La couleur va donc rendre certains modèles inaccessibles. Leur insularité et leur éloignement les rapprochent paradoxalement d'une culture française imaginaire facilitée par les structures de diffusion culturelle complice. La relation commune qui est celle de l'image corporelle manque et l'identification achoppe encore sur la couleur, alors que l'identité de culture est manifeste, trop même peut-être pour la Martinique et la Guadeloupe. Nous ne disposons pas encore sur ce dernier échantillon des travaux très avancés, mais nous avons été frappés par la très grande fréquence des thèmes de persécutions de couleur dans les délires.
Prothese mammaire et fatigue
La recherche de modèles d'identification peut résumer beaucoup des tensions qui animent ces deux départements français recherche d'identité nationale. Lors des débuts de nos recherches sur les adaptations en France des populations de couleur originaires des Caraïbes, nous avions été très vite frappés par la différence de comportement des Haïtiens d'une part et des Antillais français d'autre part. Pour schématiser, nous pourrions résumer ceci en ce que, en face des attitudes de refus de la couleur noire du milieu d'accueil, les sujets haïtiens établissaient une distance protectrice face au rejet social en se déclarant étrangers.
Le Martiniquais ou le Guadeloupéen doit assumer sa couleur différente dans une certaine confusion aggravée par le contexte historique et culturel. Un phénomène semblable a été observé dans notre étude au niveau des communautés malgaches. Schématiquement, on peut les diviser en deux groupes en s'appuyant sur l'histoire, l'écologie et la couleur. Le premier, primitivement constitué par les Hovas, est originaire des Hauts Plateaux du pays, relativement moins sombre de peau, et constitue depuis longtemps une caste où se recrute une certaine élite dominante intellectuelle, financière et administrative.
Le deuxième groupe réunit, sous le vocable de Côtiers, des populations beaucoup plus foncées car d'origine mélano-africaine et non asiatique, et a, jusqu'à ces dernières années, fait figure de dominé par la caste des Hovas. Mais la réaction des deux groupes se révèle là aussi différente et n'est pas sans présenter d'analogies avec les milieux des Antillais français et des Haïtiens.
Skin pigmentation as an adaptation
La listance au néo-modèle d'identification est ainsi diminuée et souvent l'écart les séparant de leurs compatriotes côtiers les rapproche des Blancs français par un aménagement subtil de la couleur.
Tout au contraire, la situation du deuxième groupe est vécue sur un mode plus dramatique. Il y a en effet un double effet de rejets et les modèles d'identification à réussir seront recherchés ailleurs avec tous les aléas que cela peut représenter si le sujet est fragile. Là encore, nous trouvons un indice de décompensation névrotique plus élevé et l'explicitation de la situation de rejet par la couleur est beaucoup plus fréquemment utilisée.
Nous pourrions développer également le problème des sociétés métissées où la valorisation d'une couleur — le blanchissement par exemple — facilite les comportements des plus clairs et handicape ceux des plus sombres. Essayons pour terminer d'envisager s'il est fait usage thérapeutique dé la couleur. Se reconnaître Noir, c'est s'exclure. La synthèse présuppose de solides défenses du moi et un cadre socio-culturel où elles puissent s'épanouir et éventuellement l'action d'un psychothérapeute ou d'une institution spécialisée.
Mais dans les cas où la décompensation signe l'incapacité à maîtriser des conduites aussi divergentes, le psychothérapeute, par sa neutralité bienveillante, devra servir de facilitateur, aidé en cela par les phénomènes de transfert. Ce sont évidemment des conclusions en rapport avec le milieu français.
Cette situation conflictuelle est bien précaire.
Le milieu d'accueil qui. Mais de là à admettre la causalité couleur, il y a un long chemin à parcourir. The subjects of this study are the black or coloured minorities living in a white host society. Their processes of adaptation, and the assimilation of cultural models in the dialogue between the two are analysed.
Colour is an active factor, a tactical variable, which has great importance in the social phenomena of acculturation.
