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- History of the U.S. Color Line
Essays on the Color Line and the One-Drop Rule
by Frank W Sweet
December 15, 2002
he growing consensus for an out-of-Africa scenario of modern human dispersal has produced a two-part puzzle of regional variation. Since the last glacial maximum, Europeans developed fairer complexions than any other group on earth, fairer even than others at the same or higher latitudes. Equatorial Native Americans, in contrast, failed to turn very dark over the same time period, despite living at the same latitudes as Africans and Melanesians. This essay suggests a two-part solution. Compared to other high-latitude dwellers, European diet was uniquely cereal-based and so deficient in vitamin D. Compared to other low-latitude dwellers, the ancestors of Native Americans had already lost the alleles necessary for dark brown skin before they crossed Beringia. This essay comprises two parts. Part 1 introduces the puzzle. Part 2 presents the solution.
I. The Puzzle: Skin Tone
Two factors combine to turn global human skin tone variation into a puzzle of adaptation: exceptions to the latitude rule and lack of time. The first factor is that regional variation of human skin tone, which has been known and mapped for centuries, corresponds roughly to latitude—the lower the latitude, the darker the native complexion. And yet, Europeans and equatorial Native Americans are exceptions. Both are much lighter than other groups at the same latitudes. The second factor is that the traditional explanation, that Europeans had more time to adapt, no longer holds water. Out-of-Africa scenarios, wherein modern humans colonized the globe starting 50-60 millennia ago but did not interbreed with prior humans occupying Eurasia, dramatically shorten the time available for regional variations to evolve anywhere. Consider first the latitude rule and its exceptions. We shall discuss the impact of out-of-Africa later.
Actual Skin Tone Variation
The most straightforward way of showing precisely how skin color (or any other trait, for that matter) varies regionally is with a map of iso-lines—contours of the trait’s values. The following three maps illustrate this point. The first two show actual skin color variation. The third map reveals that European hair color varies in step with European skin color.
At left is the most widely published map of native skin darkness variation around the world. It was originally produced in 1941 using complexion data collected in the 1930s by an Italian anthropologist (Biasutti 1941, 192). The map appears with little change in at least a half-dozen recent works, including (Jurmain and others 2000, 421) and (Cavalli-Sforza, Menozzi, and Piazza 1994, 145). The version at right is taken from Jurmain.
It may be of interest to know that the band of Berbers who appear as fairer than Spaniards in the above figure do not really exist. The light-colored intra-coastal stripe from Casablanca to Tunisia is merely an accidental artifact of the copying process from Biasutti. It appears in no other publication of this figure. Also, the reader should notice the small text note within the above figure from Jurmain. It says that: “these [Biasutti’s] data are, unfortunately, the best available.” As it turns out, this complaint is not entirely accurate.
C. Loring Brace and Ashley Montagu updated Biasutti’s data in 1977. The following three corrections are among their most important. They more precisely depicted different complexion shades among Australian Aborigines. They showed that the northern Lapps (Saami) of Finland are swarthier than Scandinavians. They completely redrew central Africa and repositioned the iso-complexion gradient contours within Madagascar, Indonesia, Burma, and elsewhere. This more up-to-date and more accurate map is depicted below, as taken from (Brace 2000, 296). It also appears in (Robins 1991, 187), incorrectly attributed to Biasutti. A similar map appears in (Blum 1961) from (Fleure 1945).
The most eye-catching feature on the above maps is that the lightest complexion on earth is native only to the region within 600 miles of the Baltic and North seas. The feature is unique on the globe. One can further grasp its uniqueness by examining a similar plot of iso-color contour lines for the color gradients of human head hair. The figure below appears in (Cavalli-Sforza, Menozzi, and Piazza 1994, 267), although it was first published in (Coon 1939, 270-71). This map of head hair gradients shows that blondes are also native only to the region within 600 miles of the Baltic and North seas. With two minor exceptions, the genetic trait for blonde hair precisely matches that for fair complexion. Even the black-haired and beige-skinned Saami of northern Lapland can be discerned in both maps. (The two minor exceptions are the fair-skinned but brown-haired people of Bordeaux and the blonde but swarthy descendants of the Volga Rus.)
