Human Variation

Studying Human Variation

Anthropologists investigate human variation by focusing on:

  • Who: Populations

  • What: Human variation

  • Where: Globally

  • When: Past and present

  • How: This is where problems can arise

History of the Race Concept

  • Early Examples: As early as 3100 BCE, Egyptians used different colors to distinguish between men, women, and various ethnicities.

  • Renaissance Influence: The Renaissance period (1500s-1700s) saw increased global travel, contributing to evolving perceptions of human diversity.

  • Early Classifications:

    • 1775: Blumenbach, drawing on Linnaeus' work, proposed four races, later expanded to five in 1795: European, Asian, African, and American, categorized as Caucasian, Mongolian, Ethiopian, American, and Malay.

    • 1839: Samuel Morton's "Crania Americana" classified humans into Caucasian, American, and Negro groups.

  • Eugenics Movement: The early 1900s marked the rise of the Eugenics movement.

This image shows a slide from a presentation about the Eugenics Movement. The slide's title is "Eugenics Movement". The bullet points describing the movement are difficult to read due to blurriness.

👨‍🏫 Franz Boas and the Disproving of Race

Franz Boas, often called the father of American anthropology, challenged and disproved the concept of race through research commissioned by the U.S. Congress.

  • Commission and Publication: Commissioned in 1908 and published in 1911.

  • Cephalic Index Study: Boas and his students measured the cephalic index (a ratio of head breadth to head length) in nearly 18,000 immigrant families.

  • Findings: Children of immigrants had larger head sizes, differently shaped skulls, and were taller on average than their parents who immigrated.

  • Conclusion: The environment significantly influences cranial shape and size, as well as body size, indicating that these traits are not solely determined by race.

🧬 Further Deconstruction of Race

  • Lewontin's Genetic Work (1970s): Explored genetic diversity among "races."

  • Key Finding: Genetic diversity was greater within groups than between them.

  • Implication: This finding challenges the biological basis for race.

  • Biological Determinism: Race is often associated with physical characteristics, which can lead to biological determinism:

Thinking that a person’s physical traits determine who they are

  • Clines, Not Races: Human variation exists in clines, not distinct races. A cline refers to the gradual change in a trait or allele frequency over geographic space.

  • Example: Skin color variation as an example of natural selection.

🌡 Adaptations vs. Acclimatizations

Human variation includes responses to environmental conditions that can be classified as adaptations or acclimatizations:

  • Adaptations:

Changes in physical structure, function, or behavior that allow an organism to survive and reproduce in a certain environment. Adaptations occur before adulthood and can be genetic or developmental.

*   **Genetic Adaptation Example**: Sickle-cell allele in malaria-prone areas.
*   **Developmental Adaptation Example**: Barrel-shaped chest in high altitudes.

  • Acclimatizations:

Short-term physiological responses to environmental stressors that demonstrate human plasticity. These can occur at any point in a lifetime.

*   **Example**: Increased red blood cell production after living in a high-altitude environment for an extended period. This is why Olympic athletes often train in Denver (a high-altitude city) versus sea-level cities.

⚖️ Maintaining Homeostasis

Adaptations and acclimatizations play a role in maintaining homeostasis:

Normal function of bodily systems, such as temperature and oxygen levels

🥶 Thermal – Cold Responses

  • Shivering

  • Vasoconstriction

  • Increased basal metabolic rate (BMR)

  • Lewis waves (Cold-Induced Vasodilation): Pattern of constriction then dilation of blood vessels in extremities to prevent frostbite within 10 minutes of cold exposure.

    • Present in European, Native Alaskan, and some Asian-derived populations, but often lacking in African-derived populations.

    • Potential explanation for higher blood pressure among African American populations.

    • Lacking in hypothermia situations because general survival is more important than saving a limb.

🔥 Thermal – Heat Responses

  • Sweating/Evaporation

  • Vasodilation

  • Loss of body hair

  • Conduction: Moving heat from the core to the periphery (e.g., sweaty palms and feet in the summer).

  • Humans' ability to sweat, combined with the loss of body hair, relates to our evolutionary history in Africa.

đź“Ź Body Adaptations to Thermal Environments

  • Bergman’s Rule: Body mass tends to be higher in populations residing in colder climates. As body mass increases, surface area decreases, leading to slower heat loss.

  • Allen’s Rule: Shorter limbs are favorable in colder climates to reduce heat loss, while longer limbs help heat dissipate faster due to increased surface area.

🏔 Altitude

Stressors at altitudes above 10,000 feet:

  • Hypoxia (reduced oxygen)

  • Increased UV radiation

  • Colder temperatures

  • Poor vegetation growth

Adaptations:

  • Increased lung capacity (barrel-shaped chest)

  • Large heart size (to pump more blood for faster oxygen movement)

  • Slowed growth and development (until the body develops enough to generate oxygen, delaying puberty)

Acclimatizations:

  • Increased respiration (trying to catch breath)

  • Increased RBC production (takes longer than increased respirations to take effect)

🎨 Skin Color

  • Measurement: Skin color is measured by skin reflectance, which is the amount of light the skin reflects, related to UV radiation. Dark skin has low reflectance and high UV exposure, while light skin has high reflectance and low UV exposure.

  • Melanin Production: Color is produced by melanin. All humans have the same number of melanocytes (melanin-producing cells), but they differ in the amount and size of melanin granules.

  • Melanin as Sunscreen: Melanin acts as a natural sunscreen. Darker skin is more protected from UV radiation.

  • Vitamin D Synthesis:

    • Lighter skin is needed at higher latitudes to absorb enough radiation to produce Vitamin D.

    • Insufficient Vitamin D leads to poor bone mineralization, causing rickets.

  • Folate Storage:

    • Folate is necessary for DNA replication & repair but is not produced by the body; it must be obtained through diet.

    • Excessive UV radiation exposure degrades folate, leading to neural tube defects (like spina bifida) in a fetus.

🛡 Cultural Adaptations

  • Coats, heat, mittens, scarves, etc., for cold weather.

  • Air conditioning and fans for hot weather.

  • Antibiotics for disease (although overuse can lead to antibiotic-resistant superbugs).

🍎 Nutrition

  • Basal Metabolic Requirement:

Minimum amount of energy needed to stay alive.

  • Malnutrition:

A lack of necessary nutrients or caloric needs on a long-term basis.

  • Nutrition Transition: Shift from eating locally grown, low-processed foods to foods high in fats and sugars but low in fiber and other nutrients.

  • Overnutrition/Dietary Excess: Can result in obesity, high cholesterol, and Type II diabetes.