It appears to be the most emphasized of the physical differences between races. Many interesting studies have shown the emotive signifiance of colour in fields of both animal and human psychology. In the human field history has shown the bad connotations of black, whereas white has always had good connotations. When a black or coloured person arrives in France, he is not only subjected to various bio-climatic stresses due to the change of climate, food and other environmental factors, but he is also subjected to a colour shock.
He may already be aware that he is black, and the world in which he is to live is white, but going from the field of concept to that of actual experience is almost always overwhelming, and creates a severe impact of agression. This traumatic experience can have various expressions. This dual process of shock, which is experienced simultaneously by the subject, and is not understood.
This leads to the interpretation of all the subject's difficulties in terms of colour. The bioclimatic shock engenders physiological changes in the body which create specific sensations of anxiety, which in turn produce modifications, new associations, new needs, and ultimately a new anxiety reducing attitude — as the attitude is produced in anticipation of the object of need.
This involves the reconstitution of the corporal image, and a new fantasy of objective relationship to colour. Adaptation, which has been studied in great depth, shows how the language of the body is rationalised by the patient so that the body becomes what it imagines itself to be, in order to anticipate what it may do.
The subject constructs its own corporal image during the course of adaptation, and appears as the centre of reference of the attitude.
Naevus dermique pigmenté
The change of needs, expressions, values and relationships is dealt with by constructing new patterns of behaviour and a model of identity based on socially acceptable models presented by normal subjects in his own group.
But the element of colour, however, presents a formidable obstacle in the process. Surveys of mental illness among coloured people in Paris have shown, surprisingly, that coloured people who come to Paris for a few years solely to save money to take back home, have the lowest instance of mental break-down. These people, who are usually in the lowest income group, group themselves into a new community, institute a process of retribalisation amongst themselves, and create their own homeostasie at group level.
Thus requirements today are increasingly being defined on the basis of the intake required to maintain optimal functioning of the systems in which the nutrients operate. Factors influencing calcium requirements Calcium occupies a nearly unique position among the essential nutrients.
Like many of the vitamins and trace minerals, it has key roles in signal transduction and catalytic protein activation at the most fundamental levels of cell biology. Furthermore, a constantly maintained concentration of calcium ions in the extracellular fluid ECF is critical for optimal functioning of the neuromuscular and blood coagulation systems, among others.
These roles are so critical to life, both of the cell and of the organism itself, that systems have evolved for extraordinarily tight regulation of ECF calcium ion concentration. Aquatic vertebrates had two mechanisms to buffer calcium ion concentration: transfer of ions across the gill membrane and transfer of ions in and out of bone.
The first predominated, with the calcium-rich external aquatic environment serving both as the reserve to be called upon in situations of need and the sink to be used for disposal of excess. As vertebrate evolution progressed to life outside a supporting aquatic medium, the bone mechanism assumed increased importance. It was now the only source and sink for buffering ECF calcium ion concentrations, and at the same time it provided the internal structural rigidity needed for locomotion and gravity-resisting activity.
With the exception of energy stores of which serve as thermal insulationcalcium is the only nutrient known for which the reserve has acquired a function in its own right, distinct from its fundamental metabolic role. In brief, humans walk about on their calcium reserve. In adult humans, total body calcium amounts to about mmol per kilogram, of which less than 0.
Thus, the reserve is vast relative to the metabolic pool of the nutrient, and for all practical purposes metabolic calcium deficiency probably never exists, at least not as a nutritional disorder.
However, because of the structural significance of the nutrient reserve, i. Thus, unlike requirements for all other nutrients, the requirement for calcium relates not to maintenance of the metabolic function of the nutrient, but to the maintenance of an optimal reserve and the support of the reserve's function.
Small fluctuations around the optimum reserve size have no practical structural significance, but protracted, unbalanced withdrawals from the skeletal reserves produce inescapable structural consequences. It is now quite clear that this occurrence explains the rapid bone loss and at least some of the skeletal fragility of the elderly Chapuy et al. Because the effect of unbalanced withdrawals from the calcium reserve accumulates over many years, the frequency of osteoporotic fractures tends to rise with age.
