Information

How Dark skin protects humans in sunnier climates?

How Dark skin protects humans in sunnier climates?


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

According to Physics, Black Body absorb all incoming light reflect nothing when compared to White body. This phenomenon is called Black-body radiation. So Melanin should turn your skin into white for protect the body from harmful rays in sunnier regions, that is not happening in Biology of Human Beings.

This is why Most of School Uniforms are specifically white on Saturdays (Because of intensity of sunlight is more on that day)

Then How Dark skin protects humans in sunnier climates?

My Question is different from this question because I mentioned physics subject also in my question.


Your question is based in a wrong assumption: that the epidermis should be white or that a white epidermis would reflect electromagnetic radiation and, therefore, protect the DNA against the ionizing portion of that radiation.

Before anything else: human epidermis without pigments (like melanin) has no color, that is, it's transparent. The white color we see in people with low melanin concentration is mostly due to the collagen in the underlying connective tissue1.

So, we have two possibilities:

  1. A transparent epidermis, which allows ionizing radiation reaching the living layers of epidermis (stratum granulosum and below, see the image) and dermis;
  2. An epidermis with a pigment that somehow "blocks" (absorbing or reflecting) the ionizing radiation.

Of course, it's not a good idea allowing all that ionizing radiation getting to the DNA, and because of that possibility #2 was the selected one. Therefore, there is production/accumulation of a pigment in the epidermis.

But which one, a white one or a black one?

Two important pieces of information may help you to understand this point.

First, the white pigment assumption doesn't mean the pigment need to be white: the pigment only have to reflect the ionizing (UV) section of the spectrum. It could be of any color, even almost black. Actually, melanin does reflects some UV radiation2.

Second, and the most important: if a given pigment just reflects the radiation, this radiation can still go to other nuclei and reach DNA, just like the scattered blue light in the sky goes everywhere, up and down. It's way more logical (I'm not implying that evolution operates logically, I'm just addressing your question) having an pigment that absorbs the ionizing radiation, which is the case of melanin.

By absorbing ionizing radiation, melanin (partially) avoids this ionizing radiation reaching the DNA.

References:

  1. Skin: A Natural History; Nina G. Jablonski, ISBN: 9780520275898, February 2013

  2. Brenner M, Hearing VJ. The Protective Role of Melanin Against UV Damage in Human Skin. Photochemistry and photobiology. 2008;84(3):539-549. doi:10.1111/j.1751-1097.2007.00226.x.


  1. Melanin is an antioxidant and absorbs free radicals associated with UV rays, iron, metals etc. This explains why anemia correlates with melanin.

  2. Wetter climates have more diseases and free radicals, which is why moister climates are melanizing as well.

  3. Melanin can potentially crowd in the body's own oxidants, such as testosterone, because it removes external oxidants.


Climate may have shaped the evolution of the human nose

Pixabay

In the late 1800s, British anthropologist and anatomist Arthur Thomson posited that people with ancestral origins in cold, arid climates were likely to have longer, thinner noses, while those who came from warm, humid regions were inclined to have noses that were shorter and thicker.

His theory was that climate has a profound influence on the shape of the human nose, more so than any other evolutionary factor, because one important job of the nose is to warm and humidify air inhaled through the nostrils. This suggests it is an advantage for people in colder climates to have narrower nostrils, and vice versa.

Over the years, scientists have tested Thomson’s Nose Rule, as it came to be known, with skull measurements, but until recently no one had ever studied these dimensions in live people.

Pennsylvania State University researchers did just that in a study published Thursday in PLOS Genetics, confirming that Thomson was onto something. They concluded that the size and shape of noses evolved, at least in part, as a response to local climate conditions.

Pixabay

“We are primarily interested in understanding how human variation arises,” says Arslan Zaidi, a postdoctoral fellow in biology and the study’s lead author. “The questions we ask are: why do we look different from one another? Why do males and females look different? Why are there differences among humans from different populations? We focused on the nose because there is a huge body of work suggesting that it may have evolved in response to climate.”

The research is important because studying human evolution and adaptation can have significant implications for human health. For example, people of Northern European ancestry — because of their light skin — carry an increased risk of sunburn and skin cancer when they are near the equator. Similarly, dark-skinned individuals carry an increased risk of vitamin-D deficiency at higher latitudes.

“These risks are a direct consequence of our evolutionary history,” Zaidi says. “Dark skin evolved to protect us from overexposure to ultraviolet radiation, and lighter skin evolved to allow us more absorption of UV so that we can synthesize more vitamin D. If nose shape evolution has indeed been driven by climate, does moving to a different climate increase our risk of respiratory disease? This is unclear at this point, but important to pursue.”