To be sure, not every researcher is comfortable with the above complexion maps. Their data were collected by comparing people with von Luschan’s ceramic tiles, numbered 1-36 from white to black. Some prefer supposedly more objective readings, taken with portable reflectance spectro-photometers (Robins 1991, 98-99). Others object to the whole idea of interpolating sample points to derive cline contours. As Robins put it, “for regions where no information existed, Biasutti simply filled in the map by extrapolation [sic: Robins clearly meant interpolation] from findings obtained in other [adjacent] areas!” (Robins 1991, 188) Hence, to avoid criticism, some researchers prefer simply to present tables or maps with unconnected sample points, such as the one below (Jablonski and Chaplin 2000). Unfortunately, this makes it nearly impossible to spot patterns.
In any event, tables with unconnected data points, and maps with or without interpolated cline contours, all tell the same story: Europeans have lighter skin (and hair) than any other group on earth. Conversely, equatorial Native Americans are not even remotely as dark as other groups at the same latitude. The traditional explanation was that Europeans had had more time to adapt. The traditional explanation no longer works.
Strong OOA2 Creates the Puzzle
Why did Europeans become pink even as Mongols and Inuits at the same or higher latitudes remained brown? Why did Mayas and Incas fail to become as dark brown as Africans or Melanesians of the same latitude? The older scenario of multiregional evolution avoided the puzzle by affirming that the ancestors of modern Europeans had lived there for 250 millennia (ten thousand generations), whereas northeast Asia and the New World were populated, respectively, only 20 and 12 millennia ago (a few hundred generations), not enough time for natural selection to act. A review of the multiregional scenario and of three increasingly strong out-of-Africa scenarios may make this clearer.
In 1960, scholarly consensus had settled that the first members of genus Homo (erectus or ergaster, depending on terminology) emerged in Africa about two million years ago and dispersed throughout Eurasia starting about one million years ago. It was believed that modern humans evolved from older forms simultaneously in China, Southeast Asia, Europe, and Africa. Because this happened over a period of one million years, regional variation did not need explanation. Indeed, forty years ago, it was routinely believed that each of the “races” was an “incipient subspecies” that had evolved independently (Jordan 1968, 584). Modern Chinese have a high incidence of shovel-shaped incisors because their ancestral H. erectus had shovel-shaped incisors (Coon 1962, 454). Australian Aborigines have sloping foreheads because their ancestral H. erectus had sloping foreheads (Klein 1999, 504). Modern Europeans have fair
complexion because their ancestral H. neanderthalensis had fair complexions (Brace 2000, 300), and so forth. Observed regional variation in skin tone did not pose a puzzle because it supported
and was supported by multiregional evolution.
Since 1990, widespread consensus has been reached that a second dispersal out of Africa occurred between 50 and 60 millennia ago and that this dispersal comprised anatomically modern humans. That this second dispersal actually took place, even though modern humans may have already evolved in Eurasia, is the weakest form of a hypothesis hereinafter called Out-of-Africa 2 (OOA2, for short). A very few scholars still insist that no such dispersal as OOA2 ever happened (Wolpoff, Hawks, and Caspari 2000). These holdouts do not object to multiple people movements having occurred between Africa and Eurasia throughout the late Pleistocene. It is the idea of a “first” anatomically modern human dispersal that is objectionable. Either way, acceptance of this weakest form of OOA2 did not seriously challenge the prior view of regional complexion variation. Presumably, the African newcomers simply interbred with the local anatomically modern human populations, perhaps darkening them a bit, but having little other effect.
Since 1995, an even stronger OOA2 hypothesis has been adopted by a less widespread but still significant majority. This stronger version says that anatomically modern humans were a new species that first appeared only in Africa. It denies that locally evolved modern humans already lived in Eurasia. It says that OOA2 dispersal is precisely what initially seeded Eurasia with anatomically modern humans, who may have then interbred with local archaic humans. A minority opposition believes that anatomically modern humans had already evolved simultaneously throughout Eurasia and that, although the OOA2 dispersal may have actually happened, and it may have brought new cultural technologies, it was biologically insignificant (Wolpoff and others 2001).
This stronger form of OOA2 began to undermine the old explanation of regional complexion variation. If African newcomers were a new species, only marginally able to interbreed with local archaic humans, then why should their hybrid descendants inherit the local (archaic human) complexion phenotype everywhere? The answer focused on timing differences. It was said that Europeans were unique because only they had had time to adapt. They alone fully adapted to their latitude because only their Neandertal ancestors had been cold-adapted for 250 millennia. Archaic-modern hybrids did not reach northeastern Asia until 20 millennia ago and did not populate the Arctic or the equatorial New World until 12 millennia ago—not enough time for them to turn pink or dark brown, respectively.
