The Biology of Skin Color — HHMI BioInteractive Video

Summary

Highlights

• Melanin acts as a guardian by protecting DNA from UV radiation. 🛡️
• The variation in human skin color is linked to global UV exposure. 🌐
• Dark skin favored in regions with high UV to protect folate levels. 🕶️
• Lighter skin evolved in low UV areas to aid vitamin D production. 💡
• Genetic studies highlight diverse adaptations in human skin pigmentation.🔬
• Cultural adaptations can help balance skin color mismatches with environments. 🌍

Key Takeaways

• Melanin is a powerful protector against harmful UV rays. 🌞
• Skin color evolved through natural selection based on solar exposure. 🌍
• UV light is both a threat and a necessity for health. 🌈
• Folate preservation underlies the evolution of darker skin in equatorial regions. 🌿
• Vitamin D synthesis influenced the emergence of lighter skin in northern areas. ❄️
• Skin color should never be equated with moral or social value. ✊

Overview

Chapters

• 00:00 - 00:30: Introduction to Skin Color Variation This chapter introduces the topic of skin color variation among humans. Unlike other biological characteristics such as the gray color of the human brain, red color of blood, and offwhite color of bones which are consistent across individuals, skin color differs among humans. This introduction sets the stage for further exploration into why these differences exist and what factors influence the variation in skin color.
• 00:30 - 01:00: Scientific Mystery of Skin Color Variation The chapter explores the scientific mystery behind the variation in human skin color, ranging from rich dark brown to pinkish white. It discusses how, historically, the reasons for this variation were not well understood, which led to the misuse of skin color differences to falsely attribute moral values and justify discrimination and suffering.
• 01:00 - 01:30: Evolution of Human Skin Color Biological traits, including human skin color, evolved through natural selection to enhance survival and reproductive success. Recent advances in anthropology and genetics have demystified the processes behind this evolution.
• 01:30 - 03:00: Role of Melanin in Skin Color This chapter delves into the study of human evolution and physical diversity, focusing particularly on the role of melanin in determining skin color. It highlights that skin, being one of the most visible markers, serves as a significant indicator of human variability. This is explored through the lens of biological anthropology, which seeks to understand the evolutionary reasons behind our physical differences, with a particular emphasis on the pigment melanin and its impact on skin color.
• 03:00 - 04:00: Protective Functions of Melanin This chapter explores the evolutionary development of human skin color compared to our closest animal relatives, such as chimpanzees, who have pale skin under their dark fur. It traces the origins of this diversity in skin coloration to our primate ancestors, examining how human skin color ranges have evolved over millions of years. The role of light wavelengths in determining color is also highlighted, suggesting a scientific basis for understanding how different skin colors came to be.
• 04:00 - 06:00: Link Between UV Radiation and Skin Color The chapter discusses how different wavelengths of light are absorbed or reflected by pigments, specifically melanin, in the top layer of human skin. This interaction is analogous to how leaves appear green because they reflect certain wavelengths while absorbing others.
• 06:00 - 09:00: Mapping UV Radiation and Skin Color Our melanosomes, produced by melanocytes, carry our genetic makeup which determines our melanin type. Light-skinned individuals have more reddish-yellow pheomelanin, while dark-skinned individuals have more brown-black eumelanin, dictating skin color darkness.
• 09:00 - 13:00: Evolutionary Genetics of Skin Color Melanin not only colors human and animal hair and bird feathers, but its significance lies in the wavelengths of light it absorbs rather than those it reflects. This is crucial because the most important absorptions pertain to wavelengths that are invisible to us, highlighting the biological importance of these invisible rays emitted by the sun.
• 13:00 - 15:00: Folate and Skin Color Evolution The chapter discusses the impact of ultraviolet (UV) radiation on living cells, specifically how it can penetrate cells and cause mutations in skin cell DNA. Melanin in human skin acts as a protective barrier against such threats from UV radiation, highlighting its evolutionary importance.
• 15:00 - 18:00: Vitamin D and Evolution of Lighter Skin The chapter discusses the protective role of melanin in human skin, especially in relation to UV radiation. Melanin acts as a guardian molecule, forming supranuclear caps around skin cell nuclei to absorb UV rays, preventing them from damaging the DNA. These caps function like parasols, effectively shielding the genetic material from potential harm caused by ultraviolet exposure. This protection might have played a crucial role in the evolutionary changes leading to lighter skin in certain human populations.
• 18:00 - 22:00: Genetic Variations and Skin Color Molecular biologist Zalfa Aldel Malik notes the diverse benefits of melanin across species, including temperature regulation in lower vertebrates, camouflage, and species recognition.
• 22:00 - 25:00: Modern Implications of Skin Color Variations The chapter discusses the correlation between melanin, skin color, and UV protection. Initially, as humans evolved and lost their body hair, an increase in melanin production in the skin occurred as a protective measure against UV radiation. The chapter suggests there may be a connection between the intensity of UV radiation in different environments and the resulting variations in skin color among human populations. The introduction to the study of UV radiation and skin color marks the beginning of a deeper exploration into how these factors have influenced human evolution and adaptation.
• 25:00 - 28:00: Conclusion: Evolutionary Perspective on Skin Color The chapter discusses the quest for data on the global distribution of solar UV radiation. Initially, available data appeared inconsistent and incomplete. However, through persistent search efforts, the author stumbled upon the necessary raw data which had been collected by NASA. This discovery happened accidentally and was not initially intended for the author's research questions.