It must be stressed that reduced bone mass is not the only factor in osteoporotic fragility, and low calcium intake is not the only factor causing reduced bone mass. Osteoporotic fractures are complex phenomena involving not just intrinsic bone strength or fragility, but age-related neuromuscular changes that determine both the frequency and types of falls, as well as other critical factors.
All these factors increase in intensity with age. Calcium is virtually unique among nutrients in a second sense: the relationship between the size of the reserve and the environmental availability of the nutrient is asymmetric.
By contrast, energy, the fat-soluble vitamins and many trace minerals exhibit symmetrical reserve behaviour, i. In the face of environmental calcium shortage, the organism cannot build or maintain an optimal reserve; but in the face of surplus, the size of the reserve is completely determined not by the diet but by non-nutritional forces. Bone mineralization requires active metabolic work. In brief, calcium is stored not as such, but yopa dietetique 2014 bone tissue, i.
This composite is laid down as a result of cell-based activity, which, in turn, is determined by the combined effects of genetics and mechanical usage. The importance of mechanical loading for optimal bone health is now well recognized.
Bone, in essentially all vertebrates studied to date, has a density that permits bending of about 0. More massive bone is stiffer, bends less and is recognized by the organism as excessively heavy.
As a consequence, some of the excess bone is removed in an ongoing remodelling process until the stiffness reaches the evolutionary optimal figure of 0.
Conversely, bone that bends more than the reference amount is recognized as flimsy, and the organism attempts to strengthen it by adding more bone through the normal remodelling.
A surplus of calcium will not prevent the downward remodelling of excessively massive bone, but calcium deficiency will prevent the strengthening of flimsy bone. Because of the asymmetric relationship between nutrient intake and the size of the reserve, calcium functions as a "threshold" or "plateau" nutrient, illustrated schematically in Figure 1.
At suboptimal intakes the size of the reserve, i. But above the quantity of bone determined as optimal for genetic and mechanical purposes, further increases in intake are not stored. As already noted, humans can store fat and fat-soluble vitamins virtually without limit, i. Calcium, however, can only be stored as bone, and increasing calcium intake beyond the amount that produces the optimal bone mass will not result in more bone, any more than increasing iron intake beyond the amount that produces the optimal haemoglobin mass will result in more blood.
The requirement for such a nutrient can be visualized as the intake at, or just above, the knee of the curve in Figure 1. Figure 2 shows the operation of this system from experiments in growing rats. Femur calcium content is seen to be a rising linear function of intake at low intake levels, but it does not change further above a threshold intake value. This behaviour is easy to demonstrate and understand in such animal models, particularly during growth.
However, if body retention of calcium is substituted for bone mass in the figure, then the figure works for humans as well and, more important, for all ages. The concept leads to an approach to defining the calcium requirement based on maximal calcium retention. The calcium retention that is maximal will, of course, be quite different for different life stages.
It should be positive during growth and pregnancy, stable during most of the adult years and probably somewhat negative during the declining years of life as the skeleton adapts to the reduced mechanical loading associated with ageing. Whatever the retention value may be positive, zero or negativeit is maximal with respect to intake when further increase in steady-state intake produces no further increase in retention.
This approach has now been formally adopted in the United States for estimation of recommended calcium intakes United States National Academy of Sciences, Average requirements for individuals living in the United States and Canada estimated using this criterion are given in the Table. NB: recommended dietary allowances [RDAs] will be approximately 20 percent higher than the values in the Table.
Above the threshold the horizontal linebone accumulation is limited by other factors and is no longer related to changes in calcium intake. Heaney, It should be stressed that there are important genetic and environmental influences on these average requirements, which will result in substantial differences both in the requirement for various national or ethnic groups and in the relationship of calcium intake to fracture risk. The genetic influences, briefly, include such factors as bone architecture and geometry e.
Thus, while the basic relationship between bone mass and fracture risk and between bone mass and calcium intake is the same in Asians and Caucasians Hu et al.
Ignorance of the importance of these structural features has led in the past to the erroneous conclusion that calcium intake was not related to fracture risk. There are also important differences in responsiveness of bone to the hormones that mediate the function of bone as the body's calcium nutrient reserve.
