All blacks have white palms



The young woman suffers from psoriasis. She comes from South Asia. "Here in Central Europe, people of your origin easily get too little sun," she learns from the doctor.

The fact that dark-skinned groups of people live near the equator and light-skinned groups at higher latitudes has long been attributed to solar radiation by anthropologists. However, so far they have mostly assumed that the heavily pigmented skin was created to protect against skin cancer. We found another explanation. There are many indications that the degree of pigmentation is essentially based on a balance between two vitamins that has been balanced over and over again in human evolution. One is destroyed by UV radiation, the other is built up. But both are important in order to be able to father children and bring them into the world healthily.

We are the only primates with largely bare skin. Chimpanzees look light under the coat. Since they have probably changed less than we have in the last at least 7 million years since our lineages separated, one can assume that the last common ancestor was also fair-skinned.

Only the few hairless parts of the body of a chimpanzee - the face, hands and feet that still look pink in young animals - become blotchy or dark with increasing age from solar radiation. It can be assumed that the hominids only got an overall darker skin when their coat disappeared. But when did our ancestors lose their fur?

Their fossil skeletons give a pretty good picture of their way of life and behavior. "Lucy" and her contemporaries probably struggled a good 3 million years ago in a similar way to today's savanna primates: They spent most of the day looking for food in open terrain and retired to trees at night. In one day these upright prehistoric humans, the australopithecines, hardly covered much more than five kilometers.

Primitive man was black

1.6 million years ago people lived very differently. The skeleton of the "Turkana boy", a tall youth, comes from this time. He was a Homo ergasterand thus already a member of our own species homo. With his long legs, the boy could certainly stride vigorously. Early humans like him probably traveled long distances.

With so much physical exertion, however, the risk of heat stroke increased in the open savannah landscape. Above all, the brain was not allowed to overheat. The solution in early humans was to cool the body and the circulating blood through vigorous sweating. The number of sweat glands increased, and so that the sweat evaporated quickly and brought cooling, the hair had to decrease - as pointed out by Peter Wheeler from John Moores University in Liverpool.

But now the sun hit the bare skin. The short-wave portion of its radiation in particular can burn unprotected areas of the skin, destroy the sweat glands and severely damage deep-lying cells. This results in DNA damage, which the cells usually repair, but which may develop into skin cancer (see "Sunburn and Skin Cancer", SdW 6/1997, p. 74). The hairless areas of the skin of chimpanzees contain melanocytes. When exposed to UV radiation, these pigment-forming cells produce dark pigments, the melanins - large molecules that fulfill two protective functions: They intercept the devastating UV rays and neutralize the toxic free radicals that result from damage caused by such radiation.

These mechanisms became significantly more important in humans. The yellow- or red-brown to black melanins are a natural sunscreen for him. There are many indications for this. Fair-skinned people who live in regions with strong sunlight - such as Central Europeans in Australia - are particularly at risk of skin cancer. In particular, however, people also get sick Xeroderma pigmentosummuch more often from skin cancer. In this hereditary "moonlight disease", the cells lack repair mechanisms for UV damage, so that any exposure to the sun is dangerous. Usually it is a question of comparatively easily treatable squamous cell and basal cell carcinoma. The particularly dangerous malignant melanomas, which make up four percent of skin cancer cases and mainly affect whites, occur less frequently. Even so, the disease is often fatal at an early age.

Because of these and many similar findings, it was widely believed that tropical populations developed their dark skin primarily to protect against skin cancer. From an evolutionary point of view, however, this is not understandable. Most of the time, such carcinomas do not appear before middle adulthood, when people can already have some children and certainly mostly had them earlier. However, evolutionary adaptations only arise if they have a favorable effect on reproduction. Skin cancer would rarely have significantly reduced one's own chances of reproduction. There had to be at least other important reasons for pigment formation in the skin.