The researchers looked at a variety of nose measurements. Using three-dimensional facial imaging, they examined the width of the nostrils, the distance between nostrils, the height of the nose, nose ridge length, nose protrusion, external area of the nose, and the area of the nostrils. They focused on individuals of four different ancestries: South Asian, East Asian, West African and Northern European.

This diagram represents the evolution of the human nose. Blue boxes, corresponding to colder climates, represent narrower nostrils. Red boxes, corresponding to warmer climates, represent wider nostrils. Colors in between represent intermediate nose shapes. PLOS Genetics

They asked two questions: Are some aspects of nose shape more varied across populations than expected with genetic drift? (Genetic drift is a random evolutionary process leading to differences among populations over a long period of time, simply by chance.) If so, can this variation be explained by climate?

“In other words, if two populations are isolated for a long time, we expect their noses to look different just by chance, because of genetic drift,” Zaidi says. “We needed to rule this out to show that the variation among human populations was more than that expected just by genetic drift. Out of the seven measurements describing nose shape, we found two measurements related to the width of the nose to be significantly more differentiated among populations than expected by genetic drift. This means that the difference in nose width among human populations is more than is expected by random chance.”

Researchers found a positive correlation between nostril width and temperature and humidity, suggesting that natural selection likely plays a significant role in human nose evolution. Natural selection is the process by which organisms that are well adapted to their environment tend to survive and pass their traits to succeeding generations, while ill-adapted organisms tend to die off.

But humans have always moved around, and these days it’s not uncommon for someone from a long line of cold climates to live by the equator. They’ll likely sport the narrow nostrils of their forbears.

“Evolution takes a long time,” Zaidi says. “If nose shape has evolved in the past to adapt to local climate, it likely took tens of thousands of years. So, my great-great-great grandkids are likely still going to have wider noses — I’m Pakistani — even if they continue to live in a colder climate, as long as they continue to marry other South Asians.”

Moreover, “human variation does not agree with notions of race,” he adds. “There are more similarities among humans from different populations than there are differences, both genetically and phenotypically. Traits such as skin pigmentation and nose width appear more different because they are examples of external traits that are exposed to the environment, and have evolved faster than most other human traits. They are an exception rather than the rule. This is an important caveat to make, because people often tend to focus on differences and ignore the similarities.”

The researchers also noted that other factors may also be involved, such as gender differences. Men tend to be larger than women, for example, so their noses tend to be larger as well. Other variations emerge because people may prefer mates with smaller or larger noses. Still, concepts of beauty may be related to how well-adapted a nose is to the local climate, according to the scientists.

As for the future of the schnoz, Zaidi says that evolution is “a wildly random” process, making it difficult to predict what will happen to the human nose in response to global warming.

“Human evolution, at this point, is very different from evolution in the past,” he says. “Our lifestyles aren’t what they used to be, and we move around the world way too much. That makes it very complicated to predict the future evolutionary trajectory of the nose with the changing climate.”

Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.


The Ancient Origins of Both Light and Dark Skin

A study of diverse people from Africa shows that the genetic story of our skin is more complicated than previously thought.

Few human traits are more variable, more obvious, and more historically divisive than the color of our skin. And yet, for all its social and scientific importance, we know very little about how our genes influence its pigment. What we do know comes almost entirely from studying people of European descent.

To Sarah Tishkoff, a geneticist at the University of Pennsylvania, that’s a ridiculous state of affairs. “It gives you a very incomplete perspective,” she says.

To redress that imbalance, Tishkoff and her team looked to Africa—the continent where humanity is at its most physically and genetically diverse. They recruited 1,570 volunteers from 10 ethnic groups in Ethiopia, Tanzania, and Botswana, and measured the amount of the dark pigment melanin in the skin of their inner arms. Then the team looked at more than 4 million spots in the volunteers’ genomes where DNA can vary by a single letter, to identify which variations are associated with their skin color.

They found several, clustered around six specific genes: SLC24A5, MFSD12, DDB1, TMEM138, OCA2 and HERC2. And they showed that these variants collectively account for 29 percent of the variation in skin color in the three countries studied. That’s a big proportion! For comparison, a similar and much bigger study identified hundreds of genes that affect one’s height, but that collectively account for just 16 percent of the variation that you see in large populations.

Tishkoff says that her results complicate the traditional evolutionary story of human skin. In this view, humanity began with dark skin in Africa to protect against the harmful effects of the sun’s ultraviolet radiation. As people migrated to other continents, some groups evolved lighter skin, to more effectively produce vitamin D in areas where sunlight is scarce.