Since 2000, the strongest OOA2 hypothesis has emerged, advocated by a slim majority. It says that the dispersal of African-evolved anatomically modern humans represented a new bio-species. It says that modern humans totally replaced the populations of older species (H. erectus, H. neanderthalensis, H. heidelbergensis) everywhere, with little or no hybridization. A large minority opposition suggests that miscegenation could have happened, either by gene flow before OOA2 or by interbreeding after the dispersal (Relethford 2001, 54-66).
This strongest version of the OOA2 scenario is well supported by molecular evidence. Two sets of Neandertal-modern DNA comparisons have been made, one from Feldhofer in western Europe, the other from Mesmaiskaya in the northern Caucasus. Both show a neanderthalensis-sapiens split 370 to 850 millennia ago, using Pan troglodytes as outgroup (Relethford 2001, 178-87). This finding challenges the relevance of Neandertal adaptations, to modern human regional variation. Additionally, phylogeographic analyses of mtDNA and SR-Y clades, suggest that the region around the Baltic and North Seas was depopulated during the last glacial maximum and only re-colonized after 16 millennia ago (Torroni and others 2001). People took refuge in Spain and Italy during the last glaciation. Consequently, any clear regional variation, such as the complexion pattern actually seen, must have evolved after they returned. In fact, SR-Y studies (Underhill and others 2001) suggest that the New World was colonized before post-glacial Europe was re-colonized. Apparently, Beringia re-submerged before the Alpine glaciers receded.
Evidence from Art
Admittedly, the notion that circum-Baltic European lightening took place after the last glacial maximum, not before, demands independent evidence. Two lines of artistic evidence corroborate it: Magdalenian cave art and Egyptian sculpture. The hunting scene above, one of many examples showing dark-brown bowmen shooting medium-brown deer, was painted in what is now France fifteen to thirteen millennia ago. Judging by the artists’ palettes, Europeans then had not yet lost their brown complexion.
Regarding Egypt, it is alleged (Sturm, Box, and Ramsay 1998) that, “the first depiction of variable pigmentation in man dates back to about 1300 BC and was found on the walls of the tomb of Sethos I.” As it turns out, this is too late. The earliest such depiction is a statue painted in Egypt in 2613 BC, nearly five millennia ago. It portrays Prince Rahotep and his Consort Nefret, of the Old Kingdom, early Fourth Dynasty (Kahane 1967). He is brown. She is pink.
Of course, nothing above is meant to imply that pre-LGM Europeans were as dark as Africans. Evidence suggests that early modern humans had a medium complexion, like that of today’s Khoisan or Ethiopians. The very dark complexion of central Africans also seems to be a recent adaptation (Semino and others 2002). To be sure, prior studies had suggested Mbuti pygmies as most resembling the
first moderns, but current molecular evidence points to the Khoisan and Ethiopians.
Also, nothing above suggests that every group on earth has had enough time to adapt locally. Polynesians began colonizing the Pacific (Santa Cruz, Vanatu) three millennia ago, and finished up in New Zealand less than one millennium ago (Sykes 2001). The point is that, if Europeans had enough time to become pink, then so did Asians, Inuits, Aleuts, and Saami. For that matter, if Europeans had enough time to become pink, then equatorial Native Americans had enough time to become dark brown.
Summarizing, the strongest version of OOA2, supported by molecular evidence, gives Europeans no more time on site to develop their world-unique complexion than it gives to light brown Asians at European latitudes, or to light brown Native Americans at African latitudes. And so, the questions are: why are Europeans pink? Why are Mayas and Incas not dark brown? Some obviously visible regional adaptations do not seem to have had enough time to become fixed, and yet there they are. Other expected adaptations have had just as much time to unfold, and yet there they are not.
II. The Solution: Solar UV, Diet, and Genes
Three factors explain the odd distribution of global skin tone variation under the strong OOA2 scenario (as mentioned, nothing needs explaining under the MRE scenario). The first factor applies to everyone. Overwhelming clinical and experimental evidence reveals that epidermal melanin’s adaptive role is to regulate the amount of solar ultraviolet penetrating to the dermal layer—enough UV for vitamin D synthesis but not so much as to destroy folate. The second applies to Europeans. Humans ingest vitamin D from certain foods, which supplements that synthesized from solar UV. The third applies to Native Americans. The very dark complexion of Africans, Andaman Islanders, Melanesians, and Australian Aborigines requires at least five genes working in concert. Once the trait is lost in a population, it cannot be regained.
Skin Melanin Blocks Solar Ultraviolet
As mentioned earlier, the default human complexion is apparently the light brown of the Khoisan and Ethiopian peoples. It is easy to see one’s own complexion as normal and others as needing explanation, but this is an illusion. The map below, for example, shows that C.S. Coon saw everyone from Norway southwards to and including Senegambia, Ethiopia, and Sri Lanka as of the “Caucasoid subspecies” (Coon 1962, 6-7). In fact, both the deep darkness of the Bantu people and the extraordinary near-albino lightness of the Scandinavians seem to be relatively recent and selectively driven adaptations. One is a paleness adaptation. The other is a darkness adaptation. Recent evidence suggests two different mechanisms to explain the two different adaptations.
The darkness adaptation enhances folic acid (folate) synthesis. Too little epidermal melanin for low latitudes allows intense UV to penetrate the skin, preventing or degrading folic acid synthesis, thus reducing folate levels. In pregnant females this produces neural tube defects in the fetus, causing such congenital abnormalities as craniorachischisis, anencephalus, and spina bifida. High levels of distributed epidermal melanin blocks UV and enables normal gestation at low latitudes (Jablonski and Chaplin 2000). Admittedly, some prior authors (Robins 1991, 210) had not seen evidence that fair-skinned residents of low latitudes suffered worse from folate deficiency than dark-skinned ones, but a collection of recent studies cited by Jablonski and Chaplin provide just such evidence. Hence, it seems confirmed that the darkness adaptation overcomes a threat to Darwinian fitness in its most unalloyed form—rate of successful reproduction.
The lightness adaptation enhances calciferol (vitamin D) synthesis. Too much epidermal melanin for the latitude blocks UV penetration essential to the dermal synthesis of calciferol or vitamin D. Vitamin D deficiency causes skeletal neonatal abnormalities (skull, chest, and leg malformations), rickets being the best known. Again, some mid-twentieth-century authors were not convinced that dark-skinned residents of temperate regions were more susceptible to rickets than light-skinned ones. But public health studies in the U.S. and Europe collected so much evidence of this, that vitamin D is now routinely added to milk in the West for precisely this reason. Hence, the paleness adaptation also overcomes a direct Darwinian threat to successful reproduction.
Other explanations have been offered. Some have suggested that vitamin D synthesis alone suffices to produce the global pattern of skin tone (Loomis 1967), but this sole-cause hypothesis has not withstood scrutiny. Others have suggested that dark complexion reduces the incidence of skin cancer, improves thermoregulation (ability to sweat), or camouflages the hunter. Others say that light skin is less at risk from cold injury. Some speculate that skin tone is merely an unselected by-product of adaptations to disease and parasites (Robins 1991, 187-211). But such hypotheses suffer from one of three flaws. Either they propose adaptations to a non-Darwinian threat (skin cancer strikes long after offspring are on their own), they assume that one complexion extreme or the other is the norm (in fact, both extremes are adaptations), or they lack clinical or experimental evidence.
Both adaptations, paleness and darkness, are positively selected for by natural selection to allow only the most beneficial amount of solar UV to penetrate the skin. The map below (Jablonski and
Chaplin 2000) depicts: “Predicted shading of skin colors for indigenous humans based on the results of a linear regression model in which skin reflectance (at 685 nm) for indigenous peoples in both hemispheres was allowed to respond to annual average UVMED for both hemispheres.” In other words, it shows what the regional variation of complexion would look like, if skin tone depended solely on solar ultraviolet radiation. The cited paper argues that both skin tone extremes are adaptations to solar UV, and so the trait’s regional variation depends only on
sunlight intensity at ultraviolet wavelengths. On the plus side, the paper is extremely persuasive.
Compare the Jablonski-Chaplin map with actual skin tone measurements around the world, as depicted in Part I, above. The prediction is surprisingly accurate at the low latitudes of the Old World. The Jablonski-Chaplin hypothesis is confirmed in that variations in skin tone displayed by natives of lands within twenty-five degrees of the equator in Africa and Asia may indeed have evolved in response to solar ultraviolet radiation.
On the minus side, their argument suffers from three major discrepancies. First, the Jablonski-Chaplin map predicts Native South Americans of Colombia, Venezuela, and coastal Peru to be as dark as equatorial Africans. In fact, they are not much darker than native North Americans. Second, the Jablonski-Chaplin map predicts the Saami of Lapland, the Inuit people of Greenland and Canada, and the Aleuts of the Bering Sea and northern Siberia to be lighter-skinned than Scandinavians. In fact, they are darker. Third, the Jablonski-Chaplin map predicts a band of people stretching around the globe at 55 degrees north latitude (the natives of Kazakhstan, Irkutsk, Ulan Bator, northernmost Manchuria, the Aleutians, Juneau, Hudsons Bay, and Labrador) to be as fair as Danes. In fact, they are much darker.