Blacks, with substantially heavier skeletons than Caucasians or Asians, both absorb and retain calcium more efficiently than Caucasians Abrams et al,probably as a consequence of somewhat reduced skeletal responsiveness to the action of parathyroid hormone.
In other words, in the face of exogenous deficiency, the bone reserves are slightly less readily available, resulting in higher secretion of parathyroid hormone and better intestinal absorption and urinary retention both also effects of parathyroid hormone. Thus, blacks have stronger skeletons than Caucasians, despite generally lower calcium intakes.
The colours of humanity: the evolution of pigmentation in the human lineage
Each racial group exhibits the same basic relationship between bone mass and calcium intake, only the equilibrium values are different. The environmental influences on the calcium requirement include such factors as other constituents of the diet and the degree of mechanical loading imposed upon the skeleton in everyday life. As has already been noted, because calcium was a surfeit nutrient in the primitive environment, there was no need for evolving hominid physiology to develop means of conserving calcium.
This lack is expressed in a unique sensitivity of the calcium economy to other constituents of the diet.
Urinary calcium losses, for example, rise with both sodium intake by 0. At protein and sodium intakes typical of the industrialized nations, these effects result in average obligatory urinary calcium loss of 2 to 3 mmol per day, and in some individuals the loss is substantially greater.
This effect would create no problem in the face of calcium surfeit, but it can be severely limiting in individuals with low calcium intakes, since the influence of these other nutrients restricts the ability of the organism to reduce calcium losses. It is likely that the acid-ash residue of a high-meat diet has a similar effect, since substitution of chloride by organic anions in various diets has been shown to reduce urinary calcium loss dramatically Berkelhammer, Wood and Sitrin, ; Sebastian et al.
It is sometimes mistakenly said that there is a difference in this regard between animal and vegetable proteins, but the protein per se is probably not responsible for any difference that may exist. Rather, adequate protein from both sources will have approximately the same content of sulphur-containing amino acids which become oxidized to sulphate.
Instead, any difference would result from the different content of organic anions in vegetable and meat sources. Incidentally, dairy products, though animal régime politique démocratique, are alkaline-ash, rather than acid-ash, foods.
These considerations mean that the calcium requirement, i. Because net absorption of an increment in calcium intake is only about 10 percent, and because even the resulting small absorptive increase in ECF calcium concentration will cause some of that absorbed increment to be spilled into the urine, changes in obligatory losses through the skin and kidney are amplified by a factor of about fold in their effect on the requirement. Thus, at low sodium and low protein intakes, the requirement may be as low as Therefore, the recommendations produced for North America are specifically intended to describe the requirement for individuals in the population there, and should not automatically be extrapolated to other population groups.
Factors influencing vitamin D requirements The vitamin D requirement has been exceedingly difficult to define. Lesser degrees of inadequacy termed "insufficiency" in current jargon limit the ability of the organism to adapt to low calcium intake with an appropriate increase in calcium absorption efficiency, and thus aggravate the prevailing low calcium intakes of many populations.
In addition, it is likely that at high calcium intakes there is very little active transport of calcium mediated by vitamin D, but at low calcium intakes vitamin D is essential for the induction of a calcium-binding transport protein in the intestinal mucosa which enhances calcium extraction from the digestate.
Skin colour as measured by skin reflectance and levels of UVR are highly correlated. Biologically effective UVR is often described in units of minimal erythemal dose or MED, which expresses the amount of UVR radiation that will produce minimal erythema sunburn or redness caused by engorgement of capillaries in lightly pigmented human skin within a few hours following UVR exposure. If this relationship developed under natural selection, plausible causation must demonstrate real or probable enhanced reproductive success for specific skin pigmentation phenotypes under specific UVR conditions.
Many adaptive explanations for the evolution of variation in human skin colour have been put forward in the last century, as reviewed elsewhere [ 17 ], and most have suffered from a lack of evidence for likely differences in survivorship and reproduction of different skin colour phenotypes under the same UVR conditions. Skin cancers rarely cause death or adversely affect reproductive success during the peak reproductive years [ 33 ], so this explanation was dismissed long ago as being a primary cause of the evolution of dark skin pigmentation.