But most of the variants that Tishkoff’s team identified, for both light and dark skin, have an ancient African origin. They likely arose in hominids like Homo erectus long before the dawn of our own species, and have coexisted in balance for hundreds and thousands of years. In many cases, the older variant is responsible for lighter skin, not darker. That’s consistent with an idea from Nina Jablonski, an anthropologist from Pennsylvania State University, who thinks that the ancient ancestors of humans—much like other primates—had pale skin. “As our ancestors moved out of the forest and into the savannah, they lost their hair and evolved darker skin,” says Nick Crawford, a researcher in Tishkoff’s lab.

But that wasn’t an all-encompassing change. Different groups of people adapted to their own particular environments, not just around the world, but within Africa, too. “Africa is not some homogenous place where everyone has dark skin,” Tishkoff says. “There’s huge variation.” For example, her team’s measurements showed that the Nilotic peoples in eastern Africa have some of the darkest skin around, while the San of southern Africa have light skin, comparable to some East Asians.

This physical diversity is mirrored in these groups’ genes. The first gene identified as affecting human skin color—MC1R—is very diverse in European populations but remarkably similar across African ones. Based on that pattern, says Tishkoff, some geneticists have concluded that the evolutionary pressure for dark skin in Africa is so strong that any genetic variants that altered skin color were ruthlessly weeded out by natural selection. “That’s not true,” says Tishkoff—but it’s what happens when you only study skin color in Western countries. “When you look at this African-centered perspective, there’s a lot of variation.”

For example, a gene called MFSD12 has variants that are linked to darker skin these are common in dark-skinned people from East Africa, but rare among the lighter-skinned San. MFSD12 also shows how the search for pigmentation genes can reveal new insights about the basic biology of our skin. Two years ago, the gene didn’t even have a name, but it was linked to vitiligo—a condition where people develop white patches on dark skin. By deleting the gene in fish and mice, Tishkoff’s colleagues confirmed that it controls the balance between light and dark pigments.

Another gene called SLC24A5 has a variant that has traditionally been seen as “European,” because it is so starkly associated with lighter skin in Western European populations. But Tishkoff’s team showed that the variant entered the East African gene pool from the Middle East several millennia ago and well before the era of colonization. Today, it is common in Ethiopian and Tanzanian groups, but rare in other areas.

Critically, in East African groups, the variant doesn’t lighten skin color to the same degree that it does in Europeans. It’s a stark reminder that “a person can carry a gene that confers a particular trait in one population and yet not obviously show evidence of that trait themselves,” says Jablonski. “It reminds us that we can’t be cavalier about stating that a particular crime suspect has a particular skin color based on the presence of a single genetic variant in their DNA.”

Sandra Beleza, from the University of Leicester, has done one of the only other genetic studies of skin color to include people of mixed African ancestry. She says that neither her work nor Tishkoff’s have come close to identifying all the genes behind this trait. Further studies, involving other African populations that haven’t been included in genetic studies yet, may help to plug that gap.

While many have used skin color as a means of dividing people, Tishkoff sees the potential for unity and connectedness. “One of the traits that most people would associate with race—skin color—is a terrible classifier,” she says. Even without supposedly “dark” skin, there is a lot of hidden variation. “The study really discredits the idea of a biological construct of race,” she adds. “There are no discrete boundaries between groups that are consistent with biological markers.”

Jedidiah Carlson from the University of Michigan, who has been keeping tabs on how white-supremacist groups misappropriate genetic studies, agrees. “Because visually distinguishable traits common in present-day Europeans, such as light skin color, are also assumed to have arisen within European populations, white supremacists treat these traits as a proxy for superior intelligence,” he says. The history of SLC24A5 reminds us that “light skin pigmentation, and likely other ‘European’ traits, are not unique to Europeans. Human populations have been interbreeding for as long as we have existed as a species.”

White-supremacist communities “often rally around the demonstrably false claim that Africans are more genetically similar to ancestral hominids than Europeans—and these results turn the tables,” Carlson adds. At several genes that influence skin pigments, “Europeans are actually more likely to be genetically similar to great apes.”


Modern scientific explanations of human biological variation

Contemporary scientists hold that human physical variations, especially in those traits that are normally used to classify people racially—skin colour, hair texture, facial features, and to some extent bodily structure—must be understood in terms of evolutionary processes and the long-range adaptation of human groups to differing environments. Other features may simply reflect accidental mutations or functionally neutral changes in the genetic code.

In any given habitat, natural forces operate on all of the living forms, including human groups. The necessary interaction with these forces will affect the survival and reproduction of the members of these societies. Such groups already have a wide and complex range of hereditary physical characteristics indeed, human hereditary variability is a product of human sexual reproduction, whereby every individual receives half of his or her genetic endowment from each parent and no two individuals (except for identical twins) inherit the same combination of genetic features.

