Biocultural Evolution

[V] The Life Cycle #

Tuesday, April 21, 2020 — 1:30 PM

Biocultural Evolution and the Life Cycle #

• Primate life cycles and human evolution: we are unique as culture plays role in our life history traits (and their influence on our health)
• Seek to understand our life history traits (gestation, interbirth interval, infant dependency, weaning, life expectancy..)
• Anthropologists use life history theory to answer questions about nonhuman primate and human growth and aging (ie? what is length of lifespan or lactation or why menopause)

→ What is the natural evolution of life history traits?

Growth #

• influenced by genes and environment
• humans getting taller due to better nutrition
• height of potential growth — not met if there is stress
• demonstrates plasticity of human biology

Life History Theory #

• organisms have limited amount of energy
• tradeoffs occur throughout
• large influence of culture on life history traits (ex. males prioritized over females)

Human Life Cycle #

Unique human traits #

• long period of dependency

• helpless newborn

• growth spurts

• menopause (postreproductive phase)

• growth spurts

• menopause (postreproductive phase)

Prenatal Phase #

• gestation, common in mammals
• major growth of organs

Infancy Phase #

• birth – age 4 (very long compared to other mammals)
• humans: extends past brain development
• human brains develop after birth (unusual)

Juvenile Phase #

• age 4 – puberty
• still dependent on parental care (costly)
• full size growth of brain

Adolescent Phase #

• development of complex social learning
• brain changes in structure
• trends: children are growing and maturing faster
• earlier age of menarche
  • Reasons: genetics, hormones in environment/food/water

Adult Phase #

• all animals
• fully mature
• reproductive phase
• no growth + development

Post Reproductive Phase #

• unique to humans ← human lifespan exceeds reproductive capability
• limited reproductive capabilities
  • menopause: 45-50 years old

Why? #

• child rearing (k-selection theory)

  • menopause advantageous towards allowing for better attention to a few offspring

• heterochrony (non selection theory)

  • menopause is not selected for; it is simply a consequence of having a long life span

• grandmother hypothesis

  • mothers can be able to care for grandchildren

• cultural variation in social status of menarche, menopause

• reproductive patterns vary based on culture

Nonhuman Reproductive Patterns #

• primates have first birth shortly after menarche
• 5 offspring on average
• lactation for 3-4 years

Similar to pattern in non-industrial human societies

Consequences of low fertility #

• osteoporosis (bone loss)
• cancers (ovarian, endometrial, breast)

Aging #

• becoming old and showing physical signs of age
• Senescence: the process of becoming old; less capability for repair/homeostasis

Life expectancy has changed but lifespan has not #

• infant mortality decrease

Effects of aging #

• reduced cardiac output/lung function
• fewer nephrons in kidney
• reduced immune response

Why senescence? #

• decreased # of cells (more dying than being created)

• genetic/lifestyle factors

• genetic/lifestyle factors

• decrease in cell function

  • damage to DNA
  • free radicals (byproduct oxygen/electrons)
  • shortening of telomeres

• pleiotropic genes: enhance reproductive fitness early on but lead to detrimental effects in aging

Causes #

Proximal vs Ultimate Causality #

• Long-term (genetic) changes of all individuals ← skin color, ability to sweat…
• Short-term acclimatizations ← shivering, tanning…
• Developmental adjustments ← high altitude, cultural practices like foot binding

Epigenetics #

• brain plasticity / memory can be increased
• change in expression + genetic potential, but not DNA itself
• changes in chemistry

Cultural Adaptations #

• capacity to manage various environmental situations: behavioral plasticity = culture
• selected for naturally

Skin Color #

• most widely studied characteristic
• clinal distinction: gradual change, not sudden change → associated w/ proximity to equator
• hemoglobin → pink
• carotene → yellow
• melanin → brown pigment
• generally associated with solar radiation levels
• Vitamin B folate is destroyed by sun, but UV radiation needed for vitamin D metabolism ← very rare except for sun; required for bone development / mineralization
  • reason for light skin
• females tan less easily to allow for greater vit D intake

rickets: bone metabolism abnormality due to vit D deficiency → osteomalacia in young

Why are Northern Europeans so light-skinned? #

• genetic drift
• higher carotene levels in other populations
• higher cold/frostbite tolerance

Temperature Responses #

• short term acclimatization → more efficient sweat response at risk of dehydration
• developmental adjustment → vasodilation, constriction
• long-term adaptation → body shape, sweat glands

Bergmann’s Rule #

Larger size means more volume relative to surface area

• result: larger mass = conserve body heat

Allen’s Rule #

Elongated forms lose heat more quickly

• hot climate (tall, narrow)
• cold climate (compact, square)

Allen’s Rule (slide) #

• Mammal bodies are like radiators
  • applied to limbs of mammals
  • Cold climates mammals have short thick limbs as less surface area so conserve heat
  • Hot climate have long thin limbs as more surface area to radiate heat

