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Lecture 13

Lecture 13 - Human Adaptation Part 2

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Department
Anthropology
Course
ANT203H5
Professor
Esteban Parra
Semester
Fall

Description
Human Adaptation II: Skin Pigmentation UV Radiation and Skin Pigmentation - Skin pigmentation is one of the most variable phenotypes in humans. - The variation of skin pigmentation is highly correlated with latitude, as you can see in the following slides. - This was in the early 1900s, and is based on his study of pigmentation - Pigmentation is correlated with latitude: people living in the equator tend to have darker skin - And people living in higher latitudes, tend to have less melanin Skin Pigmentation and Melanin - Skin color is mainly due to the pigment melanin, which is produced by specialized epidermal cells known as Melanocytes. o Present in the skin, iris and hair o In the skin, these cells (melanocyte) is where melanin is going to be synthesize o Kerathinoxtyes is where the melanin passes through to go to other cells A Review of Melanin Synthesis - Within the melanocytes, melanin is synthesized in organelles known as melanosomes. - Melanosomes are then transferred, through dendritic structures, from the melanocytes to surrounding cells in the epidermis, known as keratinocytes. - Tyrosinase is the key enzyme involved in melanin synthesis. o This is the key enzymes (and there are more enzymes) o This is the major enzyme that is involvedd in the synthesis of melanin in the melanosomes - This is where melanin will be synthesis - Melanin will protect the cells, as it is found in the nucleus in the cells - Protects from ultra violet radiation Skin Pigmentation Variation - The number of melanocytes is similar in all human populations. Differences in skin pigmentation are mainly due to: - Type of melanin synthesized by the melanocytes o Eumelanin: brown/black. o Pheomelanin: red/yellow.  Red head synthesized pheomelanin in hair cells - Number, size and distribution of melanosomes. o In persons with light skin, melanosomes are small and packaged in groups. o In dark-skinned persons, there are more melanosomes, which are larger and packaged as a single entity. The Evolution of Skin Pigmentation - How can we explain the distribution of skin pigmentation in human populations, and more particularly, the strong correlation of skin pigmentation and latitude? - In this case, the selective factor seems to be UV-radiation: o Populations living near the equator receive high doses of UV radiation and they tend to have dark skin. o Populations living far from the equator receive much lower doses of UV radiation, and they show light skin color. - Let’s review in more detail the evolutionary factors that could be involved. Dark Skin Color and UV Radiation - Many hypothesis have been put forward to explain the evolution of dark skin color in equatorial and tropical environments with high UV radiation. o Dark skin protects against skin cancer.  This is something evolutionary biologists are spectacle about because people believe skin cancer happen relatively late in their life time where passing on genes has already occurred. o Dark skin protects against sunburn (severe sunburn can lead to infection and interfere with an efficient sweating mechanism).  Severe sunburn can be bad in some situations o Dark skin protects against folate destruction by UV radiation.  Folate is required for adequate fetus development, and also important in reproduction (lack of folate can result in fetal defects and also male infertility).  Folate is an important component related to one of the vitamins  has important role in the synthesis of DNA. Can be destroyed by high UV radiation. The major reason for this strong relationship is for the need to have the protection  because the protection of folate destruction The Evolution of Light Skin Color - When modern humans migrated out of Africa, they began to occupy regions with lower UV radiation. - The protection provided by high melanin levels was no longer necessary. o Less risk to skin cancer, sunburn and folate destruction. - Additionally, a new selective factor also played an important role, vitamin D synthesis. o Melanin will interfere with the production of vitamin D Light Skin Color and Vitamin D Synthesis - Vitamin D is necessary for a proper absorption of calcium, which is important for bone and teeth growth. - In our body, vitamin D is synthesized through the action of UV radiation. o Vitamin D is synthesized in the skin - In environments with low UV radiation, dark skin would be harmful, because it would interfere with proper production of vitamin D. - Thus, there was likely a selective pressure for lighter skin pigmentation in regions far from the equator, with low UV radiation. In Summary - Current evidence indicates that the evolution of skin color has been driven by a balance between protection against the harmful effects of UV in equatorial and tropical regions, and the need to facilitate vitamin D synthesis in regions far from the equator. - However, very little is known about the genetic basis of pigmentation variation and the evolutionary history of the genes responsible for pigmentation. o How many genes are involved? o Which variants in each gene? o Are the mutations different in different geographical regions? Some Important Genes of the Pigmentary System - Some of the genes are expressed in the melanamosomes - The
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