The global distribution of skin colour (see map ) is the best example of adaptation, and the consequences of this process have long been well known. Skin colour clines (gradations) in indigenous populations worldwide correlate with latitude and amounts of sunlight. Indigenous populations within a broad band known as the tropics (the regions falling in latitude between the Tropics of Cancer and Capricorn) have darker skin colours than indigenous populations outside of these regions.

Within the tropics, skin colours vary from light tan to very dark brown or black, both among populations and among individuals within groups. The darkest skin colours are found in those populations long residing in regions where intense ultraviolet sunlight is greatest and there is little natural forest cover. The bluish black skins of some peoples—such as some of the Dravidians of South India, the peoples of Sri Lanka and Bangladesh, and peoples of the eastern Sudan zone, including Nubia, and the grasslands of Africa—are examples of the extremes of dark skin colour. Medium brown to dark brown peoples are found in the rest of tropical Africa and India and throughout Australia, Melanesia, and other parts of Southeast Asia.

Peoples with light skin colours evolved over thousands of years in northern temperate climates. Human groups intermittently migrating into Europe and the northern parts of the Eurasian landmass over the past 25,000–50,000 years experienced a gradual loss of skin pigmentation. The changes were both physiological and genetic that is, there were systemic changes in individuals and long-range genetic changes as a result of natural selection and, possibly, mutations. Those individuals with the lightest skin colours, with lowest amounts of melanin, survived and reproduced in larger numbers and thus passed on their genes for lighter skin. Over time, entire populations living in northern climates evolved lighter skin tones than those individuals living in areas with higher levels of sunlight. Between populations with light skin and those with the darkest coloration are populations with various shades of light tan to brown. The cline in skin colours shows variation by infinite degrees any attempts to place boundaries along this cline represent purely arbitrary decisions.

Scientists at the turn of the 21st century understood why these superficial visible differences developed. Melanin, a substance that makes the skin dark, has been shown to confer protection from sunburn and skin cancers in those very areas where ultraviolet sunlight is strongest. Dark skin, which tends to be thicker than light skin, may have other protective functions in tropical environments where biting insects and other vectors of disease are constant threats to human survival. But humans also need vitamin D, which is synthesized by sunlight from sterols (chemical compounds) present in the skin. Vitamin D affects bone growth, and, without a sufficient amount, the disease known as rickets would have been devastating to early human groups trying to survive in the cold, wintry weather of the north. As these groups adapted to northern climates with limited sunlight, natural selection brought about the gradual loss of melanin in favour of skin tones that enabled some individuals to better synthesize vitamin D.

Other physical characteristics indicate adaptations to cold or hot climates, to variations in elevation from sea level, to rainforests with high levels of rainfall, and to hot deserts. Body structure and the amount of body fat have also been explained by evolutionists in terms of human adaptation to differing environments. Long, linear body builds seems to be highly correlated with hot, dry climates. Such people inhabit the Sahara and the desiccated areas of the Sudan in Africa. Short, stocky body builds with stubby fingers and toes are correlated with cold, wet climates, such as are found in Arctic areas. People adapted to cold climates have acquired genetic traits that provide them extra layers of body fat, which accounts for the epicanthic fold over their eyes. People who live in areas of high elevation, as in the mountains of Peru, tend to have an adaptive feature not found among peoples who live at sea level they have larger lungs and chest cavities. In an atmosphere where the oxygen supply is low, larger lungs are clearly adaptive.

Some adaptive variations are not obviously visible or measurable. Many peoples adapted to cold climates, for example, have protective physiological reactions in their blood supply. Their blood vessels either constrict the flow to extremities to keep the inner body warm while their surface skin may be very cold (vasoconstriction) or dilate to increase the blood flow to the hands, feet, and head to warm the outer surfaces (vasodilation).

The prevalence of diseases has been another major factor in the evolution of human diversity, and some of the most important of human genetic variations reflect differences in immunities to diseases. The sickle cell trait ( hemoglobin S), for example, is found chiefly in those regions of the tropical world where malaria is endemic. Hemoglobin S in its heterozygous form (inherited from one parent only) confers some immunity to those people who carry it, although it brings a deadly disease ( sickle cell anemia) in its homozygous form (inherited from both parents).

In the last decades of the 20th century, scientists began to understand human physical variability in clinal terms and to recognize that it reflects much more complex gradations and combinations than they had anticipated. To comprehend the full expression of a feature’s genetic variability, it must be studied separately over geographic space and often in terms of its adaptive value. Many features are now known to relate to the environmental conditions of the populations that carry them.


