A people of many colors
Human eye colors (Prof. Richard A. Sturm, University of Queensland)
Europeans have many hair and eye colors. Those two polymorphisms arose about the same time in ancestral Europeans, while not sharing the same genetic causes. They also differ significantly between men and women.
Europeans have a surprising variety of hair and eye colors. Hair is not only black but also brown, flaxen, golden, or red. Eyes are not only brown but also blue, gray, hazel, or green.
The various hair colors are produced by alleles at over 200 loci on the genome (SNPs), and the various eye colors by alleles at over 124 loci (Morgan et al. 2018; Simcoe et al. 2021). Those alleles arose over a relatively short time, certainly less than the forty to fifty thousand years that modern humans have been in Europe. Only some kind of selection, and very strong selection at that, could have caused so many alleles to proliferate over so little time.
To resolve this evolutionary puzzle, people often put forward the following scenario. As humans moved out of the Tropics and into Europe, natural selection began to favor those with lighter skin. Dark pigmentation became less necessary to protect the skin from UV, and light pigmentation became more necessary to let UV into the skin for synthesis of vitamin D.
That scenario has three problems:
The genetic linkages are weak between skin color, hair color, and eye color. Pale skin often coexists with dark hair and dark eyes.1 Thus, selection for lighter skin, by itself, should have only a weak effect on hair and eye color. Keep in mind that the genes are different in each case: European skin became white through new alleles at SLC45A2, SLC24A5, and TYRP1, hair color diversified mainly through new alleles at MC1R, and eye color diversified mainly through new alleles in the HERC2-OCA2 region.
If there had been selection only for lighter skin, and if the changes to hair and eye pigmentation were simply a side-effect, hair color would have just become less black and eye color less brown. The “side-effect explanation” does not explain the diversification into a wide range of hair and eye colors.
The new hair and eye colors are not simply more diverse. They are also “purer”—they occupy thinner slices of the visible spectrum than the original black and brown. Pure colors are unusual in nature and occur typically in situations where an animal or a plant is seeking to draw attention, whether to get pollinated, to warn predators, or to attract a mate.
The side-effect explanation also fails to explain the sex differences in hair and eye color. Using data from the UK Biobank and the 23andMe customer base, Hysi et al. (2018) found that women are much more likely than men to have red or blond hair, and three to five times less likely to have black hair. In a twin study, Shekar et al. (2008) likewise found that women are much more likely to be redheads, even when the only genetic difference is the one between men and women. In a Czech population, Frost, Kleisner, and Flegr (2017) found that 19% of the women and 11% of the men have the highest gradation of hair redness.
Hair and eye colors are also more evenly distributed among women: the less frequent colors are more common, and the more frequent ones less common (Frost 2022a, pp. 9-10, 13).
Finally, those two color polymorphisms seem to share a common developmental cause, not only with each other but also with other sexually dimorphic features. A Czech study found that face shape is more feminine in blue-eyed men than in brown-eyed men, as if a single factor alters both face shape and eye color, most likely the level of estrogen during fetal development (Kleisner et al. 2010; Kleisner et al. 2013). A hormonal cause is consistent with the sexual differentiation of hair color at puberty, when girls become lighter-haired than boys (Steggerda 1941). During adulthood, blond hair darkens with age more slowly in women than in men (Olivier 1960, p. 74).
Did sexual selection create the diverse palettes of hair and eye colors? There is a prima facie reason for thinking so. Usually, a colorful appearance is not an adaptation to the natural environment, which favors a drab and unobtrusive look to avoid detection by predators (Kirkpatrick 1987). Usually, it is an adaptation to the social environment, as a way to get noticed. It is perhaps significant that the hair and the eyes are on or near the face—the focus of visual attention.
My argument would meet with less resistance if I stuck to hair and eye color. It’s a lot harder to overturn the widespread belief that Europeans are white-skinned because their ancestors had an acute need for vitamin D.
Nonetheless, let me talk about the whitening of European skin … if only because I like the topic. That change did not begin when modern humans first entered Europe some 40,000 to 50,000 years ago. In fact, Europeans long remained brown-skinned, almost to the dawn of history in some areas. Using inferential methods, three research teams have estimated the time when alleles for white skin became more prevalent among Europeans: 19,200 to 7,600 years ago (Canfield et al. 2014); 19,000 to 11,000 years ago (Beleza et al. 2013); and 12,000 to 3,000 years ago (Norton and Hammer 2007). As a Science correspondent concluded: “The implication is that our European ancestors were brown-skinned for tens of thousands of years” (Gibbons 2007).
