from human geographic variation in hair, eye, and skin color.

The 'intensity of sexual selection' hypothesis predicts that among hunter-gatherers, and by extension early modern humans, female-female rivalry for mates should intensify with increasing distance from the equator, ultimately peaking in the steppe-tundra of the continental Arctic. Today, this environment is confined to the northern fringes of continental Eurasia and North America. During the last ice age, it lay further south and covered more territory, especially in Europe. The Scandinavian icecap had pushed the steppe-tundra zone far to the south and on to the plains stretching from southwestern France through northern Germany and into eastern Europe. This expanse of continental tundra-the largest during human prehistory-closely corresponds to the geographic distribution of hair/eye color dimorphism and extreme skin depigmentation.


Cunningham, M.R., Roberts, A.R., Barbee, A.P., Druen, P.B., and Wu, C-H. (1995). "Their ideas of beauty are, on the whole, the same as ours": consistency and variability in the cross-cultural perception of female physical attractiveness. Journal of Personality and Social Psychology, 68, 261-279.

Frost, P. (2007). Comment on Human skin-color sexual dimorphism: A test of the sexual selection hypothesis, American Journal of Physical Anthropology, 133, 779-781.

Frost, P. (2006). European hair and eye color - A case of frequency-dependent sexual selection? Evolution and Human Behavior, 27, 85-103