The Biology of Skin Color — HHMI BioInteractive Video Transcription

• 00:00 - 00:30 [Music] human brains are gray human blood is red our bones are offwhite doesn't matter where you're born or to whom but human skin is different
• 00:30 - 01:00 some of us have rich dark brown skin some of us have pinkish white skin most of us are somewhere in between for the longest time why this variation exists was a real scientific mystery that opened the door for some to invest this biological trait with moral value and then use that to justify the suffering of others
• 01:00 - 01:30 but biological traits aren't good or bad they're features that have evolved because they enhance an organisms odds of surviving and passing on its genes like other animal traits the cpia rainbow of human skin color evolved through natural selection now thanks to advances in anthropology and genetics exactly how and why it did is no longer a mystery
• 01:30 - 02:00 [Music] biological anthropologists like myself spend our lives studying how humans evolved and why we differ from one another physically our skin provides one of the most visible markers of human variability
• 02:00 - 02:30 [Music] it's something that sets us apart from our closest animal relatives under their dark fur chimpanzees have pale skin and millions of years ago that was probably also the case for the primates that were our common ancestors so where did Humanity's range of skin colors come from from physics we know that the color of any object comes from the wavelengths of light that it
• 02:30 - 03:00 reflects back to an observer's eye we see leaves as green because they reflect back the wavelengths our eyes see as green absorbing the wavelengths we see as other colors like blue or red in humans different wavelengths of light are reflected or absorbed by a pigment in the top layer of our skin that pigment is called melanin it sits inside what look like tiny grains
• 03:00 - 03:30 the melanosomes that are produced by cells called melanocytes our individual genetic inheritance determines the type of melanin inside our melanosomes the reddish yellow fom melanin is more abundant in lightly pigmented people more Darkly pigmented people have more of the brown black um melanin and the more um melanin the darker the skin
• 03:30 - 04:00 melanin also colors human and animal hair and the feathers of many birds interestingly the wavelengths of light that melanin reflects are far less important biologically than the ones it actually absorbs and of the ones it absorbs the ones that are the most important are those that we can't even [Music] see much of the light given off by the sun is invisible to our
• 04:00 - 04:30 eyes some of that is what's called ultraviolet radiation which is highly [Music] energetic so much so it can actually penetrate living cells when it does it can wreak havoc within them it can even cause mutations in skin cell DNA what stands between us and that threat is the melanin in our skin
• 04:30 - 05:00 melanin is kind of like the sensor and kind of like a guardian molecule and its main job is protection for instance it protects skin cell DNA by forming what are called supranuclear caps and absorbing UV they're like little parasols around the nucleus and UV cannot penetrate these to go and attack the DNA that's just one of the things
• 05:00 - 05:30 molecular biologist zalfa aldel Malik finds remarkable about melanin another is the broad range of benefits it provides a diverse collection of species we know that melanin in lower vertebrates is important for regulating body temperature it can also give animals camouflage and allows them to recognize other members of the species to propagate the species
• 05:30 - 06:00 in humans one of melanin's functions is clearly to protect cells from UV damage as we evolved we lost hair and increased melanin production in our skin so is there a connection between the intensity of UV radiation and skin color hi Tess I first became fascinated with UV and skin color in the 199
• 06:00 - 06:30 90s but as I searched for information about the global distribution of solar UV I discovered the available data was in fact quite spotty I began casting a wider net and almost by accident found exactly the raw data needed to fix that it hadn't been collected by anyone interested in my questions but rather by NASA have