By chance one of us (Jablonski) came across a 1978 publication by Richard F. Branda and John W. Eaton, now at the University of Vermont in Burlington and the University of Louisville, Kentucky, respectively. The two researchers discovered that fair-skinned people had unusually low levels of folate (folic acid) in their blood after exposure to strong artificial sunlight - which penetrates into the fine, superficial blood vessels of unprotected skin. As Branda and Eaton also demonstrated, the content of this vital B vitamin in human blood serum is halved within an hour if the serum is irradiated with artificial sunlight.

What this could mean for reproduction - and may have meant in human evolution - suddenly became clear to us when we heard about a study by Fiona J. Stanley and Carol Bower at the University of Western Australia at Nedlands in the late 1980s . The two scientists investigated the causes of congenital malformations in the area of ​​the spinal cord and spinal cord, for example for the so-called open back (vertebral gap, Spina bifida). Children with this severe deformity, in which sections of the spinal cord are exposed or at least not completely protected by vertebrae, often have severe physical disabilities and often also suffer from a head of water. Stanley and Bower recognized that folic acid deficiency during pregnancy increases the risk of such undesirable developments. Many studies around the world have now confirmed the connection. Folic acid deficiency, especially in early pregnancy, should also cause cleft lip and palate. It is therefore recommended that pregnant women and women who want to have children take folic acid supplements today. The B vitamin is particularly found in food in egg yolks, liver, wheat germ and leafy vegetables (spinach, endive, broccoli).

We soon came across further evidence that folic acid is important for the body in many other ways as well. When cells divide, cells need the substance to synthesize new DNA. Folic acid is therefore indispensable wherever cells multiply quickly, as is the case with the formation of sperm. This has been demonstrated in rats and mice. If a chemical agent causes a folic acid deficiency in male rodents, their sperm production is disturbed and the animals become sterile. There are no comparable studies in humans, but Wai Yee Wong and his colleagues at the University Hospital in Nijmegen in the Netherlands recently reported that the sperm count of men with reduced fertility may be increased if they are given folic acid.

The sun destroys folic acid

We therefore suspect that dark skin developed in human evolution to protect the body's folic acid reserves from UV radiation. A report published in 1996 by the Argentine pediatrician Pablo Lapunzina also fits our thesis. Three generally healthy young women whom he looked after gave birth to children Spina bifida. In the first few weeks of pregnancy they had visited solariums.

Back to our ancestors. As the homo sapiensOriginated in Africa a few hundred thousand years ago, it must have been dark-skinned, because people certainly no longer wore fur at that time. So they had adapted to the hot climate and the strong UV radiation near the equator. The so-called modern man, who appeared there 120,000 to 100,000 years ago, must therefore have looked dark (see also "The origin was in Africa", SdW 3/2003, p. 38).

But modern people soon moved away from the African continent and also settled in regions where UV radiation is seen over the year and is significantly lower, especially in the winter months. There, the thick sun protection was not only superfluous due to many dark pigments, but it may even have proven to be disadvantageous. Dark skin contains so much melanin in the outer layers that very little UV radiation penetrates deeply in temperate latitudes. Above all, UV-B, which has a shorter wavelength than UV-A, is largely intercepted.

Why skin had to get light

UV-B has mostly dangerous, undesirable effects, but this radiation also fulfills an indispensable task: It sets the synthesis of vitamin D in motion in the skin, which is involved in calcium and phosphate metabolism and thus in particular in building bones. The first step in the skin is a preliminary stage of the vitamin, which is converted into vitamin D in the kidneys. Under the strong solar radiation of the tropics, dark-skinned people still take in enough UV-B for vitamin D synthesis. But at higher latitudes the dose for dark skin would be too low for most of the year. To compensate, the skin color became lighter.

The thesis that pigmentation decreased because of vitamin D was brought up by W. Farnsworth Loomis from Brandeis University in Waltham (Massachusetts) in 1967. He recognized a connection with reproduction and immune functions. Vitamin D is necessary for calcium absorption from the intestine. Without this mechanism, the skeleton cannot develop normally because bone substance consists largely of this mineral. Calcium is essential for many basic physiological processes. The immune system is also dependent on sufficient calcium.

