Contemporary Human Diversity

Contemporary Human Diversity

• The concept of race: a biological fallacy

• Exploring relationships between evolutionary processes and cultural contexts

• The adaptive significance of some human variation

• We (humans) change/evolve, and so do other organisms

Human Variation

• The reality --> humanity lives in almost all global habitats

  • A combination of biology, culture, and technology

• Reminder: the biocultural approach recognizes the interrelationship of biology and the many facets of culture (including technology, behaviour)

  • Our biology shapes our capacity to develop culture, and our culture shapes our biologies.

• The adaptive significance of human variation

  • Adaptability: the capacity to maintain functions in response to short- or long-term stressors

The concept of race: a biological fallacy

• Long history of attempts at typologies

• People have been attempting to classify other people into discrete groups (races) for 1000s of years

  • But human population groups are not reproductively closed systems

  • Folk vs scientific classifications; colonial history

Polytypism

• Many types --> there is broad geographic variation within species, but only to a degree

  • Broad geographical clusters define our biology

  • The problem of drawing lines around these clusters

    • Who gets to draw the lines? Where do they get drawn?

    • Based on what?

    • Genetic diversity in our species larger within populations, not between them

Humans are Polymorphic

• There are alternative forms of traits

• Clines - Gradual shifts in trait prevalence through geography

  • A depiction of the frequency with which a character appears in one population compared to its occurrence

Consequently

• Biological anthropologists: concept of race cannot be applied to human variation

  • The American Anthropological Association (AAA's) statement

  • BUT: we can still recognize that there is biological diversity within the human species

Reprogenetics

• Read box 13.2

Evolutionary Processes and Adaptive Significance

• Remember: not all variations have adaptive significance

  • Some do --> but not always straightforward

• Responses to physical environmental stressors (pre-existing or things we have done to modify the environment)

Human Variation and Adaptive Significance

• Physical stressors and research

  • Extreme cold, extreme heat, UV radiation

  • Adapting to different environments

Skin Pigmentation

• Related to amount of melanin

• Many studies --> why are there variations?

  • Populations from higher latitudes generally have lighter skin (less pigmentation) than those living closer to the equator

• Initially discussed in terms of exposure to solar radiation

  • Argument that was made: more pigmentation closer to the equator --> greater protection against skin cancers triggered by UV ray exposure

  • Weaknesses with this argument

    • People with darker skin still get skin cancer

    • More importantly, skin cancer tends to affect people more later in life

• Vitamin D hypothesis

  • Vitamin D in liver, fish oil, butter, yolks, cream

  • Can also be produced by body through exposure to UV radiation

  • Uv radiation weaker further from the equator --> less skin pigmentation would be advantageous in these regions especially if Vitamin D rich foods are relatively rare

Today

• Higher rates of rickets and osteomalacia among people with more melanin living in Northern latitudes than people living at the same latitudes with less melanin

• Exceptions and diet/time

Variations in Skin Pigmentation

• Folate Degradation Argument

  • Folate: B Vitamin key to fetal neural tube development

    • Neural tube: the anatomical structure that forms the brain and spinal chord

    • Needs to be protected by bone

Folate Degradation Argument

• Folate susceptible to degradation by UV radiation

  • Darker skin (more melanin) would offer some protection from this (therefore, a selective advantage closer to the equator)

High Altitude Environments

• Higher than 2500m above sea level

• Multiple environmental stressors

  • High levels of UV

  • Dustiness, poor soils, difficult terrain

  • Limited ecosystem

  • Hypoxia

    • Inadequate oxygen in body tissues

    • High altitudes have lower barometric pressure (air pressure)

    • As barometric pressure decreases, so does the density of oxygen in the air

      • % of oxygen doesn't decrease - still at about 21% - but there are fewer oxygen molecules in any given volume of air

Health Consequences of Hypoxic Stress?