The recent argument that ancestral hominins had pale, cancer-prone skin similar to that of individuals with OCA2 albinism and that evolution of dark skin spared the human lineage from skin cancer—related mortality lacks support [ 34 ].
Other explanations have insufficient explanatory power. These include the hypothesis that the eumelanin was most important in affording protection against tropical parasites and tropical skin diseases because of its potent antimicrobial properties [ 35 — 37 ], an idea that fails to explain the near-absence of eumelanin on the primary environmental interfaces of the volar surfaces of the hands and feet and the lips.
More recently, mooted is the hypothesis that the primary function of eumelanin was augmentation of epidermal barrier function by increasing the skin's resistance to desiccation under arid conditions, and that depigmentation did not occur under positive selection for vitamin D production [ 38 — 40 ]. This hypothesis has been disproved by multiple lines of evidence, including the fact that tanning occurs in the presence of UVR, not desiccation [ 41 ], and that positive selection for depigmented skin capable of producing vitamin D under low and highly seasonal UVB conditions is now well established [ 42 ].
The strongest hypothesis for the evolution of dark skin colour is that it afforded protection against photodegradation of cutaneous and systemic folate under high UVR conditions for early members of the genus Homo. The physiological effects of photodegradation of folate were explored long before the full extent of folate's roles in DNA biosynthesis, repair, DNA methylation, amino acid metabolism and melanin production were appreciated [ 43 ].
Because folate in its main form of 5-methyltetrahydrofolate or 5-MTHF is sensitive to photodegradation [ 44 — 47 ], protective eumelanin-rich pigmentation evolved in early Homo primarily to prevent reduction of fertility due to loss of folate in cutaneous blood vessels and the systemic circulation [ 2148 ].
Folate deficiencies are associated with potentially fatal birth defects such as neural tube defects and male infertility [ 49 — 52 ].
Folate metabolism is regulated by genes and epigenomic factors, which have evolved to favour conservation of folate under conditions of longer day length and greater potential UVR-related folate loss [ 5354 ]. Recent physiological evidence also indicates the importance of folate in the form of 5-MTHF in thermoregulation, via its effect on controlling nitric oxide-mediated cutaneous vasodilation [ 55 — 58 ].
Maintaining the integrity of folate metabolism is important with respect to evolution because it directly affects reproductive success and survival early in life [ 2159 ]. Natural selection has, thus, affected varied genetic and physiological mechanisms in order to protect folate and 5-MTHF in the face of high UVR.
The primary role of constitutive dark skin colour in hominin and modern human evolution is that of a natural sunscreen to conserve folate.
Protection of epidermal DNA against strand breaks was the important secondary role played by dark skin colour. The genus Homo and the species Homo sapiens emerged in equatorial Africa under conditions of intense and relatively invariant sunlight and UVR. Dispersal r&l carriers montana hominins into non-equatorial Africa, Eurasia and the Americas involved movements into habitats with more seasonally variable patterns and differing wavelength mixtures of UVR [ 60 ].
In this review, we shall confine our discussion to the consequences for skin colour of dispersal of Homo sapiens into Eurasia and the Americas, a process which began around 55 years ago [ 61 ].
Dispersing populations were small and bottlenecks further reduced available genetic variation in populations crossing major geographic boundaries [ 62 ]. Note also that there is no evidence that dispersing archaic Homo sapiens used sewn clothing or other methods of full-coverage protection against the sun and elements.
Non-sewn animal skins probably afforded some protection, but for the most part people were subjected to the full force of UVR apart from the times when they sought natural shelter. Skin was thus the primary interface with the environment for most of human evolution. The UVR regimes faced by dispersing hominins were a major selective pressure affecting the evolution of skin pigmentation.
Outside of the tropics, consideration of the pattern of UVB is relevant because only some wavelengths of UVB between and nm, with peak synthesis between and nm catalyse production of vitamin D in the skin. Within the tropics, average UVB is high and has two equinoctial peaks, but outside of the tropics, average UVB levels are lower and exhibit but a single peak at the Summer Solstice [ 60 ].