The Biology of . . . Skin Color

Ten years ago, while at the university of Western Australia, anthropologist Nina Jablonski was asked to give a lecture on human skin. As an expert in primate evolution, she decided to discuss the evolution of skin color, but when she went through the literature on the subject she was dismayed. Some theories advanced before the 1970s tended to be racist, and others were less than convincing. White skin, for example, was reported to be more resistant to cold weather, although groups like the Inuit are both dark and particularly resistant to cold. After the 1970s, when researchers were presumably more aware of the controversy such studies could kick up, there was very little work at all. "It's one of these things everybody notices," Jablonski says, "but nobody wants to talk about."

No longer. Jablonski and her husband, George Chaplin, a geographic information systems specialist, have formulated the first comprehensive theory of skin color. Their findings, published in a recent issue of the Journal of Human Evolution, show a strong, somewhat predictable correlation between skin color and the strength of sunlight across the globe. But they also show a deeper, more surprising process at work: Skin color, they say, is largely a matter of vitamins.

Jablonski, now chairman of the anthropology department at the California Academy of Sciences, begins by assuming that our earliest ancestors had fair skin just like chimpanzees, our closest biological relatives. Between 4.5 million and 2 million years ago, early humans moved from the rain forest and onto the East African savanna. Once on the savanna, they not only had to cope with more exposure to the sun, but they also had to work harder to gather food. Mammalian brains are particularly vulnerable to overheating: A change of only five or six degrees can cause a heatstroke. So our ancestors had to develop a better cooling system.

The answer was sweat, which dissipates heat through evaporation. Early humans probably had few sweat glands, like chimpanzees, and those were mainly located on the palms of their hands and the bottoms of their feet. Occasionally, however, individuals were born with more glands than usual. The more they could sweat, the longer they could forage before the heat forced them back into the shade. The more they could forage, the better their chances of having healthy offspring and of passing on their sweat glands to future generations.

A million years of natural selection later, each human has about 2 million sweat glands spread across his or her body. Human skin, being less hairy than chimpanzee skin, "dries much quicker," says Adrienne Zihlman, an anthropologist at the University of California at Santa Cruz. "Just think how after a bath it takes much longer for wet hair to dry."

Hairless skin, however, is particularly vulnerable to damage from sunlight. Scientists long assumed that humans evolved melanin, the main determinant of skin color, to absorb or disperse ultraviolet light. But what is it about ultraviolet light that melanin protects against? Some researchers pointed to the threat of skin cancer. But cancer usually develops late in life, after a person has already reproduced. Others suggested that sunburned nipples would have hampered breast-feeding. But a slight tan is enough to protect mothers against that problem.

During her preparation for the lecture in Australia, Jablonski found a 1978 study that examined the effects of ultraviolet light on folate, a member of the vitamin B complex. An hour of intense sunlight, the study showed, is enough to cut folate levels in half if your skin is light. Jablonski made the next, crucial connection only a few weeks later. At a seminar on embryonic development, she heard that low folate levels are correlated with neural-tube defects such as spina bifida and anencephaly, in which infants are born without a full brain or spinal cord.

Jablonski and Chaplin predicted the skin colors of indigenous people across the globe based on how much ultraviolet light different areas receive. Graphic by Matt Zang, adapted from the data of N. Jablonski and G. Chaplin

Jablonski later came across three documented cases in which children's neural-tube defects were linked to their mothers' visits to tanning studios during early pregnancy. Moreover, she found that folate is crucial to sperm development— so much so that a folate inhibitor was developed as a male contraceptive. ("It never got anywhere," Jablonski says. "It was so effective that it knocked out all folate in the body.") She now had some intriguing evidence that folate might be the driving force behind the evolution of darker skin. But why do some people have light skin?

As far back as the 1960s, the biochemist W. Farnsworth Loomis had suggested that skin color is determined by the body's need for vitamin D. The vitamin helps the body absorb calcium and deposit it in bones, an essential function, particularly in fast-growing embryos. (The need for vitamin D during pregnancy may explain why women around the globe tend to have lighter skin than men.) Unlike folate, vitamin D depends on ultraviolet light for its production in the body. Loomis believed that people who live in the north, where daylight is weakest, evolved fair skin to help absorb more ultraviolet light and that people in the tropics evolved dark skin to block the light, keeping the body from overdosing on vitamin D, which can be toxic at high concentrations.

By the time Jablonski did her research, Loomis's hypothesis had been partially disproved. "You can never overdose on natural amounts of vitamin D," Jablonski says. "There are only rare cases where people take too many cod-liver supplements." But Loomis's insight about fair skin held up, and it made a perfect complement for Jablonski's insight about folate and dark skin. The next step was to find some hard data correlating skin color to light levels.