Michael Hollick from the University of Boston (Massachusetts) and his colleagues have underpinned these relationships through their medical studies over the past two decades. They also showed that sunlight at higher latitudes in winter does not contain enough UV-B radiation for vitamin D synthesis, even for fair-skinned people. In the city of Boston on the American east coast, for example, light skin can apparently only be replenished from around mid-March. Boston is located on the 42nd parallel north, roughly at the same level as Rome and northern Spain. (The southern German city of Freiburg is at the 48th parallel, Rostock on the Baltic Sea at the 54th parallel.)

We had to search for a long time until we found a source in which the UV radiation intensities of the individual regions worldwide were compiled. It wasn't until 1996 that Elisabeth Weatherhead from the University of Colorado in Boulder helped us. It provided us with data from measurements of UV radiation on the ground, which a NASA satellite had recorded between 1978 and 1993 for maps of the ozone values. From this we determined the UV radiation over the areas of the earth. From this we calculated, using known minimum values ​​for people of different skin colors, where and when enough UV-B radiation reaches the earth so that vitamin D synthesis begins in our skin.

For fair-skinned people, the earth's surface could roughly be divided into three vitamin D zones. Among other things, the radiation intensity also depends on other geographic factors, such as the height above sea level. At the level of the tropics, roughly up to about the thirtieth north and south latitude, the UV-B dose is high enough all year round. Further north (or south in the southern hemisphere), the UV-B values ​​do not reach the necessary intensity for increasingly longer time phases. Beyond the 30th parallel (at the height of North Africa), in the second zone, this is initially a winter month, further north it is almost half a year. In the third zone, which begins around the 50th parallel (near Frankfurt), this phase lasts half a year and longer. Seen over the course of the year, people north of around the 45th parallel would therefore have to suffer from vitamin D deficiency symptoms, unless they compensate for this with food.

This radiation pattern should also be a reason why people in the far north and also in northern central Europe often do not tan at all: their skin should always be able to catch as much sun as possible. The mid-latitudes, on the other hand, regularly have a paler complexion in winter than in summer. Your skin adapts to the seasons: in winter it uses the little sunlight, in summer it protects itself from too much UV radiation. In the vicinity of the equator, on the other hand, the radiation is so strong that continuous pigment protection still enables sufficient vitamin D production.

A woman's slightly lighter skin: risk to the child's welfare

Not all populations fit into this scheme. In some arctic areas there are also groups of people whose complexion is actually too dark for these latitudes according to our model. These include the Inuit (Eskimo) of Alaska and Northern Canada. On the one hand, their relatively dark skin could be explained by the fact that they immigrated from Asia to North America barely 5000 years ago. On the other hand, there was apparently no great need for her to become fairer. The Inuit traditionally eat a lot of fish and other marine animals, a diet particularly rich in vitamin D.

Our model could also explain another phenomenon of skin color that anthropologists noticed earlier: In general, women in a population have a slightly lighter complexion than men. According to the data available, female skin is three to four percent lighter. Some researchers suspect that this stems from sexual selection, namely from a preference of men for women with fair skin. This connection may well play a role, but the actual background may originally have been different. During pregnancy and breastfeeding, women need a lot of calcium and therefore a lot of vitamin D for their child, so that they can get as much of the mineral as possible from their food. Maybe her slightly lighter skin is an adaptation so that a little more UV-B gets into the skin. In areas with strong UV radiation, this appears to be a delicate balancing act in evolution. On the one hand, too much UV radiation must not penetrate in order not to endanger the womb. On the other hand, vitamin D synthesis must also be guaranteed for the child's well-being.