Because eumelanin in skin botox para eliminar arrugas a highly effective sunscreen, the potential for cutaneous vitamin D production is reduced by dark skin [ 2163 — 66 ]. Darkly pigmented hominins dispersing out of equatorial Africa thus faced conditions that significantly affected their vitamin D physiology.
Pre-vitamin D production occurs in skin of all colours, but in eumelanin-rich skin, low doses of UVB do not raise 25 OH D levels to physiologically adequate levels at which storage can take the hampshire regiment ww1 higher doses over longer periods of time are required for this, and these conditions are not met outside of equatorial latitudes. Long-term occupation of non-tropical latitudes, thus, would not have been possible without loss of some constitutive eumelanin pigmentation in order to prevent the serious sequelae of vitamin D deficiency [ 21 ].
Couleur de la peau humaine
At extreme high latitudes, year-round occupation is not possible without a diet that is centred on consumption and storage of vitamin-D rich foods such as oily fish, marine mammals, or caribou and reindeer, which concentrate vitamin D in their muscle meat and fat [ 2170 ]. Melanin produced by the tanning response is the outcome of UV-induced stress and is regulated by melanocortins in the skin [ 71 ]. Melanocortins in the skin reduce the production of reactive oxygen species, enhance repair of DNA damage caused by UVR and inaugurate eumelanin synthesis in individuals who carry specific, naturally expressed variants of MC1R [ 71 ].
Tanning is thus a response to damage caused by UVR. Contrary to popular belief, tanned skin affords little or no protection from damage from subsequent UVR exposure. Recent experimental studies have shown that a tan developed under suberythemal UVB exposure provides minimal photoprotection, but a tan developed under UVA-rich sunlamps such as those used in tanning salons provided no photoprotective benefit [ 72 — 74 ]. Depigmented skin evolved not once, but multiple times in human history, and was accomplished by different combinations of genetic mutations.
For modern European populations, different signatures of selection on the MC1R and SLC24A5 genes imply that both natural selection and genetic drift contributed to the evolution of depigmented skin. These genetic changes affected the amounts of melanin being produced in melanocytes and the size of the melanosomes in which the melanin was packaged [ 75 ].
For modern eastern Asian populations, depigmentation was not achieved through mutations at the SLC24A5 locus [ 75 ], but via a different set of genetic changes, which are still incompletely known [ 7677 ]. The fact that depigmented skin evolved independently in the ancestors of modern Europeans and East Asians suggests that at least two and probably more distinct genetic mutation events occurred and that multiple loci underwent positive selection in these two regions receiving relatively low levels of UVB [ 78 — 80 ].
The most likely reason for this was that it was associated with a loss of skin pigment that favoured vitamin D production under conditions of low UVB [ 697881 ]. Depigmented skin also evolved independently in Homo neanderthalensis [ 82 ] probably for the same reason. There has been a cause and effect relationship between UVR and skin pigmentation in human evolution, and skin colour phenotypes have been modified under the action of natural selection to maintain an optimum balance between photoprotection and photosynthesis over spatially varying conditions of UVR.
Skin colour thus evolved as the product of two opposing clines, one emphasizing dark pigmentation and photoprotection against high loads of UVA and UVB near the equator, the other favouring depigmented skin to promote seasonal, UVB-induced photosynthesis of vitamin D 3 nearer the poles [ 60 ].
Intermediate latitudes with seasonally high loads of UVB favoured the evolution of people with intermediate colour liposuccion tarif ventre of tanning [ 8485 ]. The most important points to reinforce here are that the geographical gradient of human skin colour evolved under the influence of natural selection, and that very similar skin colour phenotypes dark, light and intermediate have evolved independently numerous times under similar UVR conditions.
Diverse combinations of skin colour genes occurred during the course of prehistory as the combined result of natural selection, gene flow due to migration, and founder effect or genetic drift due to population bottlenecks occurring in the course of dispersal events [ 6286 ].
Indigenous populations of the New World have generally lighter skin colours than those of the Old World, probably because they have not resided in their homelands for as long a time and because their adaptations to the environment have been more strongly cultural than biological, as exemplified by the wearing of sewn clothing and the making of shelters [ 212287 ]. Sexual selection does not appear to have been a major influence on the evolution of human skin coloration, but it probably did increase the degree of sexual dimorphism in skin colour in some populations [ 2187 ].