• 06:30 - 07:00 ignition and lift off in the 1980s concern about the health risk posed by the depletion of UV blocking atmospheric ozone LED NASA to take millions of UV measurements from space I asked NASA to send me the data and then asked my geographer husband George Chaplan to try to visualize it it turned out to be a bigger request than I'd realized but he found a way to turn
• 07:00 - 07:30 all those data points into a map a map that showed for the very first time exactly how UV exposure varies throughout the world so this is the map most striking was the clear gradient between the Equator and the poles which was interrupted only in places where altitude increased UV exposure this is actually in the Tibetan plateau and persistent cloud cover decreased it Congo Basin so it's full of humidity and
• 07:30 - 08:00 moisture which is blocking the UV solar energy is a fundamental attribute of any environment and it's a wellestablished fact that organisms living at different latitudes adapt in some way to their local solar conditions to see how closely human skin color correlates with UV exposure I collected skin pigment measurements made by anthropologists studying indigenous peoples for many years anthropologists
• 08:00 - 08:30 have faced the challenge of how to accurately measure skin color we now use this little device called a reflectometer basically it sends out light of specific colors and then it measures the amount of light that is reflected back this tells us what color Tess's skin is and we can then compare this to people all over the world George then created a second map using measured skin
• 08:30 - 09:00 colors and environmental data it showed UV intensity does indeed predict skin color wherever UV is strong skin is dark like it is near the equator or at high altitude at the poles the skin of indigenous people is almost always lighter that suggests that variation in human skin melanin production arose as
• 09:00 - 09:30 different populations adapted biologically to different solar conditions around the world as we've noted our early ancestors probably had full body hair covering pale skin just like other primates so when did the darker shades of human skin begin to evolve DNA sequencing has made it possible to find evidence that can help answer that
• 09:30 - 10:00 question Rick KD is a geneticist who's skilled at deciphering such Clues whenever a species under goes some form of selection uh some form of natural selection evidence of that selection is found in the genome and so as geneticist we get really excited when we explore the genome for these signatures one way in which that's done is by sampling worldwide populations and looking throughout the genome at variation and
• 10:00 - 10:30 comparing across populations and it's it's a very exciting process I feel like a detective when I when I go through that that process one of the many genes that genetic detectives have linked to human pigmentation is called mc1r sampling from around the world indicates there's a fair amount of variation in the DNA sequence of that Gene but not from every corner of the globe when we look at mc1r within
• 10:30 - 11:00 African populations we don't see a lot of diversity and the particular Al that they have in those African populations is the one that codes for darker skin mc1r codes for a protein which is involved in the switch from the production of fom melanin to um melanin and we know fom melanin is the reddish yellow pigment and then the um melanin is the brown black pigment the absence of mc1r diversity in African populations in indicates that in
• 11:00 - 11:30 that part of the world there is strong negative selection against any alals that would alter dark skin and how long has this alil been fixed in African populations other genetic Studies have calculated that it has been as much as 1.2 million years since our species evolved in Equatorial Africa it's reasonable to conclude that by that time all humans were dark skinned
• 11:30 - 12:00 the fossil record supports what we've gleaned from genetic evidence but here's where we confront what was for me the heart of the mystery the evolution of dark skin in humans suggests that under strong UV light that trait provided a survival Advantage so what exactly was that Advantage it's certainly true UV damage to skin cell DNA can lead to cancer and skin cancer can be fatal for a long time
• 12:00 - 12:30 that seemed the likeliest explanation except skin cancer generally develops after a person's Peak reproductive years for that reason though it might cut your life short it's unlikely to affect your ability to pass on your jeans as I was struggling to conceive of an alternative explanation I happened to attend a lecture on severe birth defects