Altitude Responses #

Challenges: #

• lower pressure
• less oxygen → hypoxia
• less solar protection

Short term: #

increased respiration, heart rate

Medium term: #

increased lung capacity, blood cell production, heart size ⇒ stressful but reversible

Long term developmental acclimatization: #

• more extreme medium-term adaptations
• larger chests
• more efficient oxygen transport
• low blood pressure

Issues: (or adaptation?) #

• infant mortality
• low birth weight
• genetic changes with metabolism, etc

Lactose Intolerance #

• 75% of adults
• need enzyme lactase to process lactose
• not gradual loss

Reason: #

not supposed to drink milk after weaning

Evolution of lactose tolerance #

• only possible in humans in last 6–10 kya
• domestication of dairy animals → selection for mutation for lactose tolerance

Changing Adaptations of Other Organisms #

• biocultural change

• spillover of zoonotic infectious diseases

• Race, gender is a folk taxonomy that depends on developmental outcomes

  • categories created by culture, often arbitrarily
  • used to classify geographically isolated distinctions

• Human variation is non-concordant

  • height, weight not correlated w/ skin color, etc.
  • related both to genetic history and cultural influence

Race and Biology: Biological race is not real #

Genetic Variation in Humans #

• Only <5% DNA differ between all humans
• No support for races in genetic data
  • races also used as a word for subspecies
  • does not apply to humans: no subspecies due to mobility and gene flow
  • humans have a wide mix of DNA

Pre-evolutionary approach #

• Cultural factors, not biological, are used to distinguish race
• 14–15th century: increased awareness of human diversity
  • attempts to be made to understand this: Europeans seen as most advanced
  • polygenists: thought Europeans were created separately from others
  • monogenists: all humans created once but Europeans are superior

Scientific Classification #

• 1749: Buffon — first introduction of “race” / geographical features in explaining diversity
• 1758: Linnaeus — 4 groups based on location, behavior
  • Americanus, Europeaus, Asiaticus, Afer — still used to this day
• Cranial topology:
  • 1775: Blumenbach — anatomist, categorized skulls, 5 races
  • 1842: Retzius — cephalic index (thought skull shape was non-adaptive)
  • 1839: Morton — cranial volume associated w/ intellect
  • 1903: Hrdlička — founder of Smithsonian, AJPA

Biological Determinism #

• The association between biology and behavior

• Implies cultural/behavioral variations are inherited

• Suggests superiority of races over others

• implies cultural/behavioral variations are inherited

• suggests superiority of races over others

Eugenics Movement #

• late 19th century
• “betterment” of species through selective breeding
• regulation of marriage and fertility
• died out after WWII
• validated Nazi genocide
• Francis Galton: created IQ test to study intelligence and racial variation in intelligence

Racism Today #

• idea that behavior, culture is inherited physically
• cultural differences lead to conflict (rather than biological)
• health disparities seen as racial inequality
• well defined biological differences across races:
  • risk of disease
  • low birth weight

Race vs Racism #

health differences are not biological: due to social inequalities

• individual discrimination: blood pressure issues, etc
• institutional racism: different jobs, healthcare…

New Approaches #

• rather than classifying, more research is done on measurement of genetic characteristics
• 1950s: blood component phenotypes
• 1990s: direct genotype observation
• Polymorphism: trait related to many different genes

ex. ABO blood systems #

• clinal distribution
• varying frequencies based on group
• give immunity w/ different microbes

Biocultural Evolution #

(For the psychology-side framing of this idea, see culture — which develops Dual Inheritance Theory — and nature and nurture for the broader nature/nurture/culture decomposition. The sickle-cell polymorphism below uses the same genotype/heritability machinery as 2-3 genetics.)

• Culture is a human adaptive strategy
• Example: sickle cell anemia
  • Alteration in structure of hemoglobin
  • Blood cells are misshaped
    • Homozygous recessive for sickle shape: cannot transport oxygen, internal damage
    • Heterozygous: slight reduction in oxygen transport but resistant to malaria
      • Caused by protozoan parasite
      • Requires healthy blood cells to operate
  • Balanced polymorphism: heterozygous advantages but homozygous disadvantages

Diagram: Agricultural practices → Mosquitoes spread → Human malaria → Spread of sickle-cell mutation; Mosquitoes spread ↔ DDT spraying ↔ Adaptation: DDT-resistant strains

More on Malaria #

• Lots of variations on hemoglobin
• Thalassemia: defection in hemoglobin, most common in Mediterranean
• G6PD deficiency: no protection of red blood cell membrane

Summary of Human Variation #

• Non-concordant

• Cannot be described or categorized easily

• Must consider social and evolutionary complexity

• Cannot be described or categorized easily

• Must consider social and evolutionary complexity of variations

• Humans are very biologically similar

Vestiges of race #

• forensic anthropology: poor racial predictions/indicators
• sociobiology: study of the biological basis of behavior