Those estimates are consistent with DNA retrieved from human remains. European hunter-gatherers already had white skin and a variety of hair and eye colors 9,500 to 6,000 years ago in Scandinavia and 7,460 to 5,360 years ago in the East Baltic (Günther et al. 2018; Mittnik et al. 2018). Elsewhere, Europeans remained brown-skinned for longer, as shown by DNA dated to 8,000 years ago from Luxembourg, 7,000 years ago from Spain, and 5,000 to 4,000 years ago from England (Brace et al. 2019; Lazaridis et al. 2014; Olalde et al. 2014).
That earlier physical appearance may be dimly recalled in mythology. An ancient Norse poem, the Rigsthula, describes how the god Rig created a class of black-haired, swarthy, and flat-nosed thralls (Jonassen 1951; Karras 1988).
What about vitamin D?
When Mathieson and Terhost (2022) examined DNA from Bronze Age Britain, they found that the whitening of the skin coincided with a metabolic change for increased synthesis of vitamin D. The authors attributed both changes to natural selection, although a genetic change due to natural selection cannot be easily distinguished from a genetic change due to in-migration. In this case, we know that the Yamnaya people expanded westward from eastern Europe to the British Isles during the Bronze Age. They ended up providing perhaps 90% of the British gene pool (Chintalapati et al. 2022; Furtholt 2019).2
Nonetheless, Mathieson and Terhost (2022) attribute the metabolic change to evolution within the British Isles, specifically the shift from the Mesolithic to the Neolithic, and the accompanying shift from hunting and fishing to farming. “The Mesolithic inhabitants of Britain may have avoided this problem through consumption of vitamin D rich marine resources, but later Neolithic and Bronze Age populations including those in our study relied on agricultural products for their subsistence, leading to a need for genetic adaptation.” We know, however, that the Mesolithic peoples of Scandinavia and the East Baltic already had white skin, even though they depended heavily on marine resources.
To make sense of all this, we should first understand that different human populations have different levels of vitamin D in their bodies. That level is universally low in humans with dark skin, which blocks the UVB needed for vitamin D synthesis. This has been shown in African Americans with varying degrees of African ancestry: with each 10% increase in African ancestry, the vitamin D level decreases by 2.5 to 2.75 nmol/L (Signorello et al. 2010). Yet few African Americans show signs of vitamin D deficiency. They have “a lower prevalence of osteoporosis, a lower incidence of fractures and a higher bone mineral density than white Americans, who generally exhibit a much more favorable vitamin D status” (Robins 2009). Among women 65 years of age, the risk of a hip fracture by age 80 is only 4% for African Americans versus 11% for European Americans (Harris 2006).
Dark-skinned humans cope with reduced synthesis of vitamin D by using it more sparingly or more efficiently, i.e., through higher uptake of calcium from the intestines, through a higher rate of conversion of vitamin D to its most active form, through stronger binding by proteins that transport vitamin D via the bloodstream, and perhaps through greater use of alternative pathways for calcium uptake (Frost 2022b).
When Europeans became white-skinned, they could more easily synthesize vitamin D and thus use it less sparingly and less efficiently. The whitening of European skin was therefore a cause, and not an effect, of changes to vitamin D metabolism.
Then what caused Europeans to become white-skinned? Most likely, the same cause that created their new hair and eye colors. All of those changes happened at roughly the same time.
Why would sexual selection whiten the skin?
Unlike the hair and the eyes, the skin did not develop a color polymorphism among Europeans. Instead, it became unusually pale. The reason may be that sexual selection was guided by a pre-existing dimorphism. In all populations, men are browner and ruddier than women, who by comparison are fairer (Edwards and Duntley 1939; Edwards and Duntley 1949; Edwards et al. 1941; Manning, Bundred, and Mather 2004; van den Berghe and Frost 1986). Fairer-skinned women are seen as more feminine in traditional cultures and preferred as mates (van den Berghe and Frost 1986). Sexual selection, if strong enough, would have therefore drained the European gene pool of alleles for dark skin.
Women have a lighter complexion for the same reason they have a smaller nose and chin, smoother, more pliable skin, and a higher vocal pitch. These are visual, tactile, and auditory cues that identify the human infant to an adult, who responds by feeling less aggressive and more willing to provide care and nurturance (Frost 2010, pp. 134-135; Frost 2011; Lorenz 1971, pp. 154-164; Wickler 1973, pp. 255-265).
A lighter complexion identifies an infant as an infant not only in our species but also in other primates, particularly langurs, baboons, and macaques. The color is initially pink and then darkens, becoming almost black in adulthood. A mother thus has a means to find her wayward offspring. Later, as their skin darkens with age, she loses interest in seeking and holding them (Alley 1980; Alley 2014; Booth 1962; Jay 1962).