Until the 1980s, researchers could only estimate how much ultraviolet radiation reaches Earth's surface. But in 1978, NASA launched the Total Ozone Mapping Spectrometer. Three years ago, Jablonski and Chaplin took the spectrometer's global ultraviolet measurements and compared them with published data on skin color in indigenous populations from more than 50 countries. To their delight, there was an unmistakable correlation: The weaker the ultraviolet light, the fairer the skin. Jablonski went on to show that people living above 50 degrees latitude have the highest risk of vitamin D deficiency. "This was one of the last barriers in the history of human settlement," Jablonski says. "Only after humans learned fishing, and therefore had access to food rich in vitamin D, could they settle these regions."

Humans have spent most of their history moving around. To do that, they've had to adapt their tools, clothes, housing, and eating habits to each new climate and landscape. But Jablonski's work indicates that our adaptations go much further. People in the tropics have developed dark skin to block out the sun and protect their body's folate reserves. People far from the equator have developed fair skin to drink in the sun and produce adequate amounts of vitamin D during the long winter months.

Jablonski hopes that her research will alert people to the importance of vitamin D and folate in their diet. It's already known, for example, that dark-skinned people who move to cloudy climes can develop conditions such as rickets from vitamin D deficiencies. More important, Jablonski hopes her work will begin to change the way people think about skin color. "We can take a topic that has caused so much disagreement, so much suffering, and so much misunderstanding," she says, "and completely disarm it."


Always Revealing, Human Skin Is an Anthropologist’s Map

In an era of academic hyper-specialization, Dr. Nina G. Jablonski has an amazingly broad résumé. At 53, she heads the anthropology department at Pennsylvania State University. She’s also a primatologist, an evolutionary biologist and a paleontologist.

Last year, Dr. Jablonski led an expedition to China, where she dug for human fossils in an attempt to learn how early man coped with climate change. This month, she’s in Kenya, where she and Meave Leakey are putting together a study on prehistoric monkeys.

For more than a decade, Dr. Jablonski has been trying to get her arms around a ubiquitous and yet mysterious topic: the biology, evolution and social function of human skin. The results of her studies have been published by the University of California Press as “Skin: A Natural History.”

“Skin has been studied to absolute death by dermatologists,” Dr. Jablonski said jokingly during a recent visit to New York City. “They know it inside and out from the point of view of diseases that afflict it. What we wanted to learn was how human skin came to be as it is and what that meant for humanity.”

Q. What set you off on writing a natural history of human skin?

A. I had an insight in 1981, when I was teaching gross anatomy to medical students at the University of Hong Kong. The students had been presented with a cadaver to dissect, and they were tremendously frightened of it. However, their attitude changed the very moment they cut through the skin. With the skin gone, they began seeing it as a mere body devoid of a personal history, and they could get on with their work.

That moment showed me how much of what we consider our humanity is imbued in our skin. It stayed with me for a long time. Then about 15 years ago, I joined a project studying the natural history of skin color. The topic was so engrossing that I began looking into the larger question of what our skin does and is.

Q. And what have you found?

A. That skin is the most underappreciated of our organs. Unless we’re having the sort of problem that brings us to a dermatologist, we take our skin for granted. We never think of it as working very hard for our body or doing valuable things for us socially.

But when you really start thinking about it, it’s a factory that produces vitamin D, sweat, hormones, oils, wax, pigments — substances we need. Skin is a raincoat in that it protects us from water, bugs and noxious chemicals. It’s also a billboard which we adorn with powder, tattoos, piercing and scars to give off instant messages about our history, health, values and availability for mating.

On an evolutionary level, there are three remarkable facts about skin. It comes in colors, of course. Compared to other mammals, our skin is relatively hairless. And it’s sweaty. In the last few million years, humans became the sweatiest of mammals.

Q. Is that important?

A. Absolutely. It’s often said that our large brains are what made it possible for us to evolve from ape to human. But those big brains could never have developed if we didn’t have exceptionally sweaty skin.

It happened this way. There was a tremendous takeoff in human evolution about two million years ago when primates who could no longer be called apes appeared in the savannahs of East Africa. These early humans ran long distances in open areas. In order to survive in the equatorial sun, they needed to cool their brains. Early humans evolved an increased number of sweat glands for that purpose, which in turn permitted their brain size to expand. As soon as we developed larger brains, our planning capacity increased, and this allowed people to disperse out of Africa. There’s fossil evidence of humans appearing in Central Asia around this time.

Q. In a nutshell, what has your research shown about why humans have varying skin colors?

A. That it’s not about race — it’s about sun and about how close our ancestors lived to the Equator. Skin color is what regulates our body’s reaction to the sun and its rays. Dark skin evolved to protect the body from excessive sun rays. Light skin evolved when people migrated away from the Equator and needed to make vitamin D in their skin. To do that, they had to lose pigment. Repeatedly over history, many people moved dark to light and light to dark. That shows that color is not a permanent trait.