• 12:30 - 13:00 that talk was about a research project that had found evidence that certain birth defects are far more common among pregnant women with diets deficient in a B vitamin called folate only weeks before I'd come across a paper that described how strong sunlight breaks down folate circulating in skin blood vessels here was a direct link between UV radiation skin color and reproductive
• 13:00 - 13:30 success it was a small Eureka moment for me in the years since we've learned that folate is not only essential for normal embryonic development it's even needed for healthy sperm production in males folate is biological gold it is an essential nutrient and it needs to be protected from UV radiation as it circulates in the blood vessels in the skin skin that is what melanin
• 13:30 - 14:00 does I felt I was halfway home on my quest to understand human skin color variation but a big question remained why aren't we all dark skinned it turns out there's another side to our relationship with UV light UV light is not all bad in fact the small portion of it known as UVB is critical For the synthesis in our bodies
• 14:00 - 14:30 of vitamin D a process that starts in the skin without vitamin D humans cannot absorb calcium from our diet to build our bones and for a healthy immune system back when all our ancestors liveed close to the equator there was no problem getting enough UVB through dark skin to make the vitamin D needed but then some populations start moving North
• 14:30 - 15:00 where the UV striking earth's surface is much weaker in Northern latitudes dark skin makes it hard to produce the vitamin D that human bodies really need the consequences of vitamin D deficiency include rickets a bone development disease that can the young in higher latitudes with less UV The Selective pressure on mc1r that produced dark skin in our ancient an ancestors began to Abate when we look at
• 15:00 - 15:30 the early movement Out of Africa when that constraint was relaxed we then see a plethora of variation in European and Asian populations geneticists have discovered greater variation in the mc1r gene but less variation in several other genes ones associated with lighter skin types different environments led to other genes being selected for and and
• 15:30 - 16:00 being important uh for those populations in terms of skin color selection for light skin Gene variants occurred multiple times in different groups around the world some of it in just the last 10,000 years support for the idea that the UV vitamin D connection helped Drive the evolution of paler skin comes from the fact that indigenous peoples with diets in this essential vitamin have dark
• 16:00 - 16:30 pigmentation the tension between these two aspects of our biological inheritance on the one hand the need to protect ourselves from most ultraviolet radiation and on the other the need to use some ultraviolet radiation for our own benefit these forces drove the evolution of the wonderful variation in
• 16:30 - 17:00 human skin color that we see around us today it's the legacy of an evolutionary Balancing Act necessitated by the different environmental conditions people have faced around the globe the thing is where once human migrations took many generations we now move about the planet at the speed of sound that means increasing numbers of us have pigment ation that's not a good
• 17:00 - 17:30 match with where we live people with fair skin and red hair your phenotype is telling you you have a high risk of skin cancer if you're out in the sun if you're a dark- skinned individual living for example in Scandinavia or in Minnesota you're not going to have optimal exposure to UV for optimal vitamin D synthesis and you need to take supplements we now know that we need to make cultural adaptations like these to stay heal healthy but that's not all
• 17:30 - 18:00 we've learned with the knowledge we now have about Evolution we also know that skin color is a flexible trait that has changed through time as various groups of people moved to sunny or less Sunny parts of the world and we know that skin color is inherited independently of other traits and is not associated with other aspects of a person's appearance or behavior
• 18:00 - 18:30 skin color is a product of evolution and should never have been judged as something good or bad we are a very clever and adaptable species and we are one under the sun [Music] [Music]