In our species, the infant’s lighter complexion is especially noticeable in those societies where the adult is much darker. A new Kenyan mother may tell her neighbors to come and see her mzungu, i.e., “European” (Walentowitz 2008). When Zambian girls were asked to describe how Africans look, some wrote: “At birth African children are born like Europeans, but after a few months the color changes to the color of an African” (Powdermaker 1956). The infant’s coloration is often attributed to a previous spirit 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).
The same point is made by the anthropologist Jean-Thierry Maertens: “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).
The Eurasian steppe-tundra during the last ice age. It supported a larger human population at its western end because the climate was milder and wetter there.
When and where did the sexual selection happen?
Inferential methods suggest that European skin became white between ten thousand and twenty thousand years ago. Ancient DNA shows that white skin and diverse hair and eye colors were already prevalent in northeastern Europe 9,500 years ago. For earlier times, we have only one piece of DNA evidence: an allele for blond hair from a central Siberian site dated to 18,000 years ago (Mathieson et al. 2018, supp. p. 52).
It looks like ancestral Europeans acquired their current appearance during the last ice age, on the plains stretching from the Baltic to central Siberia. That region offered humans a unique food source: large wandering herds of reindeer and other game. But there was almost nothing else to eat.
Dependence on a single food source had two consequences:
The male death rate was higher. Men had to hunt over long distances of cold, unstable terrain that offered no alternative sources of food. There was thus a correspondingly higher risk of death from starvation and other hazards of hunting.
The polygyny rate was lower. Only the ablest hunters could provide for more than one woman and her children, since the latter could not provide for themselves. Women had no food autonomy.
The result: a surplus of women on the mate market; intense rivalry among them for male attention; and strong selection for eye-catching female features. Such features became more frequent with succeeding generations, eventually forming what is now seen as the European phenotype.
What about central Siberia? Why didn’t the same phenotype evolve there? Actually, it did. According to DNA from human remains found in south-central Siberia, dated to a period extending from the third millennium BC to the fourth century AD, most of the individuals had blue or green eyes, blond, red, and light brown hair, and fair skin (Bouakaze et al. 2009). Indeed, old Chinese records mention south Siberians with “green eyes” and “red hair” (Keane 1886, p. 703).
Those people died out. The steppe-tundra was colder and drier in northern Asia than in Europe. Humans were thus fewer in number and likelier to go extinct or be absorbed by incoming groups.
If you wish to learn more, please read my recently published book: European Hair, Eye, and Skin Color: Solving the Puzzle. Washington: Academica Press.
There is no softcover version. The publisher is willing to sell a PDF version for $50 US and a hardcover version for $70 US, but only on a referral basis. Buyers must state that they have been referred by Peter Frost. Email your request to:
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“If we were to take all the human beings in the world who have dark brown eyes and black or dark brown hair, we would not only have the vast majority of the human species, but would have a group which shows virtually the complete range of human skin color, from black to almost completely depigmented” (Brues 1975).
If ancient DNA shows a gene pool changing over time, we cannot easily tell how much of the change was due to natural selection and how much to in-migration, especially if the evidence from cultural artefacts is limited or open to interpretation. Some researchers simply assume that the genetic change is either 100% due to in-migration or 100% due to natural selection.
For that reason, I’m wary of any claims that a European population is X% derived from Anatolian farmers, Y% from Yamnaya pastoralists, and Z% from indigenous hunter-gatherers. Such claims attribute all genetic change to in-migration. That assumption is all the more doubtful because cultural diffusion tends to cause convergent genetic evolution. If a population adopts dairy farming, it will undergo natural selection for lactose tolerance, and certain alleles will become more prevalent. Those alleles, however, are not evidence for in-migration from a population of dairy farmers.
Whenever I make the above point, I end up being told that convergent evolution cannot explain all of the genetic change. Well, of course not. My point is simply that the magnitude of in-migration is exaggerated. The Yamnaya certainly had a big impact on the British Isles, but I doubt that they ended up becoming 90% of the population.
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I've always wondered. Why are the genes for blue/green eyes or red/blonde hair recessive? Do you know the genetic explanation why we can't have blue eye genes to be dominant?
You should write about African hair sometime. Most Black women in the US spend a lot of money trying to make their hair straight. A lot of them just wear wigs or straight hair extensions (yes Michelle Obama also). I've personally observed Black women just hate staring at White and Asian women's straight hair.
Heat in Africa can't be the only explanation. Australian aborigines or South Asians live in hot areas and they don't have those tight curls.