Q. Did early humans decorate their skin?

A. We don’t know. There’s no human skin in the fossil record. The oldest preserved skin we have is that of Ötzi, the Neolithic iceman whose mummified body was found in the Alps in 1991. Ötzi lived about 5,000 years ago. Interestingly, he has tattoos. But we can only guess what they mean.

Modern humans, we love to alter our skin. You’ll find very few people walking around today with unadorned skin. They might make permanent changes — piercing, scarring, tattooing — to memorialize events and announce their identity. Or they might use cosmetics for temporary alterations to announce their attractiveness, mood or sexual availability. The bottom line: humans are the self-decorating ape.

Q. I get the feeling that you think cosmetic use is some kind of ancient evolutionary behavior. Are we reading you correctly?

A. Evolution is all about attracting a mate and getting a chance to reproduce, so yes, makeup helps with that. When a woman uses eyeliner to make her eyes appear larger, she’s giving off a message: “I want you to see me as attractive.” Large eyes in a woman are almost universally seen as appealing. This is not just a girl thing. Male body paint in East Africa emphasizes forbidding facial expressions. They announce a man’s prowess as a warrior and as a mate.

Q. How do you feel about your own skin?

A. I like it. It is my unwritten biography. My skin reminds me that I’m a 53-year-old woman who has smiled and furrowed her brow and, on occasion, worked in the desert sun too long. I enjoy watching my skin change because it’s one of the few parts of my body that I can watch. We can’t view our livers or heart, but this we can. And yes, I use cosmetics. Like other humans, I have a penchant for changing my appearance easily and quickly. It also helps me feel more confident. That may seem silly, but I still do it.

Q. You made news in 2004 when you discovered the world’s oldest chimpanzee fossil. These were chimp teeth about a half-million years old. Where did you find them?

A. In a drawer at the National Museum of Kenya in Nairobi. I was rummaging through this bag labeled “fossil monkeys” and I saw it. “This doesn’t look like monkey,” I thought. It turned out they were from an early chimp. That find proved important because there had been no chimpanzee this old in the fossil record. By analyzing it, we’ve learned that chimpanzees in their current form have probably existed for longer than previously thought. (Laughs) Since my find, people have been rummaging through dusty museum drawers everywhere!


Understanding skin – Skin’s protective barrier

Skin, the body&rsquos largest organ, is our first and best defence against external aggressors. When skin is healthy, its many layers work hard to protect us, but when its condition is compromised, its ability to work as an effective barrier is impaired. Superior skincare choices, and the use of products that help to restore and maintain skin&rsquos optimum pH, help to protect skin and support its natural defenses making it more resilient and less sensitive. They keep skin looking and feeling its natural best and help it to do its job of protecting us.

The role of skin

Skin is the ultimate multi-tasker, performing many functions that are essential to our overall wellbeing.

It plays an important psychological role. As the most visible indication of health, the condition of our skin affects how we feel about ourselves and how others view us.

Skin is our first line of defence against external aggressors.

Why does skin need protection?

Skin works hard to protect our bodies, but the external forces it is subjected to can impact on its condition and impair its natural defence. This can affect our overall health as we become prone to injury and infection.

A stable horny layer and intact hydrolipid film work together to limit the penetration of harmful substances and excess water loss.

Epidermal lipids

These are responsible for binding in moisture and creating skin&rsquos permeability barrier, helping to prevent bacteria and viruses from penetrating the skin&rsquos surface.

The hydrolipid film

An emulsion of water and lipids (fats) which covers the surface of the skin and acts as a further barrier against toxins.

The acid mantle

The water part of the hydrolipid film. It gives skin its mildly acidic pH &ndash the perfect environment for skin-friendly microorganisms (known as skin flora) to thrive and harmful microorganisms to be destroyed. Read more in Skin&rsquos pH.

Humans come in a rainbow of hues, from dark chocolate browns to nearly translucent whites.

This full kaleidoscope of skin colors was a relatively recent evolutionary development, according to biologists, occuring alongside the migration of modern humans out of Africa between 100,000 and 50,000 years ago.

The consensus among scientists has always been that lower levels of vitamin D at higher latitudes — where the sun is less intense — caused the lightening effect when modern humans, who began darker-skinned, first migrated north.

But other factors might be at work, a new study suggests. From the varying effects of frostbite to the sexual preferences of early men, a host of theories have been reviewed.

Vitamin iDea

Vitamin D plays an important role in bone growth and the body's natural protection against certain diseases, and the inability to absorb enough in areas of less-powerful sunlight would have decreased life expectancies in our African ancestors. The further north they trekked, the more vitamin D they needed and the lighter they got over the generations, due to natural selection.