As long as anatomically modern humans exist and they have been moving around the world, i.e. for at least 100,000 years, their complexion will probably adapt to the regional solar radiation again and again. The old African populations had the longest time to do this, because the modern one was on their continent homo sapiensoriginated. Only then did modern people come to different regions of Asia. Finally, tens of thousands of years ago, they also reached Australia, Europe and, lastly, America. It is partly understandable how their skin color gradually changed as they moved north or south again. However, one must also take into account that these people already protected themselves from the elements with clothing and dwellings, so they were no longer exposed to the local conditions without restrictions. In some places they probably compensated for the vitamin D deficiency with food, similar to the Inuit. The food, clothing and roofed-over apartments presumably determine how quickly and how strongly skin pigmentation changed evolutionarily in each case.

Even in Africa itself with its many different regions, such adaptations can still be seen today. Apparently there was a series of migrations away from the equatorial areas to the south. For example, the ancestors of the Khoisan - the Hottentots and Bushmen - have lived in southern Africa for a long time. They have significantly lighter skin than equatorial Africans. This should be an adaptation to the lower UV radiation in South Africa.

Interestingly, today's South African populations who speak Bantu languages ​​are much darker than the Khoisan. It has been historically proven that they came to the region relatively recently from areas near the equator in West Africa. That was probably a thousand years at the most, a very short period of time by evolutionary standards.

Especially in recent times, cultural traits are likely to have strongly influenced whether skin colors had to adapt or not. For example, very dark people live on the African side of the Red Sea, and significantly lighter ones on the Arabian Peninsula. The populations on the west side speak Nilo-Saharan languages ​​and have likely lived in the region for 6,000 years or more.They are extremely slender and long-limbed, so physically well equipped for the hot climate and the strong radiation of this area.

Skin color not a racial characteristic

In contrast, the farmers and shepherds on the east bank, whose ancestors came from the north, have only lived there for about 2000 years. Actually, these descendants of the first Arabs are too light for these latitudes. But they compensate for this with thick clothing that protects them almost completely from the sun, and they use tents that they carry with them. It appears that the cultural adaptation becomes more pronounced than the biological when a population has not long been at home in an area.

In our time, however, the cultural changeover to the solar conditions in a new country often does not take place quickly enough, often out of ignorance. The result can be diseases that the relevant groups of people were previously unfamiliar with or that were considered to have been overcome. Fair-skinned former Northern Europeans are increasingly paying for sunbathing in Florida or Northern Australia with premature aging and even skin cancer. We do not know how many miscarriages or malformations in newborns are due to a sun-induced folic acid deficiency. Conversely, darkly pigmented people from South Asia or Africa in the north suffer from rickets and other vitamin D deficiency symptoms that have almost been forgotten here, as in Great Britain, especially in its north, many Indians.

In the past, scientists used to classify people into races based on skin color. This approach is now considered obsolete. The different color nuances only reflect adaptations to different environments. When measured in evolutionary time, pigmentation can change rapidly. This is why skin color is one of the least meaningful characteristics for recognizing relationships between groups of people.

Bibliography


The Evolution of Human Skin Coloration. By Nina G. Jablonski and George Chaplin in: Journal of Human Evolution, Vol. 39, Issue 1, July 1, 2000, p. 57.


Shortly


- The early hominids got dark skin when they lost their thick body hair - an adaptation to their life in the hot savannah. However, the pigmentation was not created to protect against skin cancer.

- Rather, dark skin protects against folic acid breakdown by UV radiation. Otherwise there is a risk of infertility and child malformations.

- Conversely, at higher latitudes, the skin must be just so light that enough UV rays penetrate for vitamin D synthesis. This can be a delicate balancing act for pregnancy and breastfeeding.

- Today, many people no longer live in areas to which their skin color is adapted. Older cultures offset the associated health hazards through behavior. Such knowledge is often lacking in modern migrants.

From: Spektrum der Wissenschaft 6/2003, page 38
© Spektrum der Wissenschaft Verlagsgesellschaft mbH

This article is included in Spectrum of Science 6/2003