Chromatophores, Melanocytes, and Skin Color
Whether it is an African savannah or Antarctic desert, the organ that provides protection and a first line of defense in animals is the skin. When we look at the basic characteristics of life, two of the important features for survival are effective intragroup communication and response to the ecological environment. Skin functions as a media of communication for almost all living organisms from lower groups to higher groups in the taxonomic tree. With its color, texture, pattern, and multiple layers of tissues and different cells, skin helps to attract a partner and offers protection against an enemy. For all five major groups of vertebrates (fishes, amphibians, reptiles, birds, and mammals), there are certain features that are shared; at the same time, there are certain features that are unique to one individual class. The protein keratin is present across the classes of vertebrates and skin color is the medley of pigments for protection and attractiveness; feathers, however, are unique to birds alone.
Scales are formed from the skin for fishes, aiding in their movement by steering and manipulating hydrodynamics. For fishes, skin helps in osmoregulation and sometimes helps in respiration. For amphibians, skin also serves as a respiratory surface for gas exchange. Poisonous glands for self-defense in animals are produced from the skin tissue in fishes and amphibians. For reptiles, there are scales, but different in structure from fishes – reptilian scales are much harder for their adaptation to life on land. Interestingly, feathers in birds, which evolved from reptile scales, are extremely light to provide more efficiencies to handle aerodynamics. Most mammals’ skins are protected by hair. For mammals and birds, other associated functions of skin include thermoregulation and absorption. Ectothermic vertebrates (fish, amphibians, and reptiles) and endothermic animals (birds and mammals) use skin for absorption of sunlight in different ways for their physiology.
The sun, being the universal source of energy for food and fuel, interestingly is also the source of most of the beautiful colors in nature. Sunlight is a form of electromagnetic energy and made up of photons. The waves of light have different levels of energy; the lower the wavelength, the shorter the waves and the more powerful they are (having higher amounts of energy), while longer wavelengths have less strength. Among all the different ranges of wavelength, only a narrow range could be detected by human eyes and that spectrum is called visible light (400 – 700 nm). Light waves with wavelengths below 380 nm are ultraviolet (UV) rays and those with wavelengths more than 700 nm are infrared rays, neither of which are visible to humans. The visible light spectrum expresses the seven colors of the rainbow (violet, indigo, blue, green, yellow, orange, and red) and for human eyes it is a perception based on the amount of light absorbed versus the amount of light reflected from any object.
Skin color in vertebrates is mostly influenced by some pigments contained within specialized groups of cells in the upper epidermal layer of skin. For ectothermic vertebrates, which includes three lower groups of vertebrates, chromatophore is responsible for their vibrant skin color; for birds and mammals or endothermic vertebrates, skin color is controlled by melanocytes. Pigments for skin color could be divided broadly into two categories: photo protective and photo reflective. Those animals who have chromatophores contain pigments that reflect light, thereby expressing different colors. In some cases, reflection of light from change in the shape of skin folds as shields results in rapid change in skin color, as seen in geckos. Animals with melanocytes have melanin, a pigment that evolved to shield the DNA from harmful radiation of sunlight.
Among the photons of different waves that reach Earth’s atmosphere, about 95-96% are infrared and visible light, and only 3-5% are UV rays. In humans, UV rays are required by the body for vitamin D production and calcium homeostasis. There are three types of UV rays emitted by the sun, classified based on their wavelengths: UVA, UVB, and UVC. UVA rays range between 320 – 400 nm and are the least harmful, UVB rays range between 290 – 320 nm, and the shortest UV rays are UVC, between 100 – 290 nm. UVC is the strongest (lowest wavelength) among the three types and therefore the most harmful. But the UVC rays are completely absorbed by the ozone layer and the upper atmosphere and therefore never reach ground level. UVB rays pose the next level of harm to humans, but 95% of UVB are also absorbed by the ozone layer. The small fraction of UVB rays that do penetrate the ozone and reach the Earth’s surface are those that are mostly responsible for sunburn. UVB exposure of extended period of time is also known to cause wrinkles and premature aging and it is now known that UVB can cause skin cancer. UVA rays have the longest wavelengths and therefore the lowest energy, but since they are not blocked by the ozone, human skin is constantly exposed to UVA rays. Despite having longer wavelengths, UVA has a strong capacity of penetrating into skin and can also cause severe DNA damage leading to skin cancer or melanoma.
Looking at evolutionary history, it has been discovered that over time humans produce increased amounts of melanin, which acts as a natural sunscreen. Melanin is produced by a particular type of skin cells called melanocyte, which shields the DNA in nucleus from the damaging effect of UV rays. This is why when we are exposed to sunlight we can get a tan after long exposure as our body will produce more melanin to shield our DNA from UV exposure. UVB and UVA rays are mutagenic and can cause skin cancer if there is overexposure to sun. This selective pressure was the main reason for darker skin color for humans when modern humans evolved in Africa. The selective pressure again shifted to the other direction when humans migrated to the north as absorption of sunlight was necessary to maintain the level of vitamin D and calcium homeostasis. Melanocytes produce melanin to offer protection against the damaging effect of the sun and is controlled by the gene MC1R (melanocortin 1 receptor). Melanocytes produce two different types of pigments in both birds and humans: eumelanin and pheomelanin. Eumelanin acts more like photo-protective pigment and is expressed as darker in color, while eumelanin is more responsible for yellowish to red color. For humans, these pigments control the skin color, hair color, and eye color.
Absorption of UV rays by skin is necessary for normal human physiology, but people have to be careful about the potential damage to the skin and body due to overexposure. Also, humans have to understand that, biologically and scientifically, skin color does not define race; skin color for humans has changed multiple times due to the selective pressure and better adaptation of modern humans in all the possible niches.