This explanation accounts for the world's gradients of skin color traveling south to north, the prevalence of vitamin D deficiency among African immigrants to higher latitudes, as well as the relatively darker skin of Canada's Inuit peoples, who have good levels of vitamin D despite living in the Arctic, due to their diet rich in oily fish.

Sounds about right . right?

In fact, there might have been a number of concurrent evolutionary pressures at work that contributed to the development of lighter skin, according to a new study published in the August issue of the Journal of Photochemistry and Photobiology B: Biology.

&ldquoIn our opinion the vitamin D hypothesis is one of the most likely hypotheses responsible for skin lightening, although there still is no consensus about it,&rdquo said study author Asta Juzeniene of the Oslo University Hospital in Oslo, Norway.

A number of competing theories were explained and evaluated by Juzeniene and her team, reopening a debate that remains one of the most interesting and controversial in biology.

Paling in comparison

Sexual selection may have played a role, for one, with males preferring paler skin in northern latitudes, the researchers surmised.

&ldquoOne of the hypotheses is that men seem to prefer women with a light skin color, which can be regarded as a sign of youth and fertility,&rdquo Juzeniene told LiveScience. &ldquoBecause light skin characterizes the early infant stage of primates, it may have become a visual cue that triggers appropriate adult behavior toward infants, i.e. decreased aggressiveness and increased desire to provide care and protection,&rdquo she said.

As lighter skin became associated with increased health in northerly latitudes, men may have preferred mates with lighter skin and produced ever-paler generations. Fertility and health statistics at different latitudes from a few thousand years ago aren't available, Juzeniene cautioned, however, so the theory is difficult to test.

Frostbite was another causal effect investigated by the researchers.

Some reports from American soldiers serving in the Korean War and elsewhere have indicated that dark skin is more prone to frostbite than white because it emits more heat. In colder climates, evolution could have negatively selected for paler skin if frostbite was significant enough to perhaps kill darker-skinned children.

Despite the anecdotal evidence, there is not enough scientific data to support frostbite as a strong enough single factor to lighten skin in places such as Europe, the researchers said.

On the farm

Another possibility noted was the switch from subsistence-based economies to agriculture approximately 10,000 years ago, which eliminated vitamin D-rich food sources from the diet. This would have had an especially potent effect in northern Europe, according to Juzeniene and her team.

&ldquoDevelopment of agriculture has occurred in several places, and did not necessarily lead to skin lightening if the ambient UVB [ultraviolet light from the sun] level was sufficiently high to allow adequate vitamin D synthesis. Cold climates and high latitudes would speed up the need for skin lightening,&rdquo however, if people were relying mainly on grains as a food source, the researchers wrote.

The main problem with this agriculture theory is that the switch from gathering to farming occurred relatively recently, and scientists question whether all of the evolutionary changes associated with skin color could have happened so quickly.

Skin lightening could also have been accelerated by something as simple as genetic drift, making it &ldquoeasier&rdquo for a pale skin mutations to succeed in northern latitudes.

Though other elements may have come into play and need to be examined further, vitamin D remains the most likely explanation, Juzeniene stressed, especially given its role in overall health.

&ldquoIf we assume that vitamin D does not play any role in the development of human skin color, neither white nor dark, many people in the world would suffer from vitamin D deficiency,&rdquo she said.

While people of all skin types have the ability to produce the same amount of vitamin D in their systems, &ldquohighly pigmented people will need to stay in the sun around 6 times longer than light people in order to synthesize the same amount of vitamin D,&rdquo Juzeniene said, and a lack of the vitamin — something occurring among many American children right now, partly because they don't get out much — can make humans more susceptible to everything from heart disease to internal cancers.


Selection for Skin Color?

Credit for describing the relationship between latitude and skin color in modern humans is usually ascribed to an Italian geographer, Renato Basutti, whose widely reproduced “skin color maps” illustrate the correlation of darker skin with equatorial proximity (Figure 2). More recent studies by physical anthropologists have substantiated and extended these observations a recent review and analysis of data from more than 100 populations (Relethford 1997) found that skin reflectance is lowest at the equator, then gradually increases, about 8% per 10° of latitude in the Northern Hemisphere and about 4% per 10° of latitude in the Southern Hemisphere. This pattern is inversely correlated with levels of UV irradiation, which are greater in the Southern than in the Northern Hemisphere. An important caveat is that we do not know how patterns of UV irradiation have changed over time more importantly, we do not know when skin color is likely to have evolved, with multiple migrations out of Africa and extensive genetic interchange over the last 500,000 years (Templeton 2002).


Watch the video: Moisture Surge I Πώς να αποκτήσετε δροσερή επιδερμίδα. (May 2022).