• Depends on altitude and rate of assent

• Depends on the individual

• Acute mountain sickness

  • People who did not grow up in high altitudes

  • Risk at over 2000m above sea level (if doing physical activity)

  • Over 3000m above sea level (if just relaxing)

Acute Mountain Sickness

• Symptoms

  • Headaches, dizziness, feeling faint: dues to lower oxygen intake

  • Nausea, vomiting

  •  due to respiratory alkalosis - body becomes more alkaline

May Also Experience

• Cheyenne-Stokes Syndrome - breathing rate oscillates during sleep - can wake up feeling like you're suffocating

• Problems with visual acuity (for ~48 hours)

Other Health Risks

• Edema: abnormal accumulation of fluids resulting in swelling of tissues

  • Particularly problematic if in lungs - high altitude pulmonary edema (HAPE) - can block oxygen transport

• High Altitude Cerebral Edema (HACE)

  • Very rare; fluid enters brain tissues

  • Behaviour disorders, convulsions, death

  • Individual must immediately be evacuated to lowlands

  • HAPE and HACE very rare in people born in highlands

Adaptations to Hypoxic Conditions?

• No cultural adaptations

• Biological possibilities

  • Increasing the efficiency and effectiveness of oxygen transportation into bodies, tissue

    • While adjusting to any body changes that might be side effects

Biological Responses to Hypoxia

• Possibilities in terms of

  • Oxygen transport from air to lungs

  • Oxygen transport from lungs to blood, blood transport to body tissues

  • Cellular changes in oxygen utilization

Possibilities

• Oxygen from air to lungs

  • Increased ventilation rate - more air to lungs

    • Tibetans (Andeans)

  • Barrel-shaped chests and air intakes?

    • Andean populations

  • Oxygen from lungs to blood and blood transport to body tissues

    • Greater blood flow, more capillaries in tissues - Tibetans

• Cellular changes

  • Cells becoming more efficient in providing energy for any given amount of oxygen

  • Andeans

  • (and lowlanders moving up to highlands??)

Timelines

• Ethiopian highlands: up to 47,000 years ago

• Tibet: at least 25,000 years ago

Variations in High Altitude Adaptations

• Andeans

  • Adapted to hypoxic conditions by developing ability to carry more oxygen in blood cells

• Tibetans

  • More breaths per minute than lowlanders; double the nitric oxide - works to dilate blood vessels and may increase oxygen uptake, higher density of capillaries

• High-Altitude Ethiopians

  • Appear to have neither set of adaptations… other mechanisms??

2014 Paper

• EPAS1 gene --> when oxygen levels drop, gene is activated (more hemoglobin)

  • For most people, variants of the gene can lead to increasing thickness of blood --> hypertension, heart attacks

  • Tibetans - allele raises hemoglobin and red blood cells only a bit at high altitude

• This variant EPAS1 is

  • … almost identical to that found in Denisovans (and different from other humans)

  • What does this suggest? Interbreeding perchance

Remember

• Adaptations not about perfect environmental fit

  • Rather, about minimizing risk of specific stressor-related death before reproducing successfully

  • Adaptation is not necessarily positive, just less negative

And…

• Sometimes even high altitude individuals cant cope with hypoxia

  • Subacute infantile mountain sickness found in some children born in high altitudes

    • Low blood oxygen levels and a high risk of death if not evacuated to sea level

  • Chronic mountain sickness: can affect those born in or long-term residents of high altitudes; must leave high altitude area

Another Key Point Re: Evolutionary processes + adaptive significance

• Some of the diversity in the human genome not just due to physical stressors like UV radiation

  • May also be responses to the environment that we have modified

Carrier of Malaria and Sickle Cell Allele

• Carriers that are heterozygotes have blood that is a deterrent to malaria parasite

  • Better resist the parasite once infected

  • Leads to a general advantage

    • But not a perfect adaptation

Another Interesting Aspect to This

• In areas with high malaria rates, traditional crops include cassava, millet, sorghum, cane sugar, dark lima beans

  • These contain cyanate and thiocyanate, which seem to inhibit the sickling of red blood cells - might reduce the symptoms of sickle cell anemia

    • Dietary changes thus can affect the sickle-cell situation

Another Example: Lactase Persistence

• Continued production of lactase enzyme after childhood; allows milk digestion

  • Pastoralism and dependence on herd animals and milk --> changes in the frequency of the lactase-persistent allele (selective pressure)

The Point?

• Our actions, decision, can shape environment around us, and can in turn shape our biologies

Finally: As we evolve, other organisms do too

• Often influenced by human actions

  • Antibiotic resistant bacteria

  • Changing viruses (and changing exposure to viruses)

  • Insect immunity to pesticides (DDT and mosquitoes)

  • Parasite immunity to anti-parasitic (malaria and Plasmodium parasite)

The Point:

• Even as we are the product of our environments, our actions also shape the environments/organisms around us