The unexposed skin of females is lighter than that of males in most populations [ 2183 ], possibly because of the greater need of females to produce vitamin D in the skin to absorb and mobilize calcium during pregnancy and lactation. A persistent, directional preference for lighter-coloured females as marriage partners has been recognized during historic times in some east- and south-Asian cultures, and it is likely that this has contributed to the greater sexual dimorphism in coloration observed [ 2187 ].
Like skin, the coloration of human hair and eyes is determined primarily by the amount and type of melanin produced and stored in melanosomes [ 8889 ]. Iris pigmentation is also influenced by structural features within the eye itself and by the degree of pupillary dilation [ 90 — 92 ]. Although skin coloration varies according to the intensity and seasonality of UVR on a global scale, no such regular geographical pattern is observed with hair and eye coloration [ 93 ].
In fact, little variation in hair and eye coloration perte de poids ig bas menu in indigenous populations outside of Europe for hair and outside of Europe, North Africa, the Middle East, Central Asia and South Asia for eyes [ 888994 ]. The lack of variation in hair and eye coloration in Africa has been assumed to be caused by the importance of eumelanin in affording protection, as it does in the case of skin coloration, but this has not been empirically established.
Hair and eye coloration appear to have not been under as strong natural selection as skin coloration, and loss of genetic variation at one or more population bottlenecks probably contributed to the patterns of phenotypic variation observed in the hair and irises of modern people [ 95 ].
Scalp hair in most non-European populations is very dark brown, with little phenotypic variation [ 96 ]. Many genes appear to contribute to the dark brown hair colour phenotype [ 8893 ], and the relative importance of different loci is not yet known. The relatively high prevalence of blond hair in Northern Island Melanesia has been traced to the 93C allele of the TYRP1 gene [ 9798 ], which has been dispersed throughout the region in the course of human colonization of the Southwest Pacific.
In Europe, blond hair has been traced to establishment of variation in a regulatory enhancer of the KITLG gene, while red hair is produced by a specific range of variants of the MC1R locus [ 99 — ].
Les facteurs de sélection dans l'évolution de la pigmentation de la peau
Sexual selection is thought to have influenced the high prevalence of blond- and red-hair phenotypes in Europe [ 93 ], but this has not been established empirically. The nature and coloration of human male facial hair—beards and moustaches—have been a matter of curiosity and speculation, but little formal research [ 16, ]. Male facial hair, manifested as moustaches, cheek hair whiskers and beards, occurs in some male primates and appears to represent secondary sexual characteristics that evolved as amplified visual signals of rank, dominance and attractiveness [ 8].
This supports claims that age-related changes in human beard and moustache coverage and colour on the male face serve as honest signals of age or social dominance [ ], and that they evolved as products of contest competition between males [ ].
Beards augment the effectiveness of human aggressive facial displays, but are rated as ambivalent or unattractive by females . Until very recently, studies of human iris coloration focused primarily on European populations, where the greatest range and variety of eye colours—from dark brown to pale blue—are found.
Iris colour phenotypes are determined by amounts of melanin and by the ratio of eumelanin to phaeomelanin in the iris, with brown eyes having a higher ratio than light eyes [ 8889 ]. Other categorical eye colours, such as blue, green and hazel, are common in Europe and parts of the Middle East, and Central and South Asia, with Europeans having the lightest eye colours [ 88 ].
The sets of genetic markers associated with variation in iris coloration in Europe, South Asia and East Asia are distinct, and relatively little is known of the combination of evolutionary forces—including natural and sexual selection—that influenced their distribution [ 89 ].
Dark iris coloration is associated with less scattering of intraocular light, a trait that may be protective under conditions of bright sunlight and high UVR. Blue eye coloration, on the other hand, is associated with greater intraocular light scattering and a higher level of melatonin suppression, traits that may have been adaptive under highly seasonal sunshine regimes in northwestern Eurasia [ ].
