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14.6: End of Chapter Content

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    • Leslie E. Fitzpatrick

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    For Further Exploration


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    Skin Color

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    Image Description

    Figure 14.3: When exposed to cold temperatures, basal metabolic rate increases, shivering begins and vasoconstriction helps maintain heat near the core of the body where the vital organs are located. When exposed to warm temperatures above 35 degrees celsius, vasodilation occurs and excess body heat is lost through sweating.

    Figure 14.4: Vasoconstriction is the constriction of peripheral capillaries in the skin. The decreased surface area of the capillaries through vasoconstriction results in less heat reaching the surface of the skin where it would be dissipated into the atmosphere. The opposite happens with vasodilation, where the peripheral capillaries in the skin are closer to the surface of the skin so that heat can be dissipated into the atmosphere, thereby cooling the body.

    Figure 14.10: Melanocytes are melanin-producing cells located in the bottom layer of the skin’s epidermis. Located in the central part of the epidermis, melanosomes are packets of color made by melanocytes. In light skin, melanosomes are small, with little pigmentation. In larger skin, they are larger and darkly pigmented. The differences in pigmentation are also due to differences in the distribution of melanosomes within the keratinocytes.

    Figure 14.12: Evolution of Skin Color Variation:

    1. Hair and Skin: Once hominins lost most of their body hair they likely had dark pigmented skin. In environments with high UV radiation, dark skin protected early humans against skin damage.
    2. Folate and UV Rays: UV rays penetrate the skin and can break down folate in the bloodstream. Folate is necessary for sperm production and for fetal development.
    3. Leaving Africa: Once some humans left Africa, they encountered environments with different levels of UV radiation. New selective pressures began to shape human skin color among these groups.
    4. Vitamin D: In low UV environments, people with dark skin could not synthesize enough vitamin D resulting in rickets and other health problems. People with dark skin had a lower chance of survival in most of these environments.
    5. Selective Pressures: Vitamin D, folate and changing UV environments were the selective pressures that resulted in the development of a variety of skin colors in different populations throughout the world.

    Figure 14.19: Life cycle of the malaria parasite: The mosquito injects the parasites when it bites the human. The parasites grow and multiply first in the liver cells and then in the red cells of the blood. In the blood, successive broods of parasites grow inside the red cells and destroy them, releasing daughter parasites (“merozoites”) that continue the cycle by invading other red cells. The blood stage parasites are those that cause the symptoms of malaria. When certain forms of blood stage parasites (gametocytes, which occur in male and female forms) are ingested during blood feeding by a female mosquito, they mate in the gut of the mosquito and begin a cycle of growth and multiplication in the mosquito. After 10-18 days, a form of the parasite called a sporozoite migrates to the mosquito’s salivary glands. When the Anopheles mosquito takes a blood meal on another human, anticoagulant saliva is injected together with the sporozoites, which migrate to the liver, thereby beginning a new cycle. (Adapted from Malaria: Biology: Lifecycle by the CDC).

    Figure 14.21: Map of the eastern hemisphere shows locations of different red blood cell abnormalities:

    • Thalassemia: most of Africa, Mediterranean, and SE Asia,
    • HbS: most of sub-Saharan Africa and smaller regions of the Mediterranean, Middle East, and India,
    • HbC: western Africa,
    • Ovalocytosis: parts of southeast Asia,
    • HbE: parts of south Asia, and
    • Pk deficiency: much of Europe.

    Figure 14.22: The effects of human horticultural activities on the balancing selection of populations in relation to sickle cell disease genotype variants: Humans arrive and clear the land to grow crops. Slash and burn horticulture leads to pools of stagnant water. Mosquitos flourish in the pools of water. Malaria increases as a result. Individuals with the sickle cell allele HbS are better at fighting malaria, leading to its increased frequency in the population. Balancing selection occurs in populations with both malaria and high incidences of sickle cell disease (from the HbSS genotype).

    Figure 14.23: A map of the eastern hemisphere with shading illustrating the population frequencies of adults with lactase persistence. Shades range from very light in populations with little lactase persistence (0-10%) to very dark (90-100%). Low frequencies were more common in southern Africa and southeastern mainland Asia. Frequencies were higher in western Africa, the Arabian Peninsula, and most of Europe. The highest frequencies were in northern Europe.

    This page titled 14.6: End of Chapter Content is shared under a CC BY-NC 4.0 license and was authored, remixed, and/or curated by Leslie E. Fitzpatrick (Society for Anthropology in Community Colleges) via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.