Incidentally, the Jabloski prediction map has been widely published in the popular press (sometimes with attribution and sometimes without). It has appeared in the February 2001 Discover magazine and in the Winter 2000 California Wild magazine, and at several Internet sites. Oddly, the popular press often labels the map as showing actual skin tone distribution. California Wild said that its “patterns illustrate three zones of human skin tone.” Discover said that the map shows “the skin colors of indigenous people across the globe.” Of course, Jablonski and Chaplin would agree that it shows no such thing. It portrays prediction, not measurement.
In short, Jablonski and Chapel convincingly demonstrate that skin tone was naturally selected, via two different adaptations, to block just enough UV penetration to enable both folic acid and vitamin D synthesis. But their explanation alone does not suffice to explain this paper’s central puzzle. Two more points are necessary. The first focuses on dietary vitamin D to explain how and why Europeans became uniquely fair-complexioned, with lighter tone than any other group on earth, regardless of latitude. The second discusses the heredity of complexion to explain why equatorial Native Americans have not become as dark as equatorial Africans.
Vitamin D was Also Available in Diet of Pre-LGM Europeans
Understanding that the paleness adaptation is designed to enhance vitamin D synthesis is key to solving the European half of the puzzle. Other animals also produce vitamin D and store it in their fat, just as humans do. The table at left shows the vitamin D content of common foods, as published by the National Institutes of Health (http://www.cc.nih.gov/ccc/supplements/vitd.html).
Prehistoric people did not consume fortified milk or cereal, of course. But, judging from their cave art and artifacts, they certainly ingested significant amounts of meat and fish. Assuming adequate caloric intake, their dietary content was well within the range of the current USDA recommended daily allowance (400 IU), especially when added to the vitamin D synthesized in the skin from sunlight. Late Paleolithic Europeans’ risk of neonatal defects caused by vitamin D deficiency was mitigated by two independent factors: solar UV and diet. Jablonski and Chaplin show that, as modern humans migrated away from the equator to Europe, Siberia, the Arctic, and Beringia, the paleness adaptation compensated for decreased solar ultraviolet. This left them with the light brown or beige complexion common to everyone above the 55th parallel except Europeans.
Then, European diet changed with farming. European agriculture began about ten millennia ago in the Near East and spread to the Baltic by five millennia ago (Cavalli-Sforza, Menozzi, and Piazza 1994, 215-16, 256-57) (Chicago 1974, 16:304). As in Asia, Africa, and America, the advent of agriculture saw a dietary shift from meat to grains. This reduced dietary vitamin D intake among farming peoples and so perhaps lightened their complexions slightly via the paleness adaptation. It was probably not significant outside Europe because domestic grains (corn, wheat, oats, sorghum, millet, rice) do not grow without intensive modern agricultural techniques above about 55 degrees of latitude. Higher latitudes are just too cold—the growing season is too short—to let crops compete successfully with herds as food source. Consequently, even post-Neolithic high-latitude peoples continued to have a diet rich in meat (and so, vitamin D). These include the Inuit (seagoing mammals), Aleuts (fish), Saami (reindeer), Mongols (horses), and Native North Americans (bison).
Only one spot on the globe enables economically competitive grain production above the 55th parallel. It is where the warm Gulf Stream washes into the North and Baltic Seas, keeping temperatures moderate despite dim near-Arctic sunlight. Around the planet, only circum-Baltic farmers could switch to a grain diet devoid of vitamin D, in a place where sunlight also lacked UV. And so, the extreme of the paleness adaptation is found only within 600 miles of this unique spot on earth.
The main objection to this hypothesis is its recency. Five or six millennia seems too short a time for such a genetic change. Three supporting arguments come to mind. First, as mentioned, acceptance of the strongest version of OOA2 unavoidably shortens the time available for any modern human regional variation, and yet variations are clearly present. Second, the European adult lactose tolerance adaptation was also inarguably caused by the Neolithic revolution—herding in this case (no one suggests that Paleolithic hunters milked their prey before spearing it)—and so it must have unfolded in the same time frame. Third, paleness and lactose tolerance are both neotenous adaptations that merely delay an existing developmental change until later in the organism’s life (until past its life-span, in these cases). Skin, like hair, normally darkens at puberty (Relethford, Lees, and Bayard 1985). And females, who have other neotenous features (associated with human sexual dimorphism) are slightly lighter-skinned on average than men (Rebato and others 1999). The point is that neotenous adaptations can be very fast indeed—as fast as one generation for some salamanders (Gould 1977, 319). [Otherwise important distinctions among neoteny, paedomorphosis, and postdisplacement are irrelevant to the point being made.]