Blue-eyed women have been found to be preferred by blue-eyed men, possibly as a manifestation of a male adaptation for the detection of extra-pair paternity based on eye colour, as a phenotypically based assurance of paternity [ ]. Arguments for the action of natural and sexual selection on iris colour need to be examined with great care as more data on the genetic basis of the trait are revealed. A recent study showing that the genetic markers associated with iris coloration are also associated with skin and hair pigmentation traits suggests that iris coloration was a pleitropic effect associated with selection on pigmentation genes whose primary effect was skin or hair pigmentation, not iris coloration [ 89 ].
The genetic basis of human coloration is complex because some genetic variants affect all pigmentary systems—skin, hair and eyes—through pleiotropic effects, while others affect only one type [ ] and because different genes and gene combinations can create similar coloration phenotypes.
Despite the technical difficulties of such studies, the fascination of humans with their own coloration phenotypes will certainly continue to propel research forward quickly. Skin, hair and eye coloration in humans is variable, and has been influenced by different combinations of evolutionary forces. Skin coloration has been strongly influenced by natural selection, globally and throughout human prehistory, because of the importance of melanin as a natural sunscreen on naked skin.
The role of natural selection in the evolution of hair and eye coloration appears to have been negligible, but genetic bottlenecks followed by sexual selection may have played more significant roles in establishing the patterns of variation recognized outside of Africa.
In recent centuries, humans have migrated faster and over longer distances than during any time in prehistory. Many of these movements have brought people into regions with markedly different solar regimes than their homelands. Many people now live under levels of solar radiation that are much stronger, or much weaker and more seasonal, augmentation mammaire bonnet c kolagen those under which their ancestors evolved.
These rapid changes in living circumstances have created significant health problems resulting from too much UVR exposure skin cancer, accelerated ageing of the skin and from too little UVR exposure vitamin D deficiency and its many sequelae that have greatly impacted individual well-being and public health.
Mitigating these problems is now the focus of considerable attention in many health professions [ — ]. Rapid, long-distance migrations have also brought people together from disparate and widely separated places, creating unprecedented and novel opportunities for gene flow.
The twenty-first century world contains a sepia rainbow of human skin colours, created from old and new combinations of skin colour—related genetic markers. The effects of these new genetic admixtures on health are not known. More significant to health and overall human well-being, however, are the problems of social segregation and behavioural bias that are rooted in cultural constructions of skin colour—based race categories [ 87 ].
Humans are visually oriented primates, and our varied colours are badges of our recently shared evolutionary history. Our skin colours unite us, not divide us. Tim and two anonymous reviewers provided excellent suggestions for improvement of this contribution.
We thank Prendre du poids en debut de grossesse Wilson of Penn State for providing comprehensive support and assistance to our research programme, including the maintenance of our reference library and preparation of this manuscript. The authors contributed equally to the conception and design of this paper and to the drafting and revising of the article for important intellectual content; they also gave the final approval of the version to be published.
National Center for Biotechnology InformationU. Published online May Nina G. Jablonski and George Chaplin. Author information Article notes Copyright and License information Disclaimer. Accepted Dec 9. This article has been cited by other articles in PMC. Abstract Humans are a colourful species of primate, with human skin, hair and eye coloration having been influenced by a great variety of evolutionary forces throughout prehistory.
Keywords: eumelanin, phaeomelanin, ultraviolet radiation, natural selection, sexual selection, genetic drift. Introduction Discussions of the cardinal features of the human lineage usually focus on bipedalism, relative brain size, language and technology, and ignore the remarkable distinctions of the integument and eyes that have figured importantly in human evolution.
Human coloration in context Like most mammals, primates have hair covering most of their bodies. Variation in human skin coloration is mostly a product of natural selection Skin colour as measured by skin reflectance and levels of UVR are highly correlated. Hair and eye coloration are not under strong natural selection Like skin, the coloration of human hair and eyes is determined primarily by the amount and type of melanin produced and stored in melanosomes [ 8889 ].
Conclusion Skin, hair and eye coloration in humans is variable, and has been influenced by different combinations of evolutionary forces. Authors' contributions The authors contributed equally to the conception and design of this paper and to the drafting and revising of the article for important intellectual content; they also gave the final approval of the version to be published. Competing interests We have no competing interests.