An alternative explanation is that the extraordinary paleness of Europeans was due to sexual selection—it was more attractive to the opposite sex (Cavalli-Sforza, Menozzi, and Piazza 1994, 145). The problem with this speculation is that sexual selection normally results in a trait’s strong sexual dimorphism. Incidentally, Cavalli-Sforza also advocates a Neolithic time frame for both the paleness and lactose tolerance adaptations, but offers no mechanism for the former. Ultimately, it all depends on evidence. The hypothesis presented here will be contradicted when someone finds evidence as early as Magdalenian cave art, that Paleolithic Europeans were as fair complexioned as Neolithic Europeans.
Once Lost, Dark Skin Could Not be Regained by Native Americans
Understanding that several genes must work together to produce the darkness adaptation is key to solving the Native American half of the puzzle. Since 1910, researchers have known that human skin pigmentation is polygenic, depending on just a few codominant additive genes of essentially two alleles each. We have known that complexion is polygenic, rather than the result of one gene with many alleles, because breeding of palest with darkest yields a spectrum of offspring genotypes from the same parents, not just the four Mendelian ones. We have known that human pigmentation genes are additive and codominant because half the offspring of differently skin-toned parents have a complexion between that of their parents, no matter how similar the parents. We have known that at least three genes are involved because histograms of population skin reflectance yield continuous, not discrete, values (Stern 1973, 443-65), (Cavalli-Sforza and Bodmer 1971, 527-31).
Where knowledge has improved over the past century has been in precisely how many genes are involved and their specific loci. As of 1998, five human pigmentation genes had been identified. Their symbols and genome loci are: “TYR” at 11q14-21, “TYRP1” at 9p23, “TYRP2” at 13q31-32, “P” at 15q11.2-12, and “MC1R” at 16q24.3 (Sturm, Box, and Ramsay 1998). Subsequent work has identified five non-synonymous polymorphisms at the MC1R site (Rana and others 1999). Polymorphisms have been related to phenotype (Harding and others 2000). And gene-enzyme-protein reaction chains have been identified (Kanetsky and others 2002).
Much of the genetic mechanism remains to be unraveled but one conclusion is pertinent to this essay. Several independent genes must work in concert to produce the deepest complexion—the
extreme of the darkness adaptation. Many things can go wrong and, when they do, the result is a lighter complexion. For instance, deleterious mutations at the five loci above result in various forms of albinism, whether the patient’s heritage is dark or pale. In other words, there are many random ways “accidentally” to evolve a light complexion. But no genetic defect can make the child of light-skinned parents come out dark. [Nelson’s syndrome does this, but it is due to a pituitary tumor, not to a mutation, nor to genetic variability (Robins 1991, 125-26).]
This essay suggests that as modern humans migrated into northeastern Asia, they became lighter in response to two selective pressures. Less darkness was needed to protect against folic acid destruction by solar UV penetrating the dermal layer and causing neonatal neural defects. And more paleness was needed to enhance vitamin D synthesis which, together with calciferol ingested in meat, prevented neonatal skeletal malformations. But these adaptations functioned by the loss of genetic coding for dark complexion. The gene pool of the Native Americans who crossed through the Beringia bottleneck and populated the New World no longer had all the needed genes. The genetic variability subsequently available to their descendants simply did not include alleles at the five loci necessary to produce dark brown offspring.
* * * * *
In conclusion, this essay has tried to show that the growing consensus for an out-of-Africa scenario of modern human dispersal has produced a two-part puzzle of regional variation. Europeans and equatorial Native Americans are both too light for their latitudes. One would expect Europeans to have a light brown complexion like everyone else at or above 55 degrees. One would expect equatorial Native Americans to be dark brown. The puzzle does not exist in a multiregional evolution scenario because MRE explains differences as either primordial (Coon 1962) or the result of differing duration of residence (Brace 2000). This essay has offered falsifiable explanations that exploit recent genetic and anthropological findings to suggest that Europeans are unique because their diet became uniquely cereal-based and so deficient in vitamin D. Native Americans had already lost the alleles necessary for dark brown skin before they crossed Beringia.